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OFFICIAL CHILEAN STANDARD NCb 2369.0f2003
NATIONAL STANDARDIZATION INSTITUTE. INN - CHILE
Seismic design for industrial structures and facilities
Earthquake resistant design of industrial structures andfadlities
First edition: 2003
Descriptors: structural design, industrial,facilities, requirements
seismic design, industrial
CIN 91.080.01
COPYRIGHT2003: NATIONALSTANDARDIZATIONINSTITUTE- INN "Not to be copied or sold"Address : Matias Cousino N° 64. 6° Piso. Santiago. ChileWeb : www.inn.c1
Member of : ISO (Intemational Organization for Standardization). COPANT (Panamerican TechnicalStandards Board)
:::::
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A ~T
('
4.3
4.4
4.5
4.6
4.7
Contents
Preamble
1 Scope and field of application
2 Regulatory references
3 Terms, definitions and symbols
3.1 Terms and definitions
3.2 Symbols
4 General application provisions
4.1 Basic principles and hypothesis
4.2 Manners in which to specifY seismic action
Classification of structures and equipment according to importance
Coordination with other standards
Load combinations
Seismic design project and review
General provision regarding the application of this standard
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.fl-
VIII
1
1
3
3
5
9
9
10
11
12
12
14
14
14
14
15
16
19
5 Seismic analysis
5.1. General provisions
5.2 Methods for analysis
5.3 Elastic static analysis
5.4 Elastic dynamic analysis
5.5
5.6
5.7
5.8
5.9
5.10
6
6.1
6.2
6.3
6.4
7
7.1
7.2
7.3
7.4
8
8.1
8.2
8.3
Vertical seismic action 21
Solid and rigid units of equipment supported on the ground 21
Design by horizontal differential displacements 21
Special analyses 22
Structures with seismic isolation or energy dissipators 23
Other structures not specifically referred to in this standard 24
Seismic deformations 37
Deformations calculation 37
Separation between structures 38
Maximum seismic deformations 38
P-De1ta effect 39
Secondary elements and equipment erected on structures 39
Scope 39
Forces for seismic design 39
Forces for anchors design 41
Automatic trip (shutdown) systems 42
Special provisions for steel structures 42
Applicable standards 42
Materials 43
Braced frames 43
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9
9.1
9.2
9.3
10
10.1
10.2
11
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
44
45
46
48
Special provisions for concrete structures 53
Reinforced concrete structures 53
Prefabricated concrete structures 54
Industrial premises comprised by projecting columns 57
Provisions with regard to foundations 59
General specifications for design 59
Superficial foundations 59
Specific structures 60
Industrial Buildings 60
Light steel Buildings 61
Industrial buildings with several floors 63
Large suspended buildings 63
Piping and ducts 63
Large mobile equipment 63
Elevated steel tanks, process vessels, and stacks 64
Vertical tanks supported on the ground 64
Furnaces and rotating driers 66
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8.4 Rigid frames
8.5 Connections
8.6 Anchors
8,7 Horizontal bracing systems
11.10
11.11
11.12
11.3
Refractory brick structures
Electrical equipment
Minor structures and equipment
Wood structures
Attachment A (regulatory) Typical details
67
67
67
67
68
Attachment B (regulatory). Design of beam to column joints in rigid steelframes 78
B.1
B.2
B.3
B.4
B.5
B.6
B.7
B.8
Overview
Design for the panel zone of moment joints
Local flexion of the column wing due to a traction force perpendicularto the column 82
Local fluence of the core due to compression forces perpendicular tothe wing
Crushing of the core due to compression force perpendicular to thewing
Buckling of the core compression
Additional requirements for continuity stiffeners
Additional requirements for reinforcement sheets
Attachment C (for information) Comments
C.1
C.2
C.3
C.4
Scope
References
Terminology and symbols
General application provision
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78
78
83
85
86
87
87
88
88
89
89
89
C.5
C.6
C.7
C.8
C.9
C.lO
C.ll
C.B
Figures
Figure 5.1
Figure 5.1
Figure 5.1
Figure 5.2
Figure 8.1
Figure 8.2
Figure Al
Figure A2
Figure A3
Figure A4
Figure A 5
Seismic analysis
Seismic deformations
Secondary elements and equipment erected on structures
Special provisions for steel structures
Special provisions for concrete structures
Foundations
Specific structures
Design of beam to column joints in rigid steel frames
References
a) Seismic zoning in Regions I, II & III
b) Seismic zoning in Regions IV, V, VI, VII, VIII, IX, X &Metropolitan Zone
c) Seismic zoning in Regions Xl & XlI
Examples for width / thickness ratio of Table 8.1
Columns base
Roof Bracing
Detail of crane carrier and columns
Extreme", wall bracing
Column joint to brick wall
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93
101
101
102
104
106
106
115
117
34
35
36
37
52
53
68
68
69
69
70
Figure A 6
Figure A 7
Figure A8
Figure A 9
Figure Al 0
Figure All
Figure A12
Figure A13
Figure B.1
Figure B.2
Figure B.3
Figure B.4
Figure C.1
Figure C.2
Figures
Table 5.1
Table 5.2
Table 5.3
Table 5.4
Table 5.5
Rigid equipment in building
Typical detail of large suspendedconnectors and anchor bolts
equipment, seismic
Typical detail of large mobile equipment
Rail wheel system
Typical detail of large tanks
Typical detail of furnaces and rotating driers
Typical detail of industrial brickworks
Typical detail of minor structures and equipment
Reinforcement plates attached
Forces in the panel zone
Response spectrum for Huachipato
Design spectrum for Huachipato
Seismic zoning by municipality for Fourth to Ninth Regions
Value of maximum effective acceleration Ao
Definition of the foundation soil types
Value of parameters that depend on the type of soil
Absorption (Damping) rates
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70
71
73
73
74
75
76
77
80
81
84
86
97
99
107
25
29
29
30
30
Table 5.6
Table 5.7
Table 7.1
Table 8.1
Maximum values for the response modification factor 31
Maximum values for the seismic coefficient 33
Maximum values for the response modification factor forsecondary elements and equipment 42
Limits for the width / thickness ratio 50
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NCh2369
OFFICIAL CHILEAN STANDARD NCh 2369.0f2003
Seismic design for industrial structures and facilities
1 Scope and field of application
1.1 This standard establishes the requirements for seismic design of industrial structuresand facilities, both light as well as heavy. It applies, both to the structures themselves, aswell as the ducts and piping systems and mechanical and electrical process units, andtheir anchors. It also applies to warehouse structures or premises with industrialapplications, and constructions structured with braced columns.
1.2 This standard does not apply to other types of structures such as nuclear plants,electrical power plants and transmission lines, presses, washing/sweeping reservoirs,bridges, tunnels, gravitational jetties, containment walls, underground duct lines, etc.
1.3 Office buildings, canteens or those for home applications may be designed accordingto NCh433.0f96.
1.4 It is supplemented by NCh433.0f96, Seismic building design. All the requirements ofsaid standard that are not specifically modified are applicable.
2 Regulatory References
The following regulatory documents contain provisions that, by means of references in thetext of the standard, constitute requirements of the standard
To the date of publication of this standard the edition shown below was current.
All standards are subject to revision and it is recommended that all parties that are toreach agreements, based on this standard, investigate the possibility of applying morerecent editions of the standards included below.
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NCh2369
Note: The National Standardization Institute keeps a record of current national andinternational standards
NCh203NCh433NCh1159
NCh1537
NCh2745ACI 318ACI 350.3
AlSC 1989
AlSC 1999
AlSC 1999
AlSI 1996
API 620
AWWA-D 100AWWA-D 110
AWWA-D 115
UBC 97
NZS 4203
ASTM A 36jA36M-97aASTM A 242jA242M-97ASTM A 325-97
ASTM A 490-97
ASTM A 500-98
ASTM A 501-98
Steel for structural use - Requirements.Seismic design for buildings.High-Strength Low Alloy Structural Steel forconstruction
Structural building design - Permanent loads and overloadsdue to use.Analysis and design of buildings with seismic insulationBuilding Code Requirements for Structural Concrete. 1996.Practice for the Seismic Design of Liquid ContainingStructures.
Specifications for Structural Steel Buildings, AllowableStress Design.Seismic Provisions for Structural Steel Buildings - Part I:Structural Steel BuildingsLoad and Resistance Factor Design Specifications forStructural Steel Buildings.Specifications for the design of Cold Formed Steel StructuralMembers.Design and Construction of Large. Welded, Low-PressureStorage Tanks.Standard for Welded Steel Tanks for Water StorageWire and Strand Wound Circular. Prestressed ConcreteWater TanksCircular Prestressed Concrete Water Tanks withCircumferential Tendons.Uniform Building Code. 1997.Seismic Design of Storage Tanks. Recommendations of aStudy Group of the New Zealand National Society forEarthquake Engineering, 1986.General Structural Design and Design Loadings forBuildings. 1992.Specifications for Carbon Structural Steel.Specifications for High-Strength Low Alloy Structural Steel.Specifications for High-Strength Bolts for Structural SteelJoints.Specification for Heat-Treated Steel Structural Bolts. 150 ksiMinimum Tensile Strength.Specification for Cold-Formed Welded and Seamless CarbonSteel Structural Tubing in Rounds and ShapesSpecification for Hot-Formed Welded and Seamless CarbonSteel Structural Tubing
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ASTM A 502-93ASTM A 572jAS572M-97c
ASTM A 588jA588M-97a
ASTM A 913j913M-97
ASTM A 992jA 992M-98
ANSjAWSA5.1-91
ANSjAWS A5.5-96
ANSjAWSA5.17-89
ANSj AWS A5.18.93
ANSjAWS A5.20.95
ANSjAWS A5.23-90
ANSjAWSA5.29.80(R 1989)
NCh2369
Specification for Steel Structural Rivets.Specifications for High-Strength Low-Alloy ColumbiumVanadium Structural Steel
Specification for High-Strength Low-Alloy Structural Steelwith 50 ksi (345 MPs) Minimum Yield Point to 4 in. (lOOmm)ThickSpecification for High-Strength Low-Alloy Steel Shapes ofStructural Quality, Produced by Quenching and Self-Tempering Process (QST).Specification for Steel for Structural Shapes for Use inBuilding FramingSpecification for Carbon Steel Covered Arc-WeldingElectrodesSpecification for Low-Alloy Steel Electrodes for ShieldedMetal Arc-WeldingSpecification for Carbon Steel Electrodes and Fluxes forSubmerged Arc-WeldingCarbon Steel Electrodes and Rods for Gas ShieldedArc-WeldingSpecification for Carbon Steel Electrodes for Flux Cored ArcWeldingSpecification for Low-Alloy Steel Electrodes and Fluxes forSubmerged Arc WeldingSpecification for Low-Alloy Steel Electrodes for Flux CoredArc Welding
NOTE - Foreign standards deemed necessary may be referenced.
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this standard, the following terms and definitions shall apply, whichsupplement the terminology of NCh433.0f1996:
3.1.1 permanent loads (CP): action of which the variation over time is negligible withregards to the mean values or those for which the variation trends towards a limit.
According to this definition the following shall be included under this concept:
Own weight of structural elements and finishingsOwn weight of fIXed units and facilitiesNormal contents of vessels, hoppers, belts and equipment.Weight of ducts without accumulations or scale. InsulationPermanent thrusts
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NCh2369
3.1.2 connection: region in which several prefabricated elements or one prefabricatedelement and an element formed on site are joined.
3.1.3 strong connection: connection that remains elastic while the predefined area ofthe plastic joint develops inelastic response under severe seismic conditions.
3.1.4. humid connection: connection that uses any of the joining systems in sections21.2.6,21.2.7 or 21.3.2.3 of ACI 318.99 to connect prefabricated elements, and useconcrete or mortar poured and formed on site to fill the space of the joint.
3.1.5 dry connection: connection between prefabricated elements that does not qualifYas a humid connection.
3.1.6 process engineer: engineer responsible for production processes, general layout ofequipment and operating structures and processes of the industry.
3.1. 7 braced frames: structural systems with diagonal braces. The members, beams,columns and diagonals work mostly by axial stress.
3.1.8 ductile frames with non structural dilated members: are those in which the nonstructural members are separated from the frame columns by a space equal to or greaterthan the dmaxvalues defined in section 6.3.
3.1.9 ductile frames with non structural non dilated members: are those in which thenon structural members are separated from the frame columns by a space that is lessthan the dmax values defined in section 6.3. In these cases the non structural membersare to be incorporated in the structural model avoiding in the design failure due toshearing of the beam to column joints.
3.1.10 rigid frame: structural system where the beam to column joint has the capacity totransmit the moment factor. Its lateral stability on the plane depends on the rigidity toflection of the component members.
3.1.11 fundamental vibration period: mode period with the greatest equivalenttranslational mass in the direction of the analysis.
3.1.12 specialist professional: acknowledged professional in structures, legallyauthorized to practice in Chile, with proven experience in earthquake resistant design ofat least 5 years.
3.1.13 seismic risk: probability that a certain seismic event will occur in a certain areaand within a preestablished time interval.
3.1.14 overloading caused by use: actions static in nature, variable over time that aredetermined as a result of the function and use of the building and its facilities. Exhibitsfrequent or continued variations that are not negligible in comparison to the mean value.
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NCh2369
According to this definition, the following is to be included under this concept:
Unifonn loads corresponding to the use of floors and platfonns and that includenonnal traffic of persons, vehicles, light duty mobile equipment and build-up ofmaterials.
Scale and accumulation of dust in ductwork, equipment and structures.
Lifting loads of cranes.
Non pennanent water or ground pressure.
Internal pressures in vessels.
Tensions from drive belts or similar.
3.1.15 special operational overloads (SO): dynamic actions produced by the nonnal useof the facilities.
According to this definition, the following is to be included under this concept:
Impacts and loads in general of a dynamic origin although they are modeled asequivalent static actions.
Braking action.
Actions caused by moving liquids or gases, i.e. water hammer.
3.1.16 accidental operational overloads (SA): actions caused by operationalphenomena that only occur occasionally during nonnal use of the facilities.
According to this definition, the following is to be included under this concept:
Extreme impacts and explosions
Short circuit loads
Overfilling loads from tanks or hoppers
3.2 Symbols
The symbols used in this standard have the meaning shown below:
Ao =maximum effective acceleration of the ground;
Ak =weighting factor for the step associated to level, Ie;
c =seismic acceleration for horizontal seismic action;
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NCh2369
Cp =coupling coefficient between modes i andj;
Cmax =maximum value of the seismic coefficient;
Cv = seismic coefficient for vertical seismic action;
CP =permanent loads;
D =outer diameter of a circular section; tank diameter or process vesseldiameter:
E =elasticity module;
Fa =admissible tension due to compression;
Fk =horizontal stress applied in level k;
Fp =seismic horizontal stress to design a secondary element or unit ofequipment;
Fv = vertical seismic stress;
Fy =flow tension;
Fyf =flow tension in the wing of the metallic frame;
H = height of the highest elevation over the base elevation; total height of thebuilding over base elevation; height of door or walkway supports;
=coefficient related to the importance, use and risk of failure of a structureor unit of equipment;
K =buckling length coefficient;
Kp = dynamic amplification factor for the design of a secondary element orunit of equipment;
L =length of an element, clearance of a door or walkway;
P =total weight of the building or structure over the base elevation;
Pk = seismic weight associated to level k;
Pp =weight of a secondary element or unit of equipment;
Qo =base shear stress of the building or structure;
Qp =base shear stress of a secondary element or unit of equipment;
Qmin =minimum value of the base shear stress;
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NCh2369
R = modification factor of the structural response;
Rl =modification factor of the structural response defined in 6.1;
Rp = modification factor of the response of a secondary element or unit ofequipment;
S =factor resulting from the spectral modal superimposing; minimum lengthof support; separation between structures;
Sa =spectral design acceleration for horizontal seismic action;
Sav = spectral design acceleration for vertical seismic action;
Se = moment, shearing stress or axial stress in the connection associated todevelopment of probable resistance (8 <II1II) in predefined locations ofstructure plastification, based on the mechanism that controls inelasticperformance;
Si =maximum value of the contribution of mode I with its symbol;
SA =accidental operational overload;
SC =overload caused by use;
SO =special operational overload;
Tl =vibration period of mode j.T' = parameter that depends on the type of soil;
T" =fundamental vibration period in the direction of the seismic analysis;
Zk =height of the k elevation over the base elevation;
a =overload reduction factor;
ap =acceleration in the support elevation of an element or equipment;
ak =acceleration in the k elevation of a structure;
b = amplification or majoration factor of loads; half the width of the wing inwelded or laminated T or double T and TL frames; rated width of the wingin laminated Channel and Angle frames; distance from the free edge of thewing to the beginning of the curve of the fold in cold formed frames;distance between the beginning of internal curves of the wing for Z, CA,and Q folded frames; distance from the free edge to the first line of
connectors or welds, or width between connector lines or sheet welds;
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NCh2369
bf =width of the wing;
d =horizontal seismic defonnation; total height of laminated and welded Tframes;
ck =horizontal seismic defonnation; calculated with seismic requirementsreduced by the R factor;
maxd = maximum admissible value of ck;
d. = maximum horizontal seismic defonnation of the i structure;
do = defonnation caused by non seismic service loads;
e = thickness of the wing of a metal frame; thickness of the tank shell,stacks or process vessels;
9
h
=acceleration of gravity;
= free distance between wings of welded frames; free distance betweenwings subtracting the dimension of the fillet in laminated profIles;distance between the closest connectors in bolted profIles; distance in thecore between the initial points of the curves of the folds in cold fonnedframes; height of the structure at a certain elevation over the baseelevation; height between two points of a structure located over a commonvertical;
k =factor that influences in the limitation of the width/thickness rate ofdouble T, T, Channel frames;
n =parameter that depends of the type of soil; quantity of levels;
'\' = tuming radius; coefficient between periods associated to two vibrationmodes;
t =thickness of the wing of a metal profIle;
tw = thickness of the core of a metal profIle;
~ =damping rate;
<l>b =reduction coefficient of the resistance stipulated in AISC - LAFD;
Ar =limit of the width-thickness rate so as not to have local buckling;
Ap =limit of the width-thickness rate to allow complete plastification of thesection.
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NCh2369
4 Local Application Provisions
4.1 Basic principles and hypotheses
4.1.1 The design provisions in this standard, applicable together with the specific designstandards for each material, are focused on meeting the objectives described below:
a) Protection of life in the industry
a.l) Avoid the collapse of structures for earthquakes that are more severe than thedesign earthquake.
a.2) Avoid fires, explosions, or the emission of gases or toxic liquids.
a.3) Protect the environment.
a.4) Ensure the proper operational condition of escape routes during a seismicemergency.
b) Continued operation of the industry
b.l) Maintain essential processes and services.
b.2) Avoid or reduce to a minimum the duration of business interruption.
b.3) Expedite inspection and repair of the damaged elements.
4.1.2 In general, it is accepted that the seismic analysis be based on the use of linearmodels of the structures, but the dimensioning of the resistant members must beperformed with the method specified in the standards for each material, which may be ofadmissible stresses or limit loads.
4.1.3 In order to meet the objective stated in 4.1.1,a.l) the structures must have anample resistance reserve and / or be capable of absorbing great quantities of energy,exceeding the elastic limit, before failure. In order to achieve this the global structuralsystem must meet the following requirements:
a) Ensure ductile performance of the resistant members and connections, in order toavoid failure due to instability or fragility, or alternatively, ensure elastic performance.
b) Provide more than one line of resistance for seismic requirements. Seismic resistantsystems must be redundant and hyperstatic. Exceptions to this rule shall be madeonly with the explicit approval of the professional specialist defined in 3.1.12.
c) Be provided systems that are simple and clearly identifiable for the transmission ofseismic stresses to the foundations, avoiding structurizations that are highlyasymmetric and complex.
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NCh2369
In order to meet the industry operational continuity objectives, as well as those stated ina.2) and a.3), the structures, units of equipment and their anchorages must be designedso that for earthquakes that are more severe than the design earthquake the followingrequirements are met, in addition to those listed in a), b) and c):
d) Limit incursions into the non elastic range, in the event that these should endangercontinued operation or the rescue operations.
e) The damage must occur in visible and accessible locations.
f) Those emergency and control units of equipment, the operation of which during theemergency must be guaranteed, must be duly certified according to internationalstandards, with the approval of the process engineers or the professional specialist.
4.1.4 In order for the earthquake resistant structure to be ductile during cyclicperformance, according to what is set forth in 4.1.3 a), the requirements from clauses 8, 9and Attachment B, must be complied with.
4.1.5 The professional specialists and process engineers defined in 3.1.12 and 3.1.6 shallestablish, for each project, the seismic design conditions of all structures, equipment andanchors, in order to meet the objectives listed in 4.1.1. In particular, the seismicclassification must be listed for each structure and unit of equipment, as well as themethods for analysis, criteria, relevant parameters and illustrative drawings, which shallbe duly recorded in the project specifications. The seismic design of the units ofequipment can be performed by the engineers of the manufacturers supplying theequipment, but the approval must be done by the professional specialist defined in 4.6.2.
4.1.6 Location
In order to determine the location of the industry the risks posed by other phenomenaassociated to seismic action must be taken into consideration, such as topographicalamplification, tidal waves, displacements due to faults and landslides, liquation ordensification of the soil. To address these issues, in addition to complying with what is setforth in 4.2 of the MCh433.0f96, it shall be necessary to execute the correspondinggeological, topographical, tidal waves and geotechnical surveys applicable, which must becarried out by experienced professionals.
4.2 Manners in which to specify seismic action
Seismic action can be specified in one of the following ways:
a) By means of horizontal & vertical seismic coefficients, applicable to the weights ofthe various parts in which the seismic system has been divided for analysis, accordingto what is set forth in 5.3, 5.5 and 5.6.
b) By means of response spectrums of linear systems with a certain degree of freedomfor horizontal and vertical translation of the foundation soil, according to what is setforth in 5.4 and 5.5.
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NCh2369
c) By providing descriptive values of the movements of the soil, such as the maximumsfor soil acceleration, velocity and displacement, both in horizontal direction as wellvertical or other similar, according to what is set forth in 5.8.1.
d) By means of actual or synthetic accelerograms duly formulated for horizontal andvertical movements of the foundation soil, according to what is set forth in 5.8.2.
When using one of alternatives a) or b) what is set forth in 4.1 of NCh433.0f96 must befollowed, regarding seismic zoning of the national territory (Figure 5.1 and Table 5.1), andin 4.2 of said standard, with regards to the effects on foundation soil (Table 5.3) and ofthe topography on the characteristics of seismic movement.
The use of alternatives c) or d) must be based on the results of seismic hazard surveys,that address regional and local seismic characteristics, the geological, geotechnical andtopographical conditions, and the direct and indirect consequences of failures instructures and equipment. In any event it is necessary to meet what is specified in 5.8.1and 5.8.2.
If it is suspected that close field effects exist, a special analysis must be made thataddresses them.
4.3 Classification of structures and units of equipment according to importance
4.3.1 Classification
For all purposes pertaining to the application of this standard, structures and equipmentare classified in categories according to their importance as follows:
Category Cl. Critical projects, due to any of the reasons shown below:
a) Vital, that must be kept in operation to control fires or explosions and ecologicaldamage, and provide attention for health and first aid requirements of thoseaffected.
b) Dangerous, where failure involves the risk of fire, explosion or pollution of the airor waters.
c) Essential, where failure may cause prolonged shutdowns and major losses inproductivity.
Category C2. Regular projects, that can experience minor failures that can berepaired quickly and do not cause prolonged shutdowns or major losses inproductivity, and that do not endanger other projects of category C 1.
Category C3. Minor or temporary projects and equipment, where seismic failurewould not cause prolonged shutdowns, and would not endanger other projects ofcategories Cl & C2.
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NCh2369
4.3.2 Importance Coefficients
Each category has a cOITesponding importance coefficient I, with the following values:
C 1 I = 1,20
C2I=I,00
C3 I = 0,80
4.4 Coordination with other standards
4.4.1 Chilean standards
The provisions in this standard must be applied together with what is provided in otherload or design standards for each type of material, according to what is set forth in 5.3 ofNCh433.0f96.
4.4.1 Foreign standards
For each of the loads or materials not provided for in 5.2 and 5.3 of NCh433.0f96,internationally recognized standards or criteria must be used, accepted by theprofessional specialist approving the project (see 4.6.2).
Furthermore. said standards and criteria must comply with the basic principles andhypotheses listed in 4.1 of this standard.
4.5 Load Combinations
The combination of seismic requirements with permanent loads and the various types ofoverloads must be made using the superimposing rules listed below:
a) When the design is made with the admissible tensions method:
i) CP + aSC + SO*) + SA*J.:!:.Horizontal Earthquake.:!:. Vertical Earthquake")
ii) CP + SA*!.:!:.Horizontal Earthquake.:!:. Vertical Earthquake")
*) The SO and SA loads are combined with the earthquake only if for these one of thetwo following conditions is confirmed:
i) The SA action is derived from the OCCUITence of the earthquake. and must beconsidered with its symbol.
ii) It is normal to expect that when the earthquake starts the SO action is ongoingand is not interrupted during the earthquake and due to its action.
If the earthquake should have an effect such that the SO or SA action isnecessarily interrupted when the basal accelerations commence, this action mustnot be taken into consideration.
**) The vertical earthquake shall be taken into considered in the cases included in5.1.1 and the magnitude shall be determined according to 5.5.
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NCh2369
In these combinations the admissible tensions can be increased by 33.3%.
b) When the design is made with the ultimate loads method:
i) 1,2 CP + aSC + SO*) + SA*) :t Horizontal Earthquake:t Vertical Earthquake")
ii) 0,9 CP + SA*J:t Horizontal Earthquake :t 0,3 Vertical Earthquake")
In which:
a= the factor that affects the determined overload SC without considering any type ofreduction. It must be taken as equal to 1,0, unless where in agreement with aprocess engineer. the reduction of the figure above is allowed. which must providefor the probability of the simultaneous occurrence of the overload together withthe seismic requirement level listed in this standard. In any case, the value of "a"shall those listed below as a minimum:
TIPE OF PREMISES ~
Warehouses and in generallaydown (storage) areas with a low rotation rate 0,50
Normal use areas, operation platforms 0,25
Diagonal braces that support vertical loads 1.00
Maintenance walkways and roofs °b= amplification factor of seismic loads, defmed as a function of the methods of
analysis currently used for various materials. Takes on the following values:
Steel structures or equipment b=1.1
Concrete structures or equipment b = 1.4
In the combinations i) listed in a) and b) above, the + and - symbols for the verticalearthquake must be applied such that an effect is achieved that is added to that producedby loads CP and SC. In the combinations ii) listed in a) and b) above. the + and - symbolsfor the vertical earthquake must be applied such that the reverse effect is achieved. thatis, reduce the effect of loads CP and SC.
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NCh2369
The seismic requirement is an eventual load that is not to be superimposed on othereventual loads. For special locations in mountainous and elevated areas, where there cannormally be wind or snow of great magnitude and duration, special surveys must beperformed in order to determine the values of these loads that have a probablecoincidence with the design earthquake.
If it is necessary to assess various levels of content in vessels, pipelines or tanks, thenumber of these combinations increases to cover the different situations.
4.6 Seismic design project and review
4.6.1 The original seismic design must be carried out by professional specialists, (see3.1.12). The only exception is for units of equipment designed by overseas suppliers.
4.6.2 The seismic design of all structures and of their equipment and anchors, whicheverthe origin may be, must be approved by professional specialists, other than the designers.
4.6.3 The drawings and engineering calculations must include as a minimum theinformation specified in 5.11 of NCh433.0f96. The drawings and engineering calculationsmust be signed by the original design engineer listed in 4.6.1, and by the professionalspecialist listed in 4.6.2.
The only exception shall be the C3 category structures and equipment, where thesubmittal of drawings signed by the original designer shall be sufficient, listing thedimensions and materials of the stress resistant members, weights, centers of gravity andanchoring details. 0
4.6.4 The revision and approval of the seismic design does not release the originaldesigners from their full responsibility regarding compliance with the standards andspecifications.
4.7 General provision regarding the application of this standard
If the type of structure is explicitly referenced in this standard the corresponding designprovisions must be used. If the structure can be associated to various classifications thatimply different design provisions, the most stringent must be applied.
5 Seismic Analysis
5.1 General provisions
5.1.1 Seismic requirement direction
The structures must be analyzed, as a minimum, for seismic actions in two approximatelyperpendicular horizontal directions.
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NCh2369
The effect of vertical seismic accelerations must be considered in the cases listed below:
a) Suspension bars for hanging units and the support members and steel beams thatare made of welded, laminated or folded materials. with or without collaboratingslab, located in seismic zone, in which the permanent loads represent over 75% ofthe total load.
b) Structure and elements of precompressed concrete (prestressed and posts tressed).
c) Foundations and anchors and support members of structures and equipment.
d) Any other structure or member in which the variation of the vertical seismic actionsignificantly affects their dimensioning, such as for example, braced structures ormembers.
e) Structures with insulation that is sensitive to vertical effects.
5.1.2 Combination of the effects of horizontal earthquake components
For the design of earthquake resistant structural members, in general. it is not necessaryto combine the effects the effects caused by two horizontal components of the seismicaction. It is possible to proceed as if those effects were not concurrent, and as aconsequence, the members can be designed for the earthquake acting according to eachof the directions of analysis considered separately.
The exception to this simplifYing rule is structures that exhibit notorious torsionalirregularities or that have, in both directions, rigid frames with common columns to twointersecting resistant lines. In such cases, the members must be designed for the stressesobtained when considering 100% of the seismic requirement that acts in one directionplus the stresses obtained from considering 30% of the seismic requirement acting in adirection that is orthogonal to the above. and vice versa. The greater stresses that are theresult of the above two combinations must be considered.
5.1.3 Seismic mass for the structural model
For the calculation of the horizontal inertia stresses during an earthquake, theoperational overloads can be reduced according to the probability of their simultaneousoccurrence with the design earthquake.
Notwithstanding the above, the use overloads can be reduced by multiplying them by thecoefficients shown below:
- Roofs, platforms and walkways both for operation as well as for maintenance :0
- Storage warehouses. filing rooms and similar : 0,5
In order to determine the effects of the vertical earthquake in the cases listed in 5. I .1, thereduction of vertical loads must not be considered, except for those listed in NCh1537 foruse overloads.
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\JCh2
5.2 Meth~ysis
5.2.1 Ovetd8'
The seisI11it8Jysis shall be generally carried out using linear methods, for a seismicaction sp~ccording to 4.2a), or 4.2b), or 4.2c).
In special~,the analysis can be based on non linear response, for a seismic actionspecified ~g to 4.2d).
5.2.2 LinC8lltthods
Three pro~ can be used:
a) Static all@sis or of equivalent stresses, only for structures 20 m high, provided thatthe seis1l'itresponse can be assimilated into a system with one degree of freedom.
b) Spectr.d:&llldal analysis. for all types of structures.
c) Specialmdhods for structures with elastic behavior, according to what is set forth in5.8.
5.2.3 Non"'" methods
The methods6f::non linear analysis represent one of the special analysis methods listed in5.8 and mcd.fIie time-history analysis conditions in 5.8.2.
According to the principles of the standard in 4.1, non linear incursions must becontrolled jn-C)Iderto guarantee the continuity of industry operation.
The non linear method must appropriately model the resistive capacity and theperformance. of the structural members backed by laboratory tests performed for thispurpose orbyDormally accepted experimental studies.
The ductiIiIJ demand must not exceed the limit established according to the damageallowed, but in no section shall it be greater than 2/3 of the local ductility available.
The ma.xim:UPlcalculated non linear displacements must not be reduced and must meetthe limits listed in 6.3.
The non Jinea1:.model may include the dynamic interaction of soil structure. but itsinJ1uence sftaJI.be limited to 75% of the results obtained from the same non linear modelbut with a ligidbase.
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NCh2369
5.3 Elastic static analysis
5.3.1 Mathematical model of the structure
5.3.1.1 The mathematical model of the structure must be capable of adequatelyrepresenting the transfer of requirements from the application points towards thesupports. To meet this objective, it is necessary to include, at least. all the members of theearthquake resistant structure, the rigidity and resistance of those relevant members inthe distribution of stresses and the correct spatial location of the masses.
5.3.1.2 In general, a three-dimensional model must be used, excepting cases where theperformance can be predicted with flat models.
5.3.1.3 In structures without rigid horizontal diaphragms, it is necessary to defme asufficient number of nodal degrees of freedom associated to translational masses.Wherever necessary, the rotational masses must also be taken into consideration.
5.3.1.4 In structures with rigid horizontal diaphragms, a model with three degrees offreedom per floor may be used.
5.3.1.5 In structures that support units of equipment that have an influence on theirresponse, the mathematical model must consider the equipment-structure assembly.
5.3.1.6 For major hanging units of equipment, the mathematical model must include thesuspension and interconnecting devices between the unit and the supporting structure.
5.3.1. 7 If the soil characteristics or the type of foundation make it necessary to considerthe effect of soil-structure interaction. uncoupled springs may be used for translation andturning.
5.3.1.8 The effects of natural torsion and accidental torsion must only be considered inthe elevations that are provided with rigid diaphragm. The effect of accidental torsion canbe included considering the possible variations in the distribution of own loads andoverloads. In the event that no information should be available to perform the above. whatis provided in 6.2.8 of standard NCh433.0f96 must be used.
5.3.2 Horizontal baseshear stress
The horizontal shear stress at the base must be calculated according to the expressionbelow:
Qo =CIP (5-1)
Where:
Qo =shear stress at the base;
C =seismic coefficient, defmed in 5.3.3;
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NCh2369
I =importance coefficient specified in 4.3.2;
P =total weight of the building over the base elevation, calculated as shown in 5.1.3. Forthe effects of this calculation, the base elevation is considered to be the plane whichseparates the foundation from the structure, excepting where the professional specialiststates the opposite.
5.3.3 The seismic coefficient is determined from:
(5-2)
where:
Ao =maximum effective acceleration defined in Table 5.2 according to theseismic zoning established in Figure 5.1 and Table 5.1;
T',n =parameters that depends on the type of foundation soil, that aredetermined from Tables 5.3 and 5.4;
T =fundamental vibration period in the direction of analysis;
R =modification factor of the response established in Table 5.6;
~ =damping rate established in Table 5.5.;
5.3.3.1 The value C does not need to be greater than what is indicated in Table 5.7.
5.3.3.2 In no case shall the value C be less than 0,25 Ao/g.
5.3.4 Fundamental vibration period
The fundamental vibration period T must be calculated with a theoretic procedure orproven empiric procedure.
5.3.5 Distribution at height
The seismic stresses must be distributed at height according to the formula below:
Equ.ation (5-3)
Equation (5-4)
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NCh2369
Where:
Fk =hOlizontal stress in level k;
Pk,F] = seismic weight in levels k and);
Ak =parameter in level, k (k = 1 in the lower level);
n =number of levels
Qo =base shear stress;
Zk , Zk-l =height over the base of levels k and k-];
H = height of the highest elevation over the base elevation;
5.4 Elastic dynamic analysis
5. ,4.1 Mathematical model of the structure
Provisions 5.3.1.1 to 5.3.1.7 from the static elastic analysis must be used.
5.4.2 Design Spectrum
The spectral modal analysis must be performed for the following design spectrum:
Equation (5-5)
where:
T =vibration mode of the considered period
However, the value of Sa must not be greater than ICmaxx g, where Cmax is determinedfrom Table 5.7.
5.4.3 Number of modes
The analysis must include sufficient vibrating modes so that the sum of the equivalentmasses, in each analysis direction, is equal or greater than 90% of the total mass.
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NCh2369
5.4.4 Model superimposition
The seismic stresses and deformations must be calculated superimposing the maximummodal values by the Complete Quadratic Superimposing method, according to theformulas below:
Equation (5-6)
Equation (5-7)
where:
S = modal combination;
S;, Sj =maximum values of the contributions of modes i and j;
Cy = coupling coefficient between modes i andj;
~ =damping rate established in Table 5.5.;
T;,7j =period of modes i and j;
5.4.5 Minimum base shear stress
If the stress in base shear Qo is less than the following value:
Equation (5-8)
all the deformations and stresses must be multiplied by the quotient Qmin/ Qo for designeffects.
5.4.6 Accidental torsion
The effect of accidental torsion must only be considered in the elevations that areprovided with rigid diaphragm. In such cases, this effect can be included considering thepossible variations in the distribution of own loads and overloads. In the event that noinformation should be available to perform the above, what is provided in 6.3.4 ofstandard NCh433.0f96 must be used.
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NCh2369
5.5 Vertical seismic analysis
5.5.1 Vertical seismic action can be considered in static manner. in the manner listedbelow:
a) For the cases listed in 5.1.1 a) and 5.1.1 b) an equal vertical seismic coefficient mustbe applied, especially elements equal to Ao/ g. In this manner the vertical seismicstress must be: Fv = .:!:(Ac/ g) /P, where P is the sum of the permanent loads andoverloads.
b) For the cases provided for in 5.1.1 c) and 5.1.1. d), the seismic coefficient must be2Ac/3g.
c) For the cases provided for in 5.1.1 e), what is set forth in 5.9 must be followed.
5.5.2 Altematively, a vertical dynamic analysis can be developed with the accelerationsspectrum indicated in the expression (5-5). for R=3 and {;=0.03. In this case, the spectralarranged does not need to be greater than lAo. Damping factors greater than 0.03 must bespecially justified.
5.6 Solid and rigid units of equipment supported on the ground
This provision refers to units with their own fundamental period lower or equal to 0,06 s.including the effect of the system that connects the unit to the foundation.
These units can be designed with the static analysis method, with a horizontal seismiccoefficient equal to 0,7 Ac/g and a vertical seismic coefficient equal to 0.5 Ac/g.
5.7 Design by horizontal differential displacements
For bridges or walkways that join buildings, towers or other units it is necessary toprovide horizontal supports that allow actual seismic displacement between structures orequipment listed in 6.2.
On no account must the support length be less than S. where:
S {cm} =20 + 0.2L + 0.5H; L ~ 60m (5-9)
where
S =minimum support length (see Figure 5,2);
L =clearance in meters of the bridge or walkway between supports;
H =height in meters of the supports of the bridge or walkway over the foundationseal of the highest structure or the.
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NCh2369
5.8 Special Analyses
5.8.1 Spectral analyses
5.8.1.1 Special spectrums can be developed applicable to a certain project, such that theytake into consideration the characteristics and importance of the works to be executed,the site geotechnical conditions, the distance from the seismogenic sources, theircharacteristics, and the local amplification or reduction factors of the intensity of themovement of the ground as a function of the location topography, of the eventual effectsof the direction of the waves, or of the configuration or constitution of the subsoil.
To this end parameters can be defined such as the maximum values for acceleration, forsoil velocity and displacement, and by means of these configure special spectrums for theviscous damping levels in Table 5.5, or define others that allow formulations similar tothose contained in NCh433.0f96.
5.8.1.2 For design purposes, the definition of maximum acceleration values, velocity anddisplacement must take into consideration the historical background and deterministicinformation that can be applied or related to the study site, which can be supplementedwith probabilistic figures obtained from seismic risk analyses developed for earthquakeswith a return period of 100 years. In the risk analysis the attenuation formulas used shallcorrespond to the values expected from acceleration, velocity or displacement, thatcorrespond to the seismogenic stresses considered in the study.
5.8.1.3 The base shear stresses obtained with the spectrum defined by means of thisspecial analysis must not be less than 75% and do not need to be greater than 125% ofthose achieved with the methods listed in 5.4.
5.8.2 Time-history analysis
5.8.2.1 For time-history analysis at least three actual records must be used, thatrepresent the seismogenic zones considered, escalated such that the spectrum resultingfrom the combination of the spectrums from each record, by means of the square root ofthe average of the squares of the individually escalated values, is not within any point inthe interest frequencies range below the design spectrum defined in 5.8.1.
5.8.2.2 Alternatively, a synthetic record can be used the spectrum of which providesvalues that are higher than those defined in 5.8.1, for the entire interest frequency range.
5.8.2.3 When three different records are used, the maximum values of the interestparameter shall be adopted for the design, obtained by the application of each one ofthese. In this definition it is understood that interest parameter is the requirement, axialstress, shear, flection moment or deformation obtained for each member in particular orfor the structure considered in a global manner.
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NCh2369
5.8.2.4 When the time-history analysis is linear the resulting stresses in the memberscan be divided by the R factors listed in Table 5.6, provided that the displacementscalculated are compatible with the limits imposed in 6.3.
5.8.2.5 The time-history analysis must be performed considering each time movements inonly one of the main structure directions, acting simultaneously with the verticalexcitation.
5.8.2.6 In the time-history analyses, damping must be taken from Table 5.5 and theduration of the record must be equal to or greater than 120 s. unless a seismic risk studyestablishes the use of a different duration.
5.8.3 Minimum base shear stress
If the base section stress determined in accordance to 5.8.1 or 5.8.2 results in a valuelower than the one below:
Equation (5-10)
all the deformations and stresses must be multiplied by the quotient Qrnin / Qo for designeffects, excepting in the event that a non linear time-history analysis.
5.9 Structures with seismic isolation or energy dissipators
5.9.1 General overview
5.9.1.1 Seismic isolation and energy dissipators are understood to be any deviceincorporated into the resistant scheme of a structure intended to modifY the dynamicproperties, either by altering its fundamental vibration period, increasing its energydissipation capacity or modifYing the distribution of stresses, in order to improve itsseismic response.
5.9.1.2 The resistant system of the structure to lateral stresses and the isolation and / orenergy dissipation system must be designed to resist the deformation demand andresistance produced by the seismic movement, in accordance with what is specified in5.9, 5.8.1 and 5.8.2 of this standard.
5.9.1.3 The mathematical model of the physical structure must represent the distributionof the masses and rigidity of the structure at a level that is appropriate for the calculationof the significant characteristics of its dynamic response. A tridimensional model of thesuperstructure must be used that includes vertical displacements in the isolators. Forcases such as those listed in 5.1.1 e) the model must include the vertical freedom degreesin the dynamic analysis. The damping rates used must be those that correspond to theisolation or energy dissipation systems.
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NCh2369
5.9.1.4
D
e analysis and verification of the isolation and energy dissipation systems mustbe perfo ed by means of a spectral modal analysis or response history over time or infrequenc es. The spectral modal analysis may only be used if the device or isolator issuscepti Ie to being modeled in a validated equivalent linear manner.
5.9.1.5
~
he spectral analyses (see 5.4 and 5.8.1) or response history (see 5.8.2) must beperforme considering the horizontal components, one at a time, acting in the plant in themost un avorable direction simultaneously with the vertical component, where necessaryaccordin to 5.1.1 e).
he constitutive relations stress-deformation considered in the analysis for theevices, must be duly established and backed by laboratory tests.
5.9.1. 7
~
structures provided with isolation and / or energy dissipation systems the baseshear li itations listed in 5.3.3.2 and 5.4.5 are not applicable. Likewise, in structureswith isol tors the limitation for maximum deformation listed in 6.3 is only applicable tothe supe structure and not to the isolation interface.
ctures with seismic isolation
ic isolation systems must be analyzed and designed in accordance with theof NCh2745.
ctures with energy dissipators
5.9.3.1
~y structure with an energy dissipation system must be designed using the
spectrums described in 5.4 or 5.8 and subsequently verified using three recordscompatib e with the level of demand implied in the design spectrum, in accordance withthe meth dology listed in 5.8.2.
5.9.3.2
Ihe seismic analysis of structures with energy dissipation systems must be
performe using dynamic analysis procedures that adequately consider the stress-deformati n constitutive relationship of the devices included in the structure.
5.9.3.3
~
he dissipation systems to be used in a structure must have been previouslysubjected to experimental studies that confirm a stable cyclic behavior of the device, aswell as p ssible variations of its properties with temperature.
If the bas~ shear stress Qo determined for these structures results in a value less than thevalue bel6w:
r structures not specifically referred to in this standard
(5-11 )
all the deformations and stresses must be multiplied by the quotient Qmin/ Qo for designeffects.
1sion does not apply to structures that are explicitly referred to in Table 5.6.
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NCh2369
Table 5.1 - Seismic Zoning by Municipalities for Fourth to Ninth Regions
- 25-
Rej!ion Zone 3 Zone 2 Zone 1
AndacolloCombarbahiCoquimboIla pelLa HigueraLa Serea
4th Los VilosMincheMonte PatriaOvallePaiguanoPuntaquiRio HurtadoSalamancaVicuna
Algarrobo Calle LargaCabildo Los AndesCalera San Esteban
CartagenaCasablancaCatemuConcanEI QuiscoEITaboHijuelasLa CruzLa LiguaLimache
5th LlayllayNogalesOlmuePanquequePapudoPetorcaPuchuncaviPutaendoQuillotaQuilpueQuinteroRinconadaSan AntonioSan FelipeSanta MariaSanto DomingoValparaisoVilla AlemanaVina del MarZapallar
NCh2369
Table 5.1- Seismic Zoning by Municipalities for Fourth to Ninth Regions (continued)
- 26-
Region Zone 3 Zone 2 Zone 1Alhue BuinCuracavi Calera de TangoEI Monte CerrillosLampa Cerro NaviaMaria Pinto Colina
Melipilla ConchaliSan Pedro EI BosqueTiltil Estacion Central
HuechurabaIndependenciaIsla de MaipoLa CisteraLa Florida
Metropolitan La GranjaLa PintanaLa ReinaLas CondesLo BarnecheaLo EspejoLo PradoMaculMaipuNuiioaPainePedro Aguirre CerdaPeiiaflorPeiialolenPirqueProvidenciaPudahuelPuente AltoQuilicuraQuinta NonnalRecoletaRencaSan BernardoSan JoaquinSan Jose de MaipoSan MiguelSan Ramon
SantiagoTalaganteVitacura
NCh2369
Table 5.1- Seismic Zoning by Municipalities for Fourth to Ninth Regions (continued)
- 27-
Reion Zone 3 Zone 2 Zone 1La Estrella ChepicaLas Cabras ChimbarongoLitueche CodeguaLoiol Coinco
Marchigiie CoitaucoNavidad Doiiihue
6th Palmilla GranerosParalillo MachaliParedones MalloaPeumo Mostazal
Pichidigua NancaguaPichilemu Olivar
Purranque PlacillaSanta Cruz Quinta de Tilcoco
RancaguaRengoRequinoaSan Fernando
San Vicente de Tagua TaguaCauquenes ColbunCharico CuricoConstitucion Linares
Curepto LongaviEmpedrado MolinaHualaiie ParralLicanten PelarcoMaule RaucoPelluhue RetiroPencahue Rio Claro
7th San Javier RomeralTalca Sagrada FamiliaVichuquen San Clemente
Teno
Villa AlegreYerbas Buenas
NCL >9
Table 5. I-Seismic Zoning by Municipalities for Fourth to Ninth Regions (conclusion)
- 28-
Rei!ion Zone 3 Zone 2 Zone IArauco AntucoBulnes CoihuecoCabrera EICannenCaiiete Los AngelesChillan MulchenCobquecura NiquenCoelemu Pemuco
Concepcion PintoContulmo QuilacoCoronel QillecoCuranilahue San FabianFlorida San IgnacioHualqui Santa Barbara
Laja TucapelLebu YungayLos Alamos
8th LotaNacimiento
NegreteNinhuePencoPortezueloQuillonQuirihueRanquilSan CarlosSan NicolasSan RosendoSata JuanaTalcahuanoTiruaTomeTreguacoYumbel
Angol Collipulli CurarrehueCarahue Cunco LonquimayGalvarino Curacautin MelipeucoLos Sauces Ercilla PuconLumaco Freire
Nueva Imperial Gorbea9th Puren Lautaro
Renaico LoncocheSaavedra PerquencoTeodoro Schmidt PitrufquenTolten Temuco
Traiguen VictoriaVilcunVillarrica
Table 5.3 -Definition of the foundation soil tType
ofSoil
I
IV
NCh2369
Table 5.2 Value of maximum effective acceleration ADSeismic Zone AD
1 O,20R;2 0.30R;3 0,40~
to be used with Table 5.4)
Description
II
Rock: Natural material with waves propagation velocity cross section in-situ Vsequal or greater than 900 mis, or resistance of uniaxial compression of intactsamples (without fissures) equal or greater than 10MPa and RQD equal or greaterthan 50%.
a) Soil with Vs equal or greater than 400 mls in the upper 10 m and increasingwith depth; or
b) Dense gravel, with dry unit weight rd equal or greater than 20 kN/m3, or
density index ID(DR) (relative density) equal or greater than 75%, or degree ofcompaction greater than 95% of the Modified Proctor Value; or
c) Dense sand, with ID(DR) greater than 75%, or Standard Penetration Index Ngreater than 40 (standardized to effective overload pressure of 0,10 MPa) ordegree of compaction greater than 95% of the Modified Proctor Value; or
d) Hard cohesive soil, with a resistance to non drained shear Su equal or greaterthan 0,10 Pa (resistance to simple compression qu equal or greater than 0,20MPa) in samples with no fissures.
In all cases, the conditions listed must be fulfilled independently from theposition of the phreatic level. and the minimum thickness of the stratum mustbe 20 m. If the thickness over rock is less than 20 m, the soil does not classifyas type I.
III a) Permanently non saturated sand, with ID(DR) between 55 and 75%, or Ngreater than 20 (standardized to effective overload pressure of 0,10 MPa); or
b) Non saturated gravel or sand, with a compaction degree les than 95% of theModified Proctor value; or
c) Cohesive soil with Su between 0,025 and 0,10 MPa (qu between 0,05 and 0,20MPa) independently from the phreatic level; or
d) Saturated sand with N between 20 and 40 (standardized to effective overloadpressure of 0,10 MPa).
Minimum stratum thickness: 10m. If the thickness over rock or soil is lessthan 10 m, the soil will classify as type II.
Saturated cohesive soil with Su equal or greater than 0,025 MPa (quequal or greaterthan 0,050 MPa).Minimum stratum thickness: 10m. If the thickness of the stratum over soil of
es I, II or III is less than 10m, the soil will c1assifvas tvDeIII.
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NCh2369
5.4 Value of parameters that depend on the type of soil
Table 5.5 - Damping Rates
- 30-
Type of Soil T'(s) nI 0,20 1,00II 0,35 1,33III 0,62 1,80IV 1,35 1,80
Resistant System
Welded steel structure: stacks, silos, hoppers, pressure tanks, processto\Vers, pipUo, etc. 0,02Bolted or riveted steel structure 0,03Welded steel frames \Vith or \Vithout bracin. 0,02Steel frames \Vith bolted field ioints, \Vith or \Vithout bracin2". 0,03Reinforced concrete and brick structures (masonrv) 0,05Prefabricated reinforced concrete structures purely ravitational 0,05Prefabricated reinforced concrete structures \Vith humid joints, not 0,05expanded from non structural members and incorporated into thestructural model
Prefabricated reinforced concrete structures \Vith humid joints expanded 0,03from non structural members
Prefabricated reinforced concrete structures \Vith dry joints, expandedand non expanded
With bolted joints and connections by means of bars embedded in fillmortar. 0,03
With \Velded connections 0,02Other structures not included or assimilated to those in this list 0,02NOTES
1) In the event that an analysis is used \Vith soil-structure interaction, \Vhere theresulting values of the damping rate of the first mode are greater than thosesho\Vll in these tables, the increase of this rate shall not be greater than 50% ofthe listed values. The values for the remaining modes must be those listed in thistable.
2) In the event that there should be doubts regardUog the classification of a resistantsystem, provision 4.7 must be applied.
NCh2369
Table 5.6 - Maximum values of the response modification factor
continued
- 31 -
Resistant System R1. Structures desiQ:ned to remain elastic 12. Other structures not included or assimilated to those in this list 1) 23. Steel structures3.1 Buildings and structures with ductile steel frames with expanded non 5
structural members
3.2 Buildings and structures with ductile steel frames with non expanded 3non structural members and incorporated into the structural model
3.3 BuildinQ:s and structures with braced frames, with ductile anchors 5
3.4 Industrial single story (floor) buildings, with or without overhead 5boom crane and with continuous roof bracinJ;!
3.5 Industrial single story (floor) buildings, without overhead boom crane 3and without continuous roof bracinJ;! , that fulfill 11.1.2
3.6 LiJ;!ht steel sheds that fulfill 11.2.1 43.7 Inverted pendulum structures 2) 33.8 Isostatic seismic structures 3
3.9 Steel sheeting or siding structures, were the seismic behavior will be 3controlled by local buckIinJ;!.
4. Reinforced concrete structures
4.1 Buildings with reinforced concrete structure ductile frames with 5expanded non structural members
4.2 Buildings with reinforced concrete structure ductile frames with non 3expanded non structural members and incorporated into thestructural model
4.3 BuildinJ;!s and structures of reinforced concrete with isolation wall 5
4.4 Industrial single story (floor) buildings, with or without overhead 5crane and with continuous roof bracinJ;!
4.5 Industrial single story (floor) buildings, without overhead crane and 3without continuous roof bracinJ;! , that fulfill 11.1.2
NCh2369
Table 5.6 - Maximum values of the response modification factor (continued)
- 32 -
continued
Resistant Svstem R4.6.Inverted pendulum structures 2) 34.7 Isostatic seismic structures 35. Prefabricated reinforced concrete structures5.1 Purely ,gravitational prefabricated structures 55.2 Prefabricated structures with humid joints with non expanded non 3
structural members and incoroorated into the structural model5.3 Prefabricated structures with humid joints with expanded non 5
structural members
5.4 Prefabricated structures with dry joints, expanded and non expanded,with:
With bolted joints and connections by means of bars embedded in 4fill mortar. 3)
With welded connections 3) 4
5.5 Prefabricated inverted pendulum structures 2)or braced pillars 35.6 Isostatic seismic structures 36. Buildings and structures of brickwork (masonry)6.1 Reinforced block masonry with all spaces filled 46.2 Reinforced block masonry without spaces filled, and reinforced 3
,grill type ceramic masonry6.3 Confined masonry 47. Tanks, vessels, stacks, silos and hoppers7.1 Stacks, silos and hoppers with continuous shell reaching the ground 3
7.2 Silos, hoppers, tanks supported on columns, with or without bracing 4between columns
7.3 Steel vertical axis tanks with continuous shell reaching the ground 47.4 Reinforced concrete vertical axis tanks with continuous shell 3
reachin,g the ,ground7.5 Tanks and ducts of compound synthetic materials (FRP, GFRP, HDPE 3
and similar)7.6 Horizontal vessels supported on beds with ductile anchors 4
NCh2369
Table 5.6 - Maximum values of the response modification factor (conclusion)
R
34354
1) Excepting where a study confinns that a value for R can be used other than 2,structures with a resistant system explicitly referenced in this table are not to beincorporated into this classification.
2) More than 50% of the mass on the upper level. A single resistive member.
3) The value R - 4 is an upper limit. If the R value is less for the equivalent reinforcedconcrete structural system, this lower value must be used.
4) In the event that there should be doubts regarding the classification of a resistantsystem, provision 4.7 must be applied.
Table 5.7 Maximum values for the seismic coefficient
- 33 -
Cmax.R = 0,02 = 0,03 ( =0,051 0,79 0,68 0,552 0,60 0,49 0,423 0,40 0,34 0,284 0,32 0,27 0,225 0,26 0,23 0,18
NOTE: The listed values are valid for seismic zone 3. For seismic zones 2 and 1, thevalues in this table must be multiplied by 0,75 and 0,50, correspondinQ:ly.
Figure 5.1 a) Seismic zoning in Regions I, II & III
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Figure 5.1 b) Seismic zoning in Regions IV, V, VI, VII, VIII, IX, X &Metropolitan Zone
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Figure 5.1 c) Seismic zoning in Regions XI & XII
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Fixed Support Mobile support
Figure 5.2
6 Seismic Deformations
6.1 Calculation of deformations
When the analysis is made with seismic requirements reduced by factor R, thedeformations must be determined from:
d=do+R]dd (6-1)
where:
d = seismic deformation;
do =deformation due to non seismic seIVice loads;
Rl = factor resulting from the multiplication of the R value obtained from Table 5.6times quotient Qo/Qmm providing Qo/Qmm is lower or equal to 1,0. However, forquotient Qo/Qmm a value lower than 0,5 should not be used. In the event that thisquotient is greater than 1,0, Rl=R should be used;
dd = deformation calculated with seismic requirements reduced by the R factor.
If anelastic methods are used, deformation d must be obtained directly from the analysis.
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6.2 Separation between structures
6.2.1 In order to avoid adjacent structures hitting one another, the separation betweenthese must be greater than the highest of the following values:
equation (6-2)
equation (6-3)
where:
dcti, dctj' = deformations of structures i and} calculated according to 6.1;
Rii .Rij =modification factors RI of the response used to design structures i and};
lu .hj = heights at the considered elevation, of structures i and} measured fromthe corresponding basal elevations.
6.2.2 The separation between the structure and non structural, rigid or fragile members,where it is desired to avoid impact, must be higher than the relative deformation betweenthe elevations where the element is located, calculated with corresponding values for d,but not less than 0,005 times the height of the element.
6.3 Maximum seismic deformations
Seismic deformations must be limited to values that do not cause damage to piping,electrical systems or other elements joined to the structure that must be protected.
However, deformations calculated with the expression (6-1) must not exceed the valuesbelow:
a) Prefabricated concrete structures exclusively comprised by an earthquake resistantsystem based on walls connected by dry joints.
dmax.= 0,002 h (6-4)
b) Structures with brickwork (masonry) walls with rigid partitions joined to thestructure.
dmax.= 0,003 h (6-5)
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c) Unbraced frames with expanded masonry filling
dmax.= 0,0075 h (6-6)
d) Other structures
dmax.= 0,015 h (6-7)
where:
h = height of the floor or between two points located on the same vertical.
The above limitations can be omitted if it is proved that a greater deformation can betolerated by the structural and non structural members.
6.4 P-Delta Effect
The P-Delta effect must be considered when the seismic deformations exceed value:
d =0,015 h (6-8)
7 Secondary elements and equipment erected on structures
7.1Scope
Secondary elements are defined as partitions and other appendages attached to theresistant structure but that are not a part of it. Units of equipment anchored at variouselevations of the structure must fulfill what is provided in 11.3.2.
7.2 Stresses for seismic design
7.2.1 According to 5.3.1.5, in the event that a secondary element or unit of equipmenthas been included in the modeling of the structure that supports it, these must bedesigned with the following horizontal seismic stress, acting in any direction:
1,2 Qp RJF = <Pp R p
p
(7-1)
where:
Qp = shear stress present in the base of the secondary element or unit ofequipment according to the analysis of the building with seismic requirementsreduced by the R factor;
Rl =factor defined in 6.1;
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Rp = modification factor of the response of the secondary element or unit ofequipment, according to Table 7.1;
Pp =weight of the secondary element or unit of equipment.
7.2.2 If the unit of equipment does not need to be included in the modeling of thestructure, excepting its mass, the design of the secondary elements and units ofequipment must be performed with the following seismic stresses:
a) When acceleration ap is known at the level of support of the element or unit ofequipment, obtained from a dynamic modal analysis of the building with seismicrequirements reduced by factor R:
3,Oap Kp P <Pp- pF-R
p p
(7 -2)
where coefficient Kp must be determined alternatively by means of one of the following twoprocedures:
i) Kp = 2,2 (7-3)
ii) Kp = (7 -4)
where:
J3 = 1 for 0,8 T*:::. Tp ~1,1 T*
J3 = 1,25 (Tp/T *J
13 = 0,91 (Tp/T *J
for Tp < 0,8 T *
for Tp < 1,1 T *
where:
Tp = own period of the fundamental vibration mode of the secondary element, includingits anchoring system, and T * is the period of the mode with greatest translationalmass equivalent to the structure in the direction in which the secondary elementcan enter into resonance. In order to determine J3 a value for T * of less than O,06scannot be used.
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b) When a dynamic modal analysis of the building has not been perfonned:
0,7 ak K p P < PpF - pp - Rp
(7-5)
where:
Qk = acceleration at elevation k where the secondary element or unit of equipment iserected, which is detennined according to 7.2.4.
7.2.3 When the characteristics of the building or the elevation at which the secondaryelement or unit of equipment is to be erected are not known, the design can be perfonnedwith the seismic stress from fonnula (7-5) using Kp = 2,2 and Qk = 4 Ao/g.
7.2.4 The acceleration at elevation k of the structure must be detennined from:
ak=i(1+3; J(7 -6)
where:
Ao =maximum effective acceleration defined in 5.3.3;
Zk = height of elevation k over the base elevation
H =total height of the building over the base elevation
7.2.5 The design seismic stress detennined according to 7.2.1 or 7.2.2 must not be lowerthan 0,8 Ao Pp/g.
7.3 Stresses for anchors design
7.3.1 All the secondary elements and units of equipment must be appropriately anchoredto the resistant structure by means of bolts or other devices. Their design must beperfonned with the seismic stresses established in 7.2, with the modifications listed in7.3.2 and 7.3.3.
7.3.2 When the anchoring system to concrete elements includes superficial anchor bolts,(those with a length/diameter rate below 8), the seismic stresses listed in 7.2 must beincreased by 50%, or alternatively must be calculated with Rp equal to 1,5. The sameprovision applies when the anchor bolts are designed without the exposed length specified8.6.2.
7.3.3 When the anchoring system is manufactured with non ductile materials, theseismic stresses listed in 7.2 must be amplified by 3, or alternatively must be calculatedwith Rp equal to 1,0.
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7.4 Automatic trip (shutdown) systems
Ducts, vessels, and equipment that contain gases or liquids at high temperature, or areexplosive or toxic, must be provided with an automatic shutdown system that fulfills whatis required in 8.5.4 of NCh433.0f96.
Table 7.1 - Maximum values for the response modification factor for secondaryelements and equipment
8 Special provisions for steel structures
8.1 Applicable standards
Until the new version of the Chilean standard for dimensioning and construction of steelstructures is made official, the provisions included in this standard must be used,supplemented by provisions from the standards listed below.
a) Load and Resistance Factor Design Specifications for Structural Steel Buildings, 1998,by the American Institute of Steel Construction (AISC); or altematively, SpecifICationsfor Structural Steel Buildings, Allowable Stress Design, 1989, by AISC.
b) SpecifICations for the Design of Cold Formed Steel Structural Members, 1996, by theAmerican Iron and Steel Institute (AISI); for the design of cold formed members notincluded in the AISC standards.
c) In matters related to seismic design, the AISC standards must be supplemented withthe provisions in Seismic Provisions for Structural Steel Buildings, Part 1: StructuralSteel Buildings, 1999, by AISC. AItematively, the provisions in clause number 8 ofAttachment B of this standard may be used.
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Secondaryelement or unit of equipment Rp- Rigid or flexible units or elements, with non ductile materials or
additions. 1,5
- Prefabricated secondary elements. Braced elements. Partitions.
- Electrical and mechanical equipment in general.
- Stacks, tanks, steel towers. 3
- Other cases not specified in this table
- Storage shelves4
- Secondary structures
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8.2 Materials
8.2.1 Structural steel must fulfill the requirements listed below:
Have a marked natural ductility meseta in the traction test with a value for thefluency limit below 0,85 of the resistance to breakage and minimum breakagelengthening of 20% in the 50 mm sample.
Guaranteed weldability according to the AWS standard.
Minimum tenacity of 27 Joules at 21 ° in the Charpy test according to ASTM A 6.
Fluency limit not over 450 MPa.
8.2.2 In addition to the conditions specified in 8.2.1, the materials must fulfill some ofthe following specifications:
ASTM A 36, A 242, A 572 Gr. 42 & 50, A 588 Gr. 50, A 913 and A 992 for frames;sheets, bars, common bolts and anchor bolts.
DIN 17 100, qualities St. 44.2, St. 44.3 and St. 52.3 for the same elements.
NCh203 A 42-27ES, A 37 -24ES and NCh 1159 A 52-34ES for the same elements.
ASTM A 500 Gr. B & C, A 501 & A 502 for structural tubes.
AWS 5 for welds.
Materials that comply with specifications equivalent to the above and that are approvedby the professional specialists of each project may be used.
8.2.3 Earthquake resistant butt-welds must be full penetration with electrodes of aminimum tenacity of 27 Joules at - 29° C in the Charpy test according to ASTM A 6.
8.3 Braced frames
8.3.1 No configurations shall be allowed with diagonal braces that work in traction only,except for light steel sheds that comply with the provisions in 11.2.
8.3.2 In a typical resistant line there must be diagonals that work in traction anddiagonals that work in compression. The resistance provided by the tractioned diagonals,for each direction of seismic action, must be as a minimum 30% of the shear stress of theresistant line at the corresponding elevation.
8.3.3 The members of vertical earthquake resistant systems working in compression,must have width / thickness rates of less than hr, according to Table 8.1 (see Figure 8.1).
The slendemess of the member must not be less than 1,5 1C~ E / Fy .
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8.3.4 Diagonals in X configuration must be connected at the crossing points. Said pointmay be considered fixed in the perpendicular direction to the plane of the diagonals forthe purpose of determining the buckling length of the piece, when one of the diagonals iscontinuous.
8.3.5 In industrial buildings with V or inverted V bracing, the beams must be continuousover the intersection points with the diagonals, and must be designed to resist the verticalloads assuming they are not supported on the diagonals. The diagonals must be capableof resisting loads caused by their own weight and overloads induced by the beam, plusthe seismic loads from the analysis amplified by 1,5. The lower and upper wings of thebeams must be designed to support a transverse load located at the intersection pointwith the diagonals, equal to 2% of the rated resistance of the wing, that is, Fy bj t, where:
8.3.6 The seismic stress in the compressed diagonals, must be les than or equal to 80%of the resistant capacity defined in the steel design specification.
8.3.7 Seismic bracing may not be installed in K, where the diagonals intersect at anintermediate point of the column, unless this point is provided with a support that is apart of the bracing system.
8.3.8 The provisions in 8.3.3, 8.3.5 and 8.3.6 do not apply to bracings with stressescaused by absolute seismic loads that are less than one third of the stresses of thecombination that controls the dimensioning.
8.4 Rigid Braces
8.4.1 The moment joints of earthquake resistant rigid braces must be of the TR type(totally rigid). Connections of the PR (partially rigid) type are not to be used. The jointsmust be designed such that the plastic (flection) joint is developed in the beam at aprudent distance from the column, which can be achieved by reinforcing the connectionor weakening the beam in the desired position for the plastic joint.
8.4.2 Abrupt changes in the width of the beam wings are not allowed in areas with thepotential to form plastic joints, or in their vicinity, unless it is a reduced beam section,appropriately designed to induce the joint in this position.
8.4.3 Transverse (cross) sections of the earthquake resistant rigid frame columns andbeams must qualify as compact, that is, must have width-thickness ratios of less than Apof Table 8.1.
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Fy =wing fluence tension
bj =width of the wing
t =thickness of the wing
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8.4.4 In structures of several stories (floors) where the entire earthquake resistancedepends on rigid frames designed with Rl values greater than or equal to 3, the sum ofthe flection resistant capacities of the columns that concur at a knot must be greater thanor equal to 1,2 times the sum of the flection resistant capacities of the connected beams.
It is not necessary to comply with this requirement in any of the cases listed below:
a) If the seismic shear stress of all the columns in which the above provision is not metis less than 25% of the seismic shear stress of the corresponding floor.
b) If the analysis and dimensioning of the structure is performed with seismic stressesequal to double the values listed in clause 5 of this standard.
c) If it is proven by means of a non linear analysis method, (see 5.2.3) that the structureis stable in the face of the deformation requirements imposed by the earthquake.
8.4.5 The design of the panel zone in the column to beam joints of earthquake resistantrigid frames must comply with the provisions in Attachment B.
8.4.6 In columns with predominant compression, the compression resistance, withoutconsidering the flector moment effect, must be higher than the axial loads obtained fromthe combinations in 4.5, where the seismic load condition of these combinations has beenamplified by 2. Predominant compression is defined as the situation in which the axialstress obtained from the combinations in 4.5 is greater than 40% of the designcompression resistance of the columns.
8.4.7 Provision 8.4.3 does not apply to rigid frame elements where the stresses comingfrom majored seismic loads are lower than one third of the stresses of the combinationthat control the dimensioning.
8.5 Connections
8.5.1 The materials must comply with the following requirements:
Earthquake resistant connection bolts must be high resistance only, ASTM A 325 orASTM A 490 quality or equivalent.
Arc welding electrodes and consumables must comply with specifications AWS A 5.1,A 5.5, A 5.17, A 5.18, A 5.20, A 5.23 and A 5.29 or equivalent.
Electrodes must have a minimum tenacity of 27 Joules at -29°C in the Charpy testaccording to ASTM A 6.
8.5.2 The connections of seismic diagonals must be designed to resist 100% of thecapacity in traction of their gross section.
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8.5.3 Moment connections between beams and columns of earthquake resistant rigidframes must have, as a minimum, a resistance equal to that of the connected members.
8.5.4 In beam to column joints of rigid frames, the upper and lower wings of must beprovided with lateral supports designed for a stress equal to 0,02 Fy bj t.
8.5.5 Butt welds in earthquake resistant joints must be full penetration.
8.5.6 High resistance bolts must be placed with the listed pre-tensioning for criticaldisplacement joints (70% of the resistance in traction for A325 and A490 bolts). However,the design resistance of bolted joints can be calculated as that which corresponds tocrush type joints. Contact surfaces must be cleaned with a mechanical roller, sandblasted or granulated; must not be painted, but galvanizing is acceptable.
8.5.7 No joints will be allowed where the resistance depends on a combination of weldingwith high resistance bolts or rivets. The only exception is modifications of existing rivetedstructures.
8.5.8 Field joints must comply with the following requirements:
a) In connections with high resistance bolts a tightening and control methodology mustbe applied that ensures that the bolts are provided with the pre-tensioning required in8.5.6.
b) Welds must be in the flat, vertical and horizontal positions, provided the welder isprotected from wind and rain.
c) Welds must be full penetration butt welds or fillet welds. Butt welds must becontrolled with ultrasound or x-rays.
8.5.9 Column junctions must comply with the conditions listed below:
a) In buildings, the distance between the column junction and the upper wing of thebeam must greater than or equal to the lower value between 900 mm and half of thefree height of the column.
b) Junctions must be dimensioned for the design stresses obtained from thecombinations in 4.5, where the seismic load condition of these combinations has beenamplified by 2.
8.6 Anchors
8.6.1 Supports of structures and equipment that transmit seismic stresses to thefoundations or other concrete members must be anchored with anchor bolts, shearing
plates, reinforcement (steel) bar or other appropriate means.
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8.6.2 Anchor bolts that are subject to traction according to the analysis procedures listedin clauses 4, 5, and 7 must have seating and the stud must be visible to allow inspectionand repair, the thread must be long enough to allow for retightening of the nuts (seeAttachment A, Figure AI). The exposed length of the bolts must not be less than 250 mmor eight times the bolt diameter, and the length of the thread below the nut must not beless than 75 mm.
Exceptions to these requirements can be made for those anchor bolts with sufficientcapacity to resist load combinations where the seismic stresses are amplified in 0,5 Rtimes, but not less than 1,5 times, with regards to the value listed in clauses 5 and 7.
In major units, such as very tall process vessels, and in the structure of major suspendedunits, such as boilers and similar units, bolts with considerable capacity for ductiledeformation must be used, easily repaired and that can eventually be replaced (seeAttachment A Figure A 7).
8.6.3 Base plates of columns and equipment in general must be provided with seismicshear or butt plates designed to transmit 100% of the base shear stress (see AttachmentA Figure AI).
The cases listed below are excepted from this requirement:
a) Supports with shear stress of less than 50 kN; in this case it will be acceptable to takethe shear with the bolts, considering that only two of them are active for this purposeand the corresponding traction-shear interaction formulas.
b) Bases of tanks and units provided with one or more bolts; in this case it will beacceptable to take 100% of the shear with the bolts; considering that one third of thetotal quantity of bolts are active, and the corresponding traction-shear interactionformulas with maximum traction and the shear calculated thus.
c) Tanks with an aspect ratio of less than one, that do not require anchoring inaccordance with 11.8. In this case the shear can be taken with conicity in the base.
For cases a) and b) the bolts must be embedded in the foundation.
8.6.4 In the design of the shear plate the resistance of the leveling mortar (grout) mustnot be taken into consideration.
8.6.5 The design of the anchor members to the (shear) shear must not consider frictionbetween the base plate and the foundation.
8.6.6 The superposition of resistance between shear plates and anchor bolts must not betaken into consideration.
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8.6.7 When bolt holes are provided in the foundation for subsequent installation of theanchor bolts, the internal walls of the bolt holes must have a minimum inclination of 5%with regards to the vertical plane, such that the bottom area is greater than the top. Boltholes must be filled with non retracting mortar.
8.6.8 The concrete for foundations must be designed to resist the vertical and horizontalstresses transmitted by metal anchoring members. The resistance of the concrete andreinforcements must be such that the eventual failure occurs in the metal anchor devicesand not in the concrete.
8.7 Vertical bracing systems
8.7.1 The following arrangements are applicable to industrial buildings and facilitiesprovided with a steel bracing system. on the ceiling or floor, the function of which is totransfer design seismic loads and/or provide structural redundancy to comply with therequirements of this standard for specific structures.
8.7.2 In floor or ceiling bracing systems, diagonal configurations that work in tractiononly shall not be allowed. excepting cases of light steel sheds that are regulated by theprovisions in 11.2
8.7.3 In floor or ceiling bracing systems the function of which is to transfer and/or shareseismic loads that control design. branches in one or more frames (beam cranes. majorsuspended equipment, etc.) to other rigid frames or adjacent or extreme bracing. thedesign provisions in 8.7.3.1 and 8.7.3.4 must be used.
8.7.3.1 Earthquake resistant diagonals and supports that work in compression. musthave width/thickness ratios of less than Ar. according to Table 8.1. (see Figure 8.1). The
slenderness of the member must not be less than 1,5 Jl:~ E / Fy .
8.7.3.2 X configuration diagonals must be connected at the cross point. Said point can beconsidered as fixed in the direction perpendicular to the plane of the diagonals for thepurpose of determining the length of the buckling of the piece. when one of the diagonalsis continuous.
8.7.3.3 Provision 8.7.3.1, does not apply to bracing where the stresses coming from thecombination that include seismic loads that are less than one third of the stresses of thecombination that controls the dimensioning.
8.7.3.4 Provision 8.7.3.1. also does not apply when the design of the bracing system isperformed for stresses coming from the combination that include seismic loads. in whichthe ultimate load has been amplified by 0,7 R.
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8.7.4 For floor or ceiling bracing systems. the function of which is to provide structuralredundancy according to what is required in specific structures, the followingrequirements must be complied with:
8.7.4.1 Horizontal bracing systems and connections must be designed according to whatis listed in 8.l.a) or b), whichever corresponds.
8.7.4.2 The seismic loads to be considered for horizontal bracing systems must not beless than the seismic tributation of an intermediate frame, in eventual premature failure(see Figure 8.2).
8.7.5 The height of the diagonal members and the supports in floor or ceiling bracingsystems must be greater than or equal to 1/90 of the horizontal projection of the length ofthe member.
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Table 8.1 - Width/thickness ratio limits(see definitions of terms in 3.2 and in Figure 8.1)
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(continued)
Requirement affectinl4 the memberFrames A Compression Flection
Ar Ar AnDouble T, laminated, welded or hybrid and channel laminatedWings. not stiffened. I & C laminated bit O,56E / Fy ,7/ i@)?, O,38E / Fyprofiles "'x,,x,
Wings, not stiffened, welded. bit -- O,38E / Fyreinforced and hvbrid profIlesCores, all **) ***) h/tw
h/tw
Cores in compound flection. all **)***)
h/tw
Stiffened cores and any othermember stiffened by a stiffener
bitcapable of providing effective edgesupport or
h/tw
Core wing or longitudinal stiffeners cltVertical core stiffeners bitFlat molding in compressed wings bitT Frames
Wings, laminated profiles bitWings, welded profiles bitCores **) d/tw
Rectangular. uniform thickness:Wings bitCore
h/tw
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Table 8.1 - Width/thickness ratio limits(see definitions of tenns in3.2andinFigure 8.1)
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Requirement affectinQ; the memberTubular Frames A Compression Flection
Ar Ar An
Rectangular welded, with wingsthicker than the core:
Wings bit
Core h/twCircular D/tSupports fonned by laminated an2"1esSimple angle wings, TL profiles with bitspreaders, XL profiles and nonstiffened members in Q"eneralTL profIle wings with angles in bitcontactCold folded profIlesC or Z non stiffened profIle wings bitStiffened wings of CA, ZA, Omega & bitHat profIlesSimple angle wings, TL & XL profIles bitwith or without spreadersC, CA. Z, ZA, Omega & Hat Profile h/twcores.Stiffened edges cltC, CA, Z, ZA, Omega & Hat ProfIlecores in compound flection
h/tw
NOTESNA: Non applicable
E, Fy: in MPa E= 200 000 MPa Ar= width -thickness ratio limit to avoid localbuckling
Ap= width -thickness ratio limit to allow totalplastification of the section.
* (see fonnula) but within the range of 0.35 < k...
**}in hybrid beams it is necessary to use Fy of the wings
***}In members with unequal wings use he instead of h i when comparing with Ap
Figure 8.1 -Examples for width / thickness ratio of Table 8.1(Flat widths hand b according to terminology definitions in 3.2)
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Frame that fails
Figure 8.2
9 Special provisions for concrete structures
9.1 Reinforced concrete structures
9.1.1 Until the new version of NCh430 which substitutes standards NCh429.0f57 andNCh430.0f61 is made official, the provisions from the ACI 318.99 code must be used,where there is no contradiction with this standard. For the application of the provisions inchapter 21 of this code (paragraph 21.2.1), it must be considered that the entire nationalterritory, with its three seismic zones, is a high seismic risk region.
9.1.2 Structural members that are a part of ductile frames intended to resist seismicrequirements must be dimensioned and detailed as special moment resistant frames,according to the provisions in sections 21.1 to 21.5 of chapter 21 of ACI 318-99.
9.1.3 Frames belonging to structures where the seismic requirements have calculatedwith an Rl factor less than or equal to 2 can be designed according to the provisions forintermediate moment resistant frames, listed in section 21.10 of chapter 21 of ACI 318-99. Frames with seismic deformations of less than or equal to 50% of the limit valuesestablished in 6.3 can also fall under this provision.
9.1.4 In the case of structures with a combination of reinforced concrete walls andframes, where the assembly of the walls takes on, at each elevation and each direction ofanalysis, a percentage of the total elevation shear stress over or equal to 75%, the designof the frames can be performed according to the provisions listed in section 21.10 ofchapter 21 of ACI 318-99, provided the frame is responsible for taking on less than 10%of the total shear stress of each of the elevations.
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9.1.5 Frames where the seismic action does not control design, and where failure doesnot compromise the stability of the structure, can be designed according to the provisionslisted in section 21.9 of chapter 21 of ACI 318-99.
9.1.6 The design of the walls does not need to meet the provisions of paragraph 21.6.6.3of chapter 21 of ACI 318-99.
9.1. 7 In structures of several floors where the seismic resistance depends on rigid framesdesigned with Rl values of over or equal to 3, it is not necessary to meet the requirementfor strong - weak column (paragraph 21.4.2. ACI 318-99), when one of the followingconditions is met:
a) the seismic shear stress of all the columns that do not meet the above provision is lessthan 25% of the seismic shear stress of the corresponding floor;
b) if the analysis and dimensioning of the structure is perfonned with double the seismicstresses listed in clause 5 of this standard;
c) if it is proved by means of a non linear analysis method (see 5.2.3) that the structureis stable in the face of the defonnation demands imposed by the earthquake.
9.2 Prefabricated concrete structures
9.2.1 Requirements for prefabricated systems
9.2.1.1 Structures that include prefabricated concrete members must be designed toresist seismic actions according to some of the criteria listed below:
a) Gravitational systems
These systems use reinforced concrete walls or structures poured on site, confined orreinforced masonry walls or braced and non braced steel frames as the earthquakeresistant system, and use prefabricated members to resist the vertical loads only.
The prefabricated members and connections that do not belong to the earthquakeresistant system must be capable of accepting seismic defonnation d, of the structureand resist the vertical (gravitational) loads for such defonnation.
Frames that belong to the prefabricated gravitational system can be designedaccording to the provisions in section 21.9 of the ACI 318-99 code.
The connections between the prefabricated gravitational system and the earthquakeresistant system are considered as part of the latter system and must be designedaccording to paragraphs b), c) or d)
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b) Prefabricated systems with humid connections
These systems imitate the performance of reinforced concrete structures erected onsite by the use of prefabricated members joined by humid connections that meet therequirements in code ACI 318-99, especially the anchoring and bar junctionrequirements.
c) Prefabricated systems with ductile connections
These systems use structures formed by prefabricated members joined by connectionsfor which it has been proven, by means of cyclic non linear analyses and tests, thatthese have a resistance and ductility greater than or equal to the monolithic joints ofstructures designed according to ACI 318-99. These tests must meet the requirementsof document ACI ITG ITl, 1-99 Acceptance Criteria for Moment Frames Based onStructural Testing, and may have been performed in national or foreign laboratoriesprovided that the results have been certified by a certified laboratory approved by theMinistry oh Housing and Development.
d) Prefabricated systems with dry connections
These systems use structures formed by prefabricated members joined by dryconnections, designed as strong connections that ensure that the possible non linearbehavior during earthquakes with higher requirements than those considered in thisstandard will cause an incursion within the non linear response range in sections atdistance from the strong joint.
In these prefabricated systems an earthquake resistant system is accepted that iscomprised solely of walls connected with dry joints, or one that is comprised solely offrames connected by dry joints.
Structures where the earthquake resistant system is comprised solely by aprefabricated system with dry connections, may only be erected up to 4 levels with amaximum height of 18 m, measured from the base elevation.
9.2.1.2 Structures that include prefabricated gravitational systems must be designedconsidering the seismic requirements that correspond to the earthquake resistant systemused.
Prefabricated systems with humid connections and ductile connections must be deignedusing the seismic requirements that correspond to a monolithic structure of reinforcedconcrete.
9.2.1.3 Prefabricated structures with dry connections must be designed with the valueslisted in Table 5.6 for reinforced concrete structures poured on site. However, the R valuemust not be greater than 4 and the damping rate must not be greater than 0,03 for boltedconnections and connections by means of bars embedded in fill mortar, nor greater than0,02 for welded connections.
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In order to meet the behavior requirements listed in 9.2.1.1 c), dry connections must meetprovisions 9.2.1.4 and 9.2.1.5.
9.2.1.4 In prefabricated systems connected with dry joints, the quotient between therated resistance of the connection and that of the member connected at the connectionpoint (Se) must be greater than or equal to 1,4.
9.2.1.5 The dry connections of prefabricated frames must be capable of developing, toflection, to shear or axial stress or a combination of these actions acting on theconnection, a probable resistance Spr, determined using a value <P = 1, that is not lessthan 125% of the fluency resistance of the connection and must be capable of developinga displacement to Spr, that is not less than 4 times the fluency displacement. Theanchoring of the connection of the prefabricated member in any side of the connectionmust be designed to develop a tension equal to 1,3 times Spr. The connection must alsomeet the requirements of confinement iffe is greater than 0,7 fe.
The behavior established above must be guaranteed by testing that includes the cyclicnature of the action. The tested samples must represent the proposed system. The testsmust meet ASTM specifications for instrumentation and execution of cyclic tests.
9.2.1.6 Steel and electrodes used in welded joints must meet the requirements listed in8.2.2 and 8.5.1.
9.2.1.7 If the base shear stress Qo is lower than the value below:
Qmin=0,401 Ao pg
(9.1)
all the deformations and stresses must be multiplied by quotient Qmin/ Qo for designpurposes.
The above provision does not apply to prefabricated concrete structures classified in9.2.1.1 as gravitational systems or prefabricated systems with humid connections andductile connections, which must meet the provision with regards to minimum base shearstress established in 5.4.5.
9.2.2 Special provisions
9.2.2.1 The design of prefabricated members and connections must include loadconditions and deformation, from the initial fabrication until the structure is completed,including removal of formworks, storage, transportation and erection.
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9.2.2.2 The design of prefabricated members and connections must include the effect offabrication tolerances.
9.2.2.3 Additionally to the requirements for drawings and specifications of this standard,the following information must be included in the shop drawings:
a) Details of the reinforcement steel (rebar), embeds and lifting devices needed to resistthe temporary stresses derived from the handling, storage, transportation anderection.
b) Concrete resistance at the established ages or construction phases.
9.2.2.4 The use of connections that are based solely on friction caused by thegravitational loads.
9.2.2.5 In order to consider a pavement slab comprised by prefabricated members as arigid diaphragm, it is necessary to be provided with a top slab that meets the provisionsin sections 21.7.2,21.7.3,21.7.4 and 21.7.5 of ACI 318-99.
9.3 Industrial buildings/bays/sheds/shops comprised by projecting columns
9.3.1 This paragraph establishes the special requirements for industrial buildings withconcrete columns poured on site or prefabricated, with or without beam crane, structuredwith columns embedded at the base and beams connected to the columns with swiveledjoints. The seismic resistance and deformation capacity of these systems stem solely fromthe columns.
9.3.2 Industrial buildings must be provided with a continuous roof plan bracing systemconnected to the columns at the upper level.
If the bracing is provided by steel frames these must comply with the provisions in 8.7.
If the bracing is provided by a different system it must have a rigidity equivalent to that ofa steel system with diagonals comprised by frames that work in traction only, that complywith 8.1 a) or b). This different system must not be comprised by members that worksolely in traction.
9.3.3 Seismic design of structures that meet 9.3.1 and 9.3.2 must be performed with R =3 and a damping ratio of 0,02.
9.3.4 The base shear stress must not be less than:
Qrnin=04/ Ao P / g (9-2)
In cases where base stress Qo is lower than the above values, the stresses and
deformations must be multiplied by Qrnin/ Qo for design purposes.
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9.3.5 the design of the members must be performed using the provisions in 9.1 if thesemembers are concrete poured on site and 9.2 if prefabricated concrete is used.
Confinement must be provided at the column bases in a length greater or equal to twicethe height of the transverse section of the column according to section 21.3.3 of ACI 318-99.
9.3.6 The maximum slenderness of the columns must meet:
A =k L/ r~lOO (9-3)
Unless adequately justified, the value for k must be 2.
9.3.7 Beams must be laterally supported to avoid them overturning due to the action ofthe rafters or secondary beams. Lateral bracing to the support beams must be providedfor this purpose.
9.3.8 It will not be acceptable for any deck sheets that are not concrete to provide lateralbracing for any member.
9.3.9 Column heads must be connected to support beams in two orthogonal orapproximately orthogonal directions.
9.3.10 The seismic loads to be considered for horizontal bracing systems must not be lessthan the seismic tributation of an intermediate frame, in eventual premature failure (seeFigure 8.2).
9.3.11 To calculate deformations it is necessary to proceed according to 6.1 and therequirements in 6.2 and 6.3 must be respected.
Maximum horizontal deformations must be calculated modifying formula 6-1 as follows:
d =do +So R) dd (9-4)
considering the following values for So:
1.00 for soil I1.25 for soil II1.50 for soil III
9.3.12 The consideration of the P-Delta effect must meet 6.4.
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9.3.13 In the design of columns and foundations supported on type III soil the rotation ofthe foundations must be considered both for calculating the stresses as well asdeformations. A geotechnical study must be performed for this purpose that indicates themaximum and minimum values for the dynamic ballast coefficient. The stress calculationmust be performed with the maximum ballast coefficient and the deformations with theminimum.
Supporting of foundations on type IV soil is not allowed.
10 Provisions with regards to foundations
10.1 General specifications for design
10.1.1 The foundations must reflect the assumptions of the model used, both in thegeometry as well as the rigidity and mass characteristics.
It can be considered that massive foundations are lacking in elastic properties, but forisolated foundation systems connected by foundation beams and foundation slabs, itmust be assumed that these are provided with both inertial as well as elastic properties.
Foundations where the deign is performed assuming them to be infinitely rigid supportedon flexible ground, must have dimensions consistent with that hypothesis.
10.1.2 The dimensioning by resistance of the foundation must be performed for all theload combinations considered in the design of the rest of the structure.
10.1.3 The verification of the foundation stresses induced to the ground, the deformationand the stability of these, must be performed for all the applicable non factored loadscombinations.
10.1.4 It must be confirmed that the foundations exhibit a satisfactory behavior both forthe action of static loads as well as for seismic loads, verifYing that the contact pressurebetween the soil and the foundation is such that the deformations induced are acceptablefor the structure.
10.2 Surface foundations
10.2.1 Unless the geotechnical survey imposes a higher restriction, at least 80% of theare under each isolated foundation or foundation slab must be subjected to compression.
This restriction does not apply to those cases in which anchors are used between thefoundation and the ground.
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10.2.2 To calculate the seismic actions that develop at the base of undergroundfoundations in level ground, the inertia forces of the structure masses below naturalground level and the seismic thrust of the ground can be dismissed, as long as thefoundation has been constructed against natural ground or the backfill installed betweenthe foundation and natural ground are adequately compacted and controlled.
10.2.3 Foundations subject to non factored load combinations that include theearthquake, that generate net tractions in the foundation, must take these tractionssolely with the own weight, guaranteeing a minimum safety factor to upheaval of 1,5.
11 Specific Structures
11.1 Industrial sheds
11.1.1 These provisions apply to industrial buildings with or without boom crane beams.
11.1.2 Buildings with transverse beams must have a continuous bracing system in theroof. When there are trusses in the roof the continuous bracing must be placed in thelower spring line plane. Buildings without boom cranes are excepted where thepermanent loads come only form the weight of the building itself (see Attachment A,Figure A-2).
11.1.3 In buildings with a boom crane the seismic analysis must be made with the mostprobable magnitude and height of suspended load during the design earthquake. For thispurpose the frequency of the design earthquake and the operating conditions of the cranemust be taken into consideration.
11.1.4 If there are several cranes, either in one building or parallel buildings, acombination of seismic loads must be considered with all cranes with no load andstationed in the most unfavorable position.
11.1.5 The lateral joint between crane support beams and columns must be flexible inthe vertical direction. Furthermore, safety devices must be considered to avoid the bogeyfalling if it derails (see Attachment A, Figure A,3).
11.1.6 In buildings with rigid frames, the bracings of the end walls that are intended toprovide lateral support to columns designed for wind, must not provide a lateral rigiditygreater than that provided by the intemal frames unless these are considered in thestructural model according to what is specified in 5.3.1.1 (see Attachment A, Figure A,4).
11.1.7 If the building is flexible and has non structural rigid masonry walls or of anotheranalogue material, joints must be designed capable of providing lateral support for thewalls and allow independent longitudinal displacement between them and the structure(see Attachment A, Figure A,5).
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11.2 Light steel buildings
11.2.1 These provisions apply to steel buildings that meet the conditions listed below:
They are structured by means of a succession of parallel frames comprised bycolumns and beams, grill type, open full core profiles or closed profiles.
The free internal height of the lateral columns must less than or equal to 15 m.This requirement can be ignored if within the load combinations listed in 4.5, theseismic stresses resulting form the analysis are amplified by 2.
The transverse distance between the axis of adjacent columns must less than orequal to 30 m. This requirement can be ignored if within the load combinationslisted in 4.5, the seismic stresses resulting form the analysis are amplified by 2.
The building may be one bay, or several parallel bays.
The earthquake resistant stru'cture is comprised by rigid parallel frames orextreme and intermediate rigid or braced frames, that receive the seismic stresseshorizontally by means of a roof bracing system.
The structures must qualifY as category C2 or C3 according to 4.3.1.
Boom cranes must have a rated capacity of less than or equal to 100 KN, forcranes with no operator cabin, and 50 KN for cranes provided with an operatorcabin.
Units of equipment supported by the structure must have a weight per frame ofless than or equal to 100 KN.
The horizontal seismic load that the garrets transmit to each column of thestructure must not be greater than 15 KN.
There are no storage shelves seismically supported in the structure.
11.2.2 In order to determine the design seismic stresses, the damping rates listed inTable 5.5 must be considered, and a response modification factor of less than or equal to4.
11.2.3 The design of light steel bays must meet the provisions in clause 8, excepting8.3.3, 8.3.5, 8.3.6, 8.4.1, 8.4.2, 8.4.3, 8.4.4, 8.4.5, 8.4.7, 8.5.2, 8.5.4 and 8.5.9, theapplication of which is not mandatory.
11.2.4 Diagonal members of the bracing system that work in traction only must beinspected and be provided with appropriate devices for initial tensioning and subsequentadjustment.
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11.2.5 Seismic diagonal braces for roofs designed solely to resist traction stresses, musthave a capacity that at least equals the sum total of initial pretension and the seismicstresses from the analysis amplified by 1,5.
11.2.6 The deck bracing system, designed to transmit horizontal stresses to the extremetransverse frames, must be continuous and be comprised by diagonal members andsupports that work both in traction as well as in compression.
11.2.7 The vertical bracing system must be comprised by diagonal members andsupports that work both in traction as well as in compression, and their slenderness
must be les than or equal to 1,5 J[ ~E / Fy. This requirement is not mandatory for bayswith a clearance between columns of less than or equal to 12 m and a shoulder height ofless than or equal to 6 m. In these cases members that work in traction only may beused, provided they meet the requirements in 11.2.4 and 11.2.5
11.2.8 The seismic design of the vertical and deck bracing system connections, must beperformed considering the load combinations listed in 4.5, with the seismic stresses fromthe analysis amplified by 1,5.
11.2.9 The seismic deformations must be determined in accordance with what is set forth
in 6.1 and be limited to values that do not cause damage to piping, lifting andtransportation equipment, electrical systems or other elements attached to the structurethat must be protected. It is not necessary to comply with what is established in 6.3 and6.4.
11.2.10 The separation between structures must meet what is listed in 6.2.1.
11.2.11 For light steel bays that do not use the system described in 11.2.6 and are notprovided with the boom cranes and equipment described in 11.2.1, the roof panel may beconsidered as a rigid diaphragm capable of transmitting seismic stresses to the lateralbracing systems, provided the capacity to transmit said shear stress is certified by meansof static tests with cyclic load. The safety factor with regards to the experimental valuecan be taken from the AISI 1996 standard listed in clause 2.
The design of the diaphragm must be performed according to what is set forth indocument AC43 Acceptance Criteriafor Steel Decks dated July 1996 from ICBO ES, and inthe AISI standard as a supplement. The load combinations listed in 4.5 must be used,with the seismic stresses resulting from the analysis amplified by 2.
The tests must be analyzed by internationally recognized competent independentinstitutions, and must be performed on samples that consider the joint action of the paneland attachment system of the panel to the supporting structure (rafters), precisely howthey are to be installed in the field.
The company that certifies the panels, must also ensure the quality and properinstallation of the attachment system.
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11.3 Industrial buildings with several floors
11.3.1 Insofar as it is possible, the floors should be rigid seismic diaphragms, which maybe concrete, or metallic with horizontal bracing, or solid floor shell. The diaphragms mustinclude joining devices to the structure capable of transmitting seismic stresses.
11.3.2 Rigid equipment or ducts that extend vertically more than one floor must beprovided with support and joining systems that prevent them from participating in theseismic resistance or rigidity of the building (see Attachment A, Figure A6). If this is notpossible, the units of equipment must be included in the earthquake resistant systemmodel
11.4 Major suspended equipment.
11.4.1 Boilers, metallurgic fumace reactors and other major units of equipmentsuspended from the structure must be attached to it with connection devices thattransmit the seismic stresses without restricting free thermal dilation (expansion), bothvertical as well as horizontal (see Attachment A, Figure A 7).
11.4.2 For electrical suspended equipment that cannot be horizontally attached to thestructure, such as electrode cages in electrostatic precipitators (precipitadores), specialisolators must be specified with ample resistance capacity and provide electrical powercutoff devices in the event of a major earthquake. If the possibility exists for the cage tostrike the casing of the unit with the collector plates, impact plates must be installed.
11.5 Pipes and ducts
11.5.1 In large dimension piping and duct systems expansion joints and supports mustbe provided that ensure seismic stability and allow simultaneous thermal dilation.
11.5.2 If the piping and ducts are light duty in comparison to the buildings andstructures they connect, the seismic analysis can be performed introducing the d.<ideformations from 6.1 of the buildings or structures, at the connection points. If thecontrary is true, analysis must be made of the structure and ducts as a single unit.
11.6 Major mobile equipment
11.6.1 Major mobile equipment such as bulk material loaders and off loaders, stackers,portal cranes, and similar units must be dynamically analyzed, considering themagnitude and most unfavorable position of the loads. For the analysis it may beassumed that the wheels are jointed in the rails or ground, but if there is a significantlifting, counterweights must be installed to avoid this (see Attachment A, Figure A8).
11.6.2 In order to reduce the possibility of the rail edges striking the wheels, the systemmust be self centering (see Attachment A, Figure A9).
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11.6.3 Special attention must be given to the seismic eccentricity tests that occur inthese systems.
11. 7 Elevated tanks, process vessels and steel stacks
11.7.1 Elevated tanks must be designed considering water mobility.
11.7.2 Process vessels must be designed paying special attention to the joint of thesupports to the side shell when it does not reach the foundations.
11.7.3 Elevated stacks must be designed with the dynamic method. When the gas duct isnot self supporting it is necessary to consider the interaction between the duct and theexternal steel or concrete structure. The internal concrete coating. if installed. must betaken into consideration for the rigidity calculation. but not for resistance.
11.7.4 The tank siding must be designed so that no local buckling exists considering theeffect of design lateral and vertical stresses and the fabrication tolerances. To achievethis, the compression tension of the sides must not exceed the lowest of the values below:
Fa =135Fy e/ D Fa :S;0,8 Fy (11-1)
where:
Fa = admissible tension in earthquake condition
Fy =fluency tension (stress)
e =thickness
D =diameter of the side
11.8 Vertical tanks supported on the ground
11.8.1 The following requirements apply to cylindrical or rectangular tanks. symmetricwith regards to a vertical axis and with the bottom directly supported on the ground. Thetanks must be fabricated in steel or reinforced concrete and may contain any type ofliquid.
11.8.2 Where the provisions of these articles are not contradicted. and the tank materialand contents of the tank . the use of the following tank design standards andspecifications is accepted: API 650 Welded Steel Tanks for Oil Storage. API 620 Designand construction of Large. Welded. Low Pressure Storage Tanks. Seismic Design of StorageTanks of the New Zealand National Society for Earthquake Engineering jointly with NewZealand Standard 4203. AWWA-D 100 Wire and Strand Wound Circular. PrestressedConcrete Water Tanks. AWWA-D 115. Circular and Prestressed Concrete Water Tanks withCircumferential Tendons, ACI 350.3 Practicefor the Seismic
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Design of Liquid Containing Structures, or other internationally recognized standards andspecifically accepted by the professional specialist who approves the project, according towhat is listed in 4.4.2. In particular, the design base shear stress must be calculatedaccording to clause 5, and shall not be lower than the value resulting from the applicationof 11.8.6, 11.8.7 and 11.8.8. Only one of the above standards may be used for the designof each tank, avoiding a mixture of provisions from different standards. It must beconsidered that New Zealand standards consider load and resistance factors, while theremainder of those mentioned above are of admissible tensions.
11.8.3 The analysis model must consider both the impulsive horizontal response, inwhich a portion of the content vibrates together with the structure, as well as theconvective horizontal response, associated to the waves on the free surface.
11.8.4 For calculation purposes of the periods and masses participating in the impulsive,convective and vertical modes, it may be assumed that tank is infinitely rigid.
11.8.5 The determination of the hydrodynamic masses and the periods associated to theimpulsive and convective mode responses must be done according to what is specified inthe design standards mentioned in 11.8.2, correspondingly.
11.8.6 For steel tanks a maximum value of R = 4 must be used of the responsemodification factor.
11.8.7 For reinforced concrete tanks a maximum value of R = 3 must be used of theresponse modification factor. This value applies to normal construction of the continuousjoint between the wall and the base. If this condition is not complied with lower R valuesmust be used that the project must justifY.
11.8.8 The design spectral acceleration or seismic coefficient of the impulsive mode forhorizontal seismic action must be equal to the maximum seismic coefficient listed inTable 5.7 for ~= 0,02 for steel tanks, and ~= 0,03 for concrete tanks. The design spectralacceleration or seismic coefficient of the convective mode for horizontal seismic actionmust be determined according to expression (5-2) considering a damping ratio of ~ =0,005; in no case must this value be less than 0,10 &/g.
11.8.9 In cases where the design standard used considers vertical action, the verticalseismic coefficient must be equal to 2/3 of the impulsive mode coefficient.
11.8.10 The design must consider the corresponding coefficients of importance accordingto 4.3.2.
11.8.11 Where applicable, the modal stresses and deformations must be superimposedaccording to the criteria specified in the design standard used.
11.8.12 In the event that design methods by load and resistance factors should be usedthe stresses must be combined according to what is set forth in 4.5.
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11.8.13 In anchored metal tanks with flat bottom, the design of the anchor bolts must beperformed so that 1/3 of the number of bolts are capable of taking on the total seismicshear stress, unless the anchoring system includes a device that guarantees that 100% ofthe bolts are active to take on the seismic shear. The design of the bolts must considersimultaneous occurrence of tensions due to traction and shearing.
11.8.14 In non anchored tanks a conical slope of 1% as a minimum must be given to thebottom of the tank.
11.8.15 In order to reduce the risk of spillage and avoid damages to the roof and upperportion of the tank wall, a separation (revancha) must be left between the free surface ofthe liquid and the roof structure greater than or equal to the wave height of the convectivemode.
Smaller separations (revanchas) may be used provided that the subpressures originatedby contact between the liquid and the roof are taken into consideration, with which theroof and connections with the rest of the structure must be designed.
11.8.16 In order to reduce the secondary damage caused by movement of the liquid, thefollowing conditions must be met:
a) in metallic tanks, the roof plates must not be welded to the rafters;
b) the normal diameter of the air vents in the roof must be duplicated;
c) in metallic tanks, allow vertical displacement of the columns on the bottom.
11.8.17 Piping systems and their connection points to the tank must be designed withample capacity for deformation in order to avoid damage due to possible rising of the tankbottom or tank displacements.
11.9 Rotating furnaces and dryers
11.9.1 The longitudinal earthquake must be resisted by wheel rims and thrust rollersarranged on each side of the rim, and placed on a single support in order to allowlongitudinal expansion.
A free space must be left between the thrust rollers and the wheel rim to facilitateoperation. The rim and rollers must consider the possibility of longitudinal impact whenthis space closes. It is acceptable for the rollers and mechanisms to be designed assacrificial elements that can fail in the event of an earthquake; in this case themanufacturer must provide detailed instructions for the repair in a short time period toavoid damage to the furnace due to cool down.
11.9.2 The transverse earthquake must be resisted by lateral wheels and rollers placedon several supports. The width of the rollers must be greater than the width of the wheelrims to prevent them falling if the thrust rollers should fail.
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11.10 Refractory masonry (brickwork) structures
11.10.1 In the design of industrial foundry furnaces or other industrial processes,comprised by steel or concrete structures combined with refractory brick masonry thatoperate at high temperatures, arrangements must be found where the structuralresistance is provided by the conventional materials and only in exceptional cases by themasonry. (For example in Figure A.12 in Attachment A, the suspended roof must bepreferred) .
11.10.2 In the event that it should not be possible to avoid the masonry being anearthquake resistant member, special analysis must be used that take into considerationthe characteristics for non linear behavior of the material.
11.10.3 The design must consider both cold furnace conditions, as well as startup andnormal operation.
11.11 Electrical equipment
11.11.1 The provisions in this standard are applicable to structural aspects of electricalequipment located inside industrial plant sites. They do no apply to generation andtransmission equipment nr to the main substations, which must be regulated by specialspecifications.
11.11.2 The electrical operability of these units of equipment during an earthquake mustbe qualified according to special standards defined by the process engineers.
11.11.3 The isolators must be designed as rupture proof with a minimum safetycoefficient of 3,0, for load combinations that include seismic requirements.
11.12 Minor structures and equipment
All structures and equipment, independent of their size and importance, must be capableof resisting the seismic stresses specified in this standard and be appropriately anchored(see Attachment A, Figure A.13).
11.13 Wood structures
Wood structures must be designed according to NChl198. The connections must have aductile behavior and a failure resistance below that of the connected wooden members, byflection or traction. The R value for cooling towers must be equal to 4.
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Attachment A(Regulatory)
Typical Details
Shear plate
Figure A.I - Columns base
Columns
Upper spong line Lower spong line
Figure A.2 - Roof bracing
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Figure A.3 - Detail of crane support and columns
Seismic clamp
Do not use
Do not use
Figure A.4 - Extreme wall bracing
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Rigid diaphragms
Figure A.5 - Column to masonry (brick) wall joint
Lateral (side) support
Expansion joint
Space
Support
Figure A. 6- Rigid unit of equipment in building
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SeismicDisplacement
Rubber or plastic
NCh2369
Hangers
Level 4
Level 3
Level 2
Levell
Level(Elevation) 0
Elevation
Casing
Tube
a) Connecting rod joint
Figure A.7 - Typical details of major suspended equipment,seismic connectors and anchor bolts
(continued)
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Thennal axis
Elastomer
bJButt joint
cJHammer head bolt
Figure A.7 - Typical details of major suspended equipment,seismic connectors and anchor bolts
(conclusion)
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NCh2369
Operational counterweight
Seismic counterweights
Stacker
Figure A.S - Typical detail of major mobile equipment
Self centering wheel
Pumped railElastomer
Figure A.9 - Rail wheel system
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Air vent
Roof sheetsbreaking
Local lowerbuckling
(elephant's foot)
Lifting
Tube
Elastomer
Do not weld
DETAIL 1Roof sheets
DETAIL 2Columns support
Figure A.I0 - Typical detail of major tanks
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Wheel rim
Thrust rollers
DETAIL ILongitudinal support
Wheel rimSide supports
StopsLateral rollers
ChCvS
=Horizontal seismic coefficient=Vertical seismic coefficient= Safety factor> 1,20
DETAIL 2Lateral supports
Figure A.II - Typical detail of rotating furnaces and driers
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Brace strut
a) Arc roof furnace
Hangers
Suspended roof
b) Suspended roof furnace
Figure A.12 - Typical detail of industrial masonry
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InitialHeatup
stop
Stops
Anchor bolts
Diagonals
a) Fast filter
b) Transfonner
c) Compact boiler
Figure A.13 - Typical detail of minor structures and equipment
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NCh2369
Attachment B(Regulatory)
Design of beam to column joints in rigid steel structures
Bl Overview
The use of the AISC, Seismic Provisions for Structural Steel Buildings, 1999, for thedesign of rigid frames is subject to the following limitations:
a) In rigid frames with no bracing the provisions in this standard must be applied, withno additional mandatory requirements from AISC, Seismic Provisions. In particular,the Seismic Provisions for special frames (paragraph 9), and intermediate frames(paragraph 1O), are not applicable, nor are laboratory tests required for the beam tocolumn joints.
b) In frames with concentric bracing the provisions in this standard, paragraph 8.3 shallapply, with no additional mandatory requirements from AISC, Seismic Provisions.
c) In frames with eccentric bracing the AISC, Seismic Provisions, paragraph 15 must beapplied.
B2 Design of the panel zone of moment joints
B.2.! The analysis can be made with elastic or plastic methods.
B.2.2 The core panels must be reinforced with plates attached or diagonal stiffeners(Figures B.l and B.2) if requirement Ru exceeds CPRv, where cP= 0,75 and Ru and Rvaredetermined as follows:
a) See equation on page 78 of the original document (B-1)
where:
Mul & Mu2 : moments of the beams in the joint due to the combination of loadsdescribed in 4.5 b), in which the condition of seismic load of thesecombinations has been amplified by 2, but not greater than thecorresponding plastic moments;
dml & dm2 : 0,95 dl and 0,95 d2, in which dl and cb are the height of the beams;
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Where:
de
dp
Fy
Pu
Py
A
NCh2369
Vu shear stress in the column at the level of the joint due to the combinationof loads described in 4.5 b), in which the condition of seismic load of thesecombinations has been amplified by 2.
b) lfPu ~ 0,75 Py
See equation on page 79 of the original document (B-2)
c) lfPu > 0,75 Py
See equation on page 79 of the original document (B-3)
bej =width of the column wing;
t:j = thickness of the column wing;
=height of the column profile;
tp =total thickness of the panel zone including attached reinforcement plates;
=highest value between dl and cb (see Figure B.2);
=fluency tension;
=design compression axial stress of the column;
=AFy axial fluency stress of the column;
=areas of the column section.
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a) Plates joined with butt weld
b) Double reinforcement plates,joined with butt or fillet weld
Figure B.l - Attached reinforcement plates
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Section Vu
Attached reinforcement plate
Continuitystiffeners
Section VU
Diagonalstiffener
Figure B.2 - Stresses in the panel zone
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B.2.3 Continuity stiffeners must always be placed in the panel zones (Figure B.2)dimensioned to resist stresses transmitted by the wings of the beam to the column.
B.2.4 The attached plates must be joined to the wing of the columns with fIllet or buttwelds with full penetration, calculated to resist design shear stresses. When they arelocated by the core of the column. they must be welded to it at the upper and lower edges.If they are separated. they must be placed symmetrically and welded to the continuitystiffeners.
B.2.5 The thickness of the core of the column or of each attached plate must meet thefollowing relation:
1;:::(d2 +w2 )/90 (B-4)
where:
t =thickness of the core or of each plate
d2 = height of the panel zone between the continuity stiffeners
W2 = width of the panel zone between the column wings
B.2.6 The joints that are welded in the field between the wings of the beam and thecolumn must be full penetration butt welds, executed in the horizontal position onbackup plates, with non destructive testing by X-ray or ultrasound.
B.2.7 The backup plates and welding initiation or completion coupons must be removed.Mter the removal of the plates. the metal will be cleaned and the root reinforced with filletwelds.
B.3 Local flection of the column wing due to a traction stress perpendicular to it
B.3.! The continuity stiffeners must be designed for a stress of Ru - cPRn , where:
traction stress perpendicular to the wing of the column, corresponding tothe MJl.moment of the beam defmed in B.2.2;
0.90;
2
6,25 t FYII
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Ru =
cP =
Rn =
NCh2369
being:
Fyf = fluency tension of the wing, MPa;
tj = thickness of the loaded column wing, mm.
B.3.2 If the width of the beam wing is less than 0,15 b, where b is the total width of thecolumn wing, it is not necessary to perform this verification.
B.3.3 If stress Ru concentrated is applied at a distance of less than 10 tj from the end ofthe column, resistance Rn above must be reduced by half.
B.3.4 Continuity stiffeners must be welded to the core and the loaded wing, in order totransmit to the core the proportion of the load taken from the stiffeners I).
B.4 Local fluency of the core due to compression stresses perpendicular to the wing.
B.4.1 Stiffeners must be installed dimensioned for a stress of Ru - cPRn , where:
Ru compression stress perpendicular to the wing of the column (see Figure B-3) , corresponding to the Mu moment of the beam defined in B.2.2;
cP = 1,0;
and Rn is determined with the following formulas:
a) If concentrated stress RI1is applied at a distance from the end of the column greaterthan its height "d"
(B-5)
1) The sentence proportion of the load takenfrom the stiffeners refers to the differencebetween the load applied and the resistance indicated in this paragraph and theones that follow for column cores. Thus, for example, if Ru is the majored loadtransmitted by the wing of a beam to the column and cP Rn min , is the lowerresistance indicated in clauses B.3 to B.6, the stiffener in the column must bedesigned for Rn. st = Ru - cPRn.min, and the minimum area of stiffener required is Ast= Rn, st / cP F Yost,with cP = 0,9. Additional instructions are provided in B.7 forstiffeners design. This note is also valid for B.3, B.5 and B.6.
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NCh2369
b) If concentrated stress R/l is applied at a distance of less than or equal to "d" from theend of the column:
(B-6)
minimum fluency stress (tension) specified of the core, MPa;
thickness of the wing of the beam that compresses the core of the column,or of the beam wing connection plates, mm. If N< k it is taken as N=k;
distance from the outer surface of the wing to the foot of the fillet weld inthe core, mm;
thickness of the column core, mm:
B.4.2 The continuity stiffeners must be welded to the loaded wing in order to transmit theproportion of the load that corresponds to the stiffener, and the weld to the core must bedimensioned to transmit the proportion of the load taken on by the stiffeners. (see B.7)
B.4.3 Alternatively, if reinforcement plates attached are required, provision B.8 applies.
Figure B.3
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where:
Fyw =
N =
k =
tw =
NCh2369
B.5 Crushing of the core by compression stress perpendicular to the wing
B.5.1 Continuity stiffeners must be installed and eventually reinforcement platesattached, dimensioned for a stress of Ru - cPRn , where:
Ru = compression stress perpendicular to the column wing, corresponding to theMu moment of the beam defined in B.2.2
cP = 0,75
And Rn is defined as follows:
a) If the concentrated compression is applied at a distance greater than or equal to dj 2from the end of the column:
See equation on page 85 of the original document (B-7)
b) If the concentrated compression is applied at a distance less than dj2 from the endof the column:
For Njd.:S 0,2
See equation on page 85 of the original document (B-8)
For Njd > 0,2
See equation on page 85 of the original document (B-9)
In formulas (B.7), (B.8), and (B.9) the following definitions apply:
thickness of the beam wing or the connection plate of the beam wing;
total height of the column profile
thickness of the column wing
thickness of the column core, or sum total of the core thicknesses andattached reinforcement plates.
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N
d =
1;[ =
tv =
NCh2369
B.5.2 The continuity stiffeners must be welded to the loaded wing and the weld to thecore must be calculated to transmit the proportion of the load taken from the stiffeners(see B.7 and B.8).
B.6 Compression buckling of the core
B.6.1 This section refers to a pair of concentrated opposite stresses, applied to both wingsin the same section (see Figure B.4.). Continuity stiffeners and attached reinforcementplates must be installed along the entire length of the height of the core. dimensioned fora stress of Ru - cPRn .where:
R" = compression stress in the column wing;
cP = 0.90
See equation on page 86 of the original document (B-I0)
Figure B.4
B.6.2 If the pair of concentrated opposite stresses that must be resisted are applied at adistance of less than dj 2 from the end of member Rn it must be reduced in 50%.Transverse stiffeners must be welded to the loaded wings and to the core so as totransmit the proportion of the load taken on by the stiffeners. The weld of the stiffeners tothe core must be capable of transmitting the load taken on by these (see B.7).Alternatively. when attached plates are required. provision B.8. applies.
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NCh2369
B.7 Additional requirements for continuity stiffeners
B.7.1 Transverse or diagonal stiffeners must also meet the following criteria:
a) The width of each stiffener plus half the column core thickness must not be less thanone third of the width of the column wing or the moment connection plate thatprovides the concentrated stress.
b) The thickness of the stiffener must not be less than the thickness of the wing or themoment connection plate that provides the concentrated stress; nor less than its
width multiplied by IF: /250 (Fy in MPa).
B.7.2 Continuity stiffeners that resist compression stresses applied to the column wingmust be verified as axially compressed columns. with an effective buckling length of 0.75h and a section comprised by: 2 stiffeners and a fraction of the core of 25 tv in width forintemal stiffeners and 12 tv for extemal stiffeners.
B.8 Additional requirements for reinforcement plates
B.8.1 Reinforcement plates attached to the core must meet the following additionalrequirements:
a) The thickness and size of the reinforcement plate must provide the necessary materialto equal or exceed the resistance requirements.
b) The plate must be welded to transfer the proportion of the total load transmitted to it.
c) Reinforcement plates attached in panel zones of earthquake resistant frames must bewelded to the column wings using full penetration butt welds or fillet welds. capable ofdeveloping the total shear resistance of the attached plate. When the attached platesare installed in contact with the core of the column these must be welded in the upperand lower edges with welds that are capable of taking on the proportion of the totalload transmitted to them. When the attached plates are installed separated from thecore of the column. they must be arranged in pairs. symmetric with regards to thecore and must be welded to the continuity stiffeners in the column core. with weldsthat are capable of taking on the proportion of the load corresponding to each one.
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NCh2369
Attachment C(Infonnative)
Comments(Each numeral refers to the corresponding number in the standard)
C.! Scope
C.I.I The reasons that were considered to prepare a special seismic standard forindustrial structures, supplementary to the building standard, were the following:
a) Industrial structures hardly never have the characteristics that buildings posses:discrete and fairly unifonn distribution of masses at height, rigid horizontaldiaphragms at various elevations, relatively reduced eccentricity and damping ofaround 5%.
b) The basic philosophy of design is different, due to the great importance that industrieshave for the countries economies. It is necessary, therefore, to add to the basicobjectives of the Building Standard (NCh433), paragraph 5.1), the reduction to aminimum of business interruptions and the facilities to perfonn inspections and swiftrepairs.
c) A very important part of industrial structures are the earthquake resistantcomponents of process equipment, frequently complex and with large dimensions,which are necessarily designed by manufacturers overseas: This introduces a factorthat is non existent in buildings.
d) The need to be provided with special standards for industries is being graduallyrecognized by industrialized countries, mainly the U.S.A, Russia, New Zealand andJapan. In Chile, although standards did not exist, since 1940 a fairly unifonn seismicdesign practice has been developed and recognized as being efficient. The standard setforth herein, is mainly based on Chilean practice (1,2) on the Chilean Building Code(3), on the North American Unifonn Building Code UBC standards (4) and theStructural Engineers Association of California SEAOC (5) as well as the New Zealandrecommendations for the petrochemical industry (6).
C.1.2 The standard applies to structures and equipment contained within the industrypremises, the object of which is to manufacture the elements or comply with theobjectives for which the facility was built. They do not apply, therefore, to elements suchas those described, that are generally outside the premises, for which special standardsapply.
NOTE: The references are shown in brackets and are summarized at the end of thecomment.
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NCh2369
C.1.3 In spite of the differences noted between this standard and NCh433, there aremany common elements in the design of buildings and industrial structures, inseismological aspects, related to other standards, other methods of analysis and similar.Hence the need for both standards to be supplementary.
C2 References
All references are included, both national as well as foreign, referred to in the standard.
C3 Terminology & Symbols
C.3.1 Terminology
NCh433, paragraph 3.1, is supplemented by the addition of industrial structures andequipment. The separation of their loads in permanent and several classes of overloadsdue to use is based upon the customary design practices used in the country.
The definition for professional specialist is added, responsible for the earthquake resistantdesign of the industrial equipment as well as for their approval, in consideration withlegal conditions and customary practices, proven as effective, from projects executed inChile and abroad.
The definition for process engineer is also added in the sense in which it is used in thestandard.
C.3.2 Symbols
The symbols in NCh.433, paragraph 3.2, have been completed with the additionalsymbols mentioned in the standard.
C4 General application provisions
C.4.1 Basic principles and hypothesis
C.4.1.1 The principles invoked, with minor variations, are common in the Chilean andNew Zealand practices and in the North American codes (3 to 7). They are supplementedby those in NCh433, paragraph 5.1.1.
C.4.1.2 Both in the Chilean and New Zealand practices as well as in the North Americancodes and in NCh433, paragraph 5.2, the elastic analysis is specified as a basic method.
C.4.1.3 The ductility and redundancy conditions are also common to Chilean and NewZealand practices as well as in the above North American codes.
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NCh2369
C.4.1.5 It is essential that process engineers and professional specialists agree on generalcriteria and details of earthquake resistant design. It is suggested that the agreements besummarized in special forms. such as the one shown below. that must be included in theproject specifications.
C.4.1.6 Topographic amplification is the name given to the increase of seismicaccelerations that occurs in special cases, that must be analyzed by GeotechnicalEngineers, between the valleys and summits of surrounding hills. (such as was observedin Villa del Mar during the earthquake of March 1985.
C.4.2 Ways in which to specify seismic action
The provisions in this standard are based on design earthquakes that have a probabilityof excedence of 10% during an exposure period of 50 years. The criteria for an excedenceof 10% during a minimum exposure period of 50 years. is the criteria adopted by NorthAmerican codes UBC and SEAOC and Chilean standard NCh433. The 50 year periodcorresponds to the useful life cycle of most buildings and industries. However. there areindustries. mainly petrochemical and mining. in which. for reasons of technologicalobsolescence or exhaustion of the sources of raw materials, the useful operational lifecycle is shorter. The New Zealand codes for the petrochemical industry are based on 15%excedence and 25 years (5). According to the same standards (6. Table 2.1 and FigureC.2.1). a reduction from 50 to 25 years with 10% reduces the seismic stresses in only12%. Due to this the Chilean standard has maintained the 50 years for industries.
a) For maximum effective seismic acceleration .Aothe following definition is suggested.initially proposed by the Applied Technology Council ATC. from the U.S.A. (7) andadopted by SEAOC and UBC (4 and 5):
.Ao= Sa /2.5
where Sa is the mean acceleration of the elastic response spectrum with 5% dampingbetween periods 0.1 s and 0.5 s.
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Structure CategoryN° Title Coefficient Analysis R 8% Reference Notes
rJ PIs.
201 Coal hoppers Cl AC-5021.2 Dynamic 3 3 515
202 Operating Platform C"2 BL-O 161.0 Dynamic 4 3 017
203 Stack Cl BL-023 Design according to1.2 Special - - 028 ASCE-75 Steel
Chimney Liners204 Temporary Building C3 AC
0,8 Static 5 5 21001211
*) See 4.3.1 and 4.3.2
NCh2369
b) The provisions have been taken from DBC and SEAOC (4 and 5).
c) All of the Chilean coast in the high intensity Seismic Zone 3 is subject to the risk oftidal waves, that have historically reached grade 3 in the Imamura scale, with somecase of the maximum grade 4 (8). The areas with the highest risk are Tarapaca,Atacama, Concepcion and Valdivia. The risk of tidal waves depends also on theseismic conditions of the marine and topographical conditions of the coast.
C.4.3 Classification of equipment according to the importance
C.4.3.1 The classification is based on the Chilean practice that corresponds, in generalterms, to the New Zealand recommendations. (6).
C.4.3.2 The importance coefficients are based on the Chilean practice and informationfrom DBC, SEAOC and the New Zealand recommendations, that are referenced below:
C.4.4 Coordination with other standards
C.4.4.1 In design of industries, however, it is necessary to use a major quantity ofmaterials and loads that are not regulated in this country, therefore the use of recognizedintemational standards is allowed. The most used in Chile are the following:
American Association of Sate highway and Transportation Officials AASHTO forbridges.
American Association of Mechanical Engineers ASME for boilers and pressure vessels.
American National Standards Institute ANSI/ ASME for piping.
American Petroleum Institute API for petroleum tanks.
American Society for Testing Materials ASTM for materials.
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Catelfories Critical Normal SecondaryIndustrial Chilean practice 1,2 to 1,3 1,0 1,00NCh.433 BuildinJ:!:s 1,20 1,0 0,60DBC and SEAOC 1,25 1,0 1,00New Zealand 1,30 1,0 0,83
NCh236~
American wttd\IDgSociety for welding.
Gennan DlNStandards, British BS Standards. French NF Standards, Japanese JISStandards orflUrostandards of the European Community.
C.4.5 Load CoDiinations
The criteria for load combinations are those recommended by the North AmericanStandards Soci. and the North American Civil Engineers Society ANSI/ ASCE (9). alsoadopted by the IPrth American Metallic Construction Institute AISC (10. 11) and theNorth AmericanCmcrete Institute ACI (12).
This paragraph fIDesnot include the wind loads nor the overloads caused by snow. thatmust be consid~ taking into consideration the design specifications for each particularcase or those .renced above. In general. the wind loads can be considered as thereplacing seismit~loads in the fonnulas. that do not coincide with them. Snow can beconsidered as altoverload. that may be nonnal or eventual.
The factor b = 1..4 for concrete structures or equipment was established taking intoconsideration tlti: load factor established in NCh433.0fl996 and the load and resistancefactors of the ACl318.99 code, therefore the factor b = 1.4 must be used together withthe resistance reduction factors listed in said edition of ACI 318.
The ACI 318 co~ in the 2002 edition adopts the load factors established by ASCE, whichconsiders a factDf of 1.0 for the amplification of seismic requirement, and the resistancereduction factor.i used in the previous editions of ACI 318 are modified to maintainequivalent safetJ factors in design. The load and resistance factors used until the 1999edition are listed in an attachment of ACI 318-02 as an altemative procedure.
C.4.6 Seismic design project and review
C.4.6.1 According to Chilean regulations. all the designs of projects that are constructedwithin the natiOB must be executed by professionals legally authorized to practice inChile. Additionally. the law makes it mandatory to review the structural design ofbuildings of any nature. These provisions have been supplemented as follows:
adding the requirement for the specialist professional to have a structuralspecialization;
allow design Qf equipment manufactured by foreign manufacturers, as a practicalnecessity. In this case, however, it is recommended that for major units such as largeboilers. tall pl'Qcess vessels and similar. the manufacturer be advised by professionalspecialists registered in Chile.
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NCh2369
C.4.6.2 The approval of the design by other professionals is an essential condition that iscontained in most of the Codes and Standards worldwide (13). In the standard the
approval by peers is recommended, who must be professional specialists registered inChile. This requirement is specially important for buildings designed outside the country.
C.4.6.3 The presentation of drawings and engineering calculations from NCh433paragraph 5.11, has been simplified for the great number of minor equipment andstructures existing in the industries, wherein the seismic factor is not determinant.
C.5 Seismic analysis
C.5.1 General provisions
C.5.1.1 Direction of the seismic requirement
The use of seismic requirements in two perpendicular directions is sanctioned by practicein all seismic standards.
The criteria for application of the vertical earthquake are based on Chilean practice (1),the New Zealand recommendations (6), NCh433, paragraph 5.8.2 and the North Americanstandards (3and 4). The vertical accelerations of 2/3 of the horizontals are prescribed inthe referenced codes and are based on actual earthquake records.
C.5.1.3 Seismic mass for the structural model
The design earthquake is an event that will occur once or twice during the lifetime of theindustry and has a duration of a few minutes as a maximum. To select the probableoverload at that moment, it is necessary to be very familiar with the operationalrequirements of the industry. It is recommended that the seismic overload be determinedjointly by the operators or process engineers and the professional specialist, and a recordbe kept of this in the drawings and engineering calculation.
C.5.2. Analysis methods
C.5.2.1 Overview
The majority of the seismic standards including NCh433, the North American and NewZealand ones, are based on elastic response spectrums with accelerations with 5%damping, a figure that is representative for buildings. Industrial structures, however,have a damping of 2%, and is the value that the Chilean practice has based upon. Adamping of 2% was recommended by J.A. Blume and other investigators after extensiveinvestigations carried out in the Huachipato Steel Plant after the great earthquakes in theSouth of Chile in May, 1960 (14).
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NCh2369
C.5.2.2 Linear methods
a) Static analysis: Static analysis is an approximate method theoretically applicable tomathematical structural models with discrete masses uniformly distributed in similarheight and rigidities among the various levels. The NCh433 standard, paragraph6.2.1, UBC and SEAOC contain criteria to determine the application limits of staticanalysis in buildings, that are not applicable to industrial structures. The NewZealand recommendations limit the static analysis to structures in which the massand rigidity at any given level have differences of less than 30% with regards toadjacent levels.
It is recommended that this method not be applied in buildings or structures higherthan 20 m, industrial steel buildings with more than 6 levels or concrete buildingshigher than 18 m or structure with an irregular plant or elevation configuration.
b) Spectral modal or dynamic analysis: dynamic analysis is applied to structures, inwhich the basic hypothesis for linear response, ductile behavior and viscous dampingare valid.
Dynamic analyses can be applied in cases where static analysis is not applicable,particularly in the following cases: buildings and structures that support heavysuspended equipment, steel or concrete chimney stacks with refractory insulation andprocess vessels higher than 20 m or a height to least transverse dimension ratio over5.
C.5.2.3 Non linear methods
Non linear analyses are reserved for structures that exhibit major variations with regardsthe basic hypothesis. Typical examples are major mobile units subject to uplift or impactin the supports, industrial masonry that do not allow traction, structures with insulationat the base and similar. The provisions are based on UBC (4)and IBC (15).
It is recommended that industrial project specifications the specialist professionalsshould indicate the analysis method used for each structure or unit of equipment (seeC.4.1.5).
C.5.3 Elastic static analysis
C.5.3.1 Mathematical model of the structure
C.5.3.1.3 In three-dimensional models each node has 6 degrees of freedom, 3translational and 3 rotation. The assignation of discrete masses to the nodes is performedpartly automatically by the analysis programs, that assign to each one half of the massescorresponding to the actual weight of the members or elements themselves that concur tothe node, and partly by decision of the designer, who assigns to some or all of the nodesof the model representative masses of the extemalloads or of the units supported by thestructure. The degrees of freedom of each node are associated, in this manner, to theinertial characteristics of the masses assigned to them. Generally, the
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NCh2369
effects of inertia on the rotation of masses of the structural members are disregardedwhen the inertial characteristics of these is established, considering only theirtranslational inertia in the three directions in space. The global effect of the rotationinertia of the assembly of masses, on the other hand, is well represented by the spatialdistribution of the total mass in a large quantity of nodes. When the assignation ofmasses that the designer makes to a node must represent the dynamic behavior of a bodythat has non disregardable inertia to rotation, it is necessary to assign to this mass arotational inertia that corresponds to the represented body. Alternatively, therepresentation of that body can be made with a group of masses with purely translationalcharacteristics, distributed and linked to each other in a manner such that the jointresponse of all of these reflects the inertial characteristics of the represented body. Allthree-dimensional analysis programs require the designer to specifY the inertialcharacteristics, both translational as well as rotational, of the masses incorporated intothe model.
C.5.3.1.4 When the structure is provided with rigid diaphragms, the massescorresponding to all the nodes linked by the rigid diaphragm, and its inertialcharacteristics, can be grouped in the masses center and be represented by a singleresulting mass, provided with translational inertia in both directions of the diaphragmplane and of the rotational inertia on the same plane, corresponding to the distribution ofthe masses within the diaphragm. With this grouping the analysis is notably simplified.However, the diaphragm usually has a reduced rigidity in the perpendicular direction toits plane, therefore the effects of the vertical earthquake cannot be properly representedwith the above simplification; in such a case, the vertical earthquake must be treated asan independent load case. Alternatively, normal masses distribution may be used forthree-dimensional analysis, and use the link option and interdependence of degrees offreedom of the diaphragm nodes (constraint) for displacements within the plane of thediaphragm; with this option it is also reduced in computational terms and the horizontaland vertical earthquake can be analyzed simultaneously.
C.5.3.1.5 When the units supported on a structure posses rigidity or inertiacharacteristics that may determine the response of a structure locally or globally, itbecomes necessary to include elements that represent the unit into the model, linked tothe structure of the node in the same fashion as the unit will be, and provided withrigidity or mass characteristics that represent those of the actual unit. This is the case,for example, of large diameter ducts that are attached to several levels of the structure, orlarge vessels that are supported by several frames and / or levels of the structure.Likewise, when it is desired to capture the response of a certain unit of equipmentsupported by the structure, although its translational and rotational inertia may be minorin comparison to those in level at which it is located, elements and masses that representit must be included in the model, linked to the structure in the same way as the unit willbe.
C.5.3.2 Horizontal base shear stress
The formula (5-1) coincides with formula (6-1) in the NCh433 and has the same format asDBC and SEAOC.
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NCh2369
C.5.3.3 Horizontal seismic coefficient
The Chilean seismic design practice for industry is based on the empirical spectrum ofelastic response proposed by J. A. Blume in 1963 (14) after analyzing 16 structures of theHuachipato Steel Plant. The structures were in general steel chimney stacks, invertedpendulum tanks and process vessels. Seven of the structures did not sustain damage inthe earthquakes of May 1960 and the remaining 9 sustained simple failures, such aselongation of the anchor bolts and buckling of the siding sheets. In Figure C.l the Blumespectrum is shown, which according to the author, is reliable in the range of periods from0,6 to 1,1 s, and has a dampening of around 1% to 2%.
Based on the studies carried out by Blume and his extensive professional career,Professor Rodrigo Flores Alvarez proposed the following seismic coefficients (16):
See 3 equations on page 96 of the original document
Standard NCh433 (3) is based on the analysis of a considerable number of records ofsubductive earthquakes recorded in Japan and the Chilean earthquake on May 3, 1985(17). The elastic response spectrum proposed by NCh433, with 5% damping is thefollowing:
See equation on page 96 of the original document (formula 6-1, NCh433)
See equation on page 96 ofthe original document (fon-nula 6-2, NCh433)
where T' and n are parameters that depend on the type of soiL
In this standard the format in formula (6-2) is proposed with a coefficient that allowsconsidering damping ratios other than 5%:
See equation on page 96 of the original document
Figure C.l shows the previous spectrums for the Huachipato Plant, zone 3 and soil typesII of Table 4.2 of NCh433. Blume's empirical spectrum is also shown as well as the one inthe DBC 93 and SEAOC 92 standards.
It can be noted that the coincidence is satisfactory.
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NCh2369
Figure C.1Response spectrum for Huachipato(Zone 3 Ao = 0,4 g Soil II I = 1,0)
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NCh2369
Soil parameters
The classification and soil parameters in Tables 5.3 and 5.4 have been taken fromNCh433, Tables 4.2 and 6.3.
Damping values and coefficient R.
The damping values and structural coefficient R shown in Tables 5.5 and 5.6 have beendetermined from a study of many actual cases of structures in all types of soils andseismic zones, that have been subjected to the major earthquakes of 1960 and 1985, aswell as comparative analyses with the DBC and SEAOC codes.
In Figure C.2 there is a comparison between the design spectrums of R. Flores (RFA) andthose in this standard for R = 2 and R = 3 and soil II. It can be appreciated that thestandard, considering the minimum value, is safe and adequate.
C.5.3.3.1 and C.5.3.3.2 Limit values of the seismic coefficient
Some maximum and minimum values are shown below for the seismic coefficient ofseveral standards and the Chilean practice, for I = 1:
Maximum0,350,24
0,3670,2750,34
Reference
34,54,5
The values of the Chilean practice are within the range of the other codes and have beenproved to be effective in 5 major earthquakes of magnitudes between 7,5 and 9,5 from1960 to 1985.
C.5.3.5 Distribution at height
The formulas proposed are from NCh433 (3), formulas (6-4) and (6-5-)
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NCh2369
Figure C.2 - Design spectrum for Huachipato(Zone 3 Ao = 0,4 g Soil II I = 1,0)
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NCh2369
C.5.4 Elastic dynamic analysis
C..5.4.2 Design spectrum
See C.5.3.3
C.5.4.3 Number of modes
The condition of taking sufficient modes to have 90% of the total mass is contained inNCh433, DBC, SEAOC and the New Zealand recommendations (3, 4, 5 and 6).
C.5.4.4 Modal Superimposition
The complete quadratic superimposition and the formulas proposed are taken fromNCh433, paragraphs 6.3.6.2.
C.5.4.5 Minimum baseshear stress
See C.5.3.3.2.
C.5.4.6 Torsion in plant
The recommendations are based on Chilean practice.
C.5.5 Vertical Seismic action.
The justification for the need to consider vertical seismic action is given in C.5.1.1. Theprovisions apply to structural provisions described in 5.1.1 a), b), c), d) and e), where thevertical stresses have a special importance and have caused damage in earthquakes.
C.5.6 Solid and rigid units of equipment supported on the ground
The units, generally very rigid, are numerous in industrial facilities. The provision isbased on the recommendations from SEAOC and DBC dated 1997.
C.5.8 Special Analyses
The special analyses apply to cases in which the basic hypothesis from the linearanalyses described in 5.2.2 are not complied with.
The standard identifies two basic procedures, spectral and time - history.
The spectral analysis is based on the preparation of spectrums that address non linearityof the structural response, considering the maximum values of the seismic factors in thelocation and type of soil.
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NCh2369
The time - history analysis is based on a step by step analysis of the structural responsefor a minimum of 3 historical records or one synthetic record. The provisions are basedon studies made within the country considering the provisions of the New Zealand andNorth American, DBC and SEAC standards.
C.5.9 Structures with seismic isolation or energy dissipators
The provisions for seismic isolators are based on DBC 97 with minor modifications. Moreinformation can be found in reference 19.
C.6 Seismic Deformations
C.6.1 Calculation of Deformations
The formula (6-1) initially proposed in ATC-3, has been adopted by DBC, SEAOC and theNew Zealand recommendations (4, 5, 6, 7). This is a recognition of the fact that thereduction in stresses between an elastic response spectrum and one of design does notapply to deformations.
C.6.2 The separations s =dl + d2, contained in the New Zealand recommendations (6), isconservative because dl and d2 do not generally occur at the same instant. In the Chilean
practice the following expression has been mainly used s=.Jd[ +d~ ' which is moreprobable, but does not have a safety margin. The values 0,004 hand 30 mm have been ofnormal application within the country.
C.6.3 In the Chilean practice, in general, the horizontal seismic deformations have notbeen limited in industrial constructions, excepting in cases where they could damageelements joined to the structure, such as piping or ducts. The DBC, SEAOC standardscontain the 0,04 h/R limitation; in the May 1960 earthquakes deformations wereobserved of hi 75 = 0,0133 h in industrial buildings with boom cranes (16), a similarvalue to the proposed formula.
C.6.4 The P-Delta effect is rarely important in industrial structures but can be in rigidframe structures.
C.7 Secondary Elements and equipment erected on structures
C.7.1 Scope
Clause 8 of NCh433, based on ATC-3 (7) mainly refers to the secondary elements ofbuildings. In this clause the basic theory has been maintained, but minor modificationshave been made to adapt the requirements to industry.
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NCh2369
C. 7.2 Stresses for seismic design
Formulas (7-1) to (7-6) and Table 7.1 correspond to an improved version of clause 8 ofNCh433.
C. 7.3 Stresses for anchors design
One of the most frequent causes of seismic failure in minor equipment is the lack orinsufficiency of anchors resulting from the application of normal practices in non seismiczones.
In general, en units the anchor bolts are sufficient and it is not necessary to recur tospecial provisions such as shear plates.
C.7.4 Automatic shutdown systems
The recommendation is taken from NCh433, paragraph 8.5.4.
C.8 Special provisions for steel structures
C..8.1 General provisions
The special provisions are based on Chilean experience and the North Americanrecommendations made after the earthquakes of Lorna Prieta and Northridge andintroduced into their codes.
The Chilean experience has been proven in six major earthquakes between 1960 and1985 of magnitudes on the Richter Kanamori scale of 7,5 to 9,5.
The North American recommendations have been summarized in the standards andseismic recommendations from AISC (l0, 11 and 20). The recommendations from AISI(21) were also taken into consideration for slender members not included in AISC.
C.8.2. Materials
The specifications of steel and welds, included in American codes (4, 5, 15) are intendedto avoid failures due to fragile rupture. These are based on the numerous investigationsperformed after the earthquakes of Lorna Prieta and Northridge. In Chile there have beensome failures due to fragile rupture of high strength steel and low tenacity in bridges, nonseismic conditions.
C.8.3 Braced frames
The provisions on bracing are based on the Chilean experience, with some modificationstaken from AISC. In general, it has been considered that the maximum seismicdeformation of our standards is approximately half of that used in the United States,which reduces the risks of failure due to local buckling or anelastic.
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Provisions 8.3.2 about the use of diagonals that work in compression and traction, aretaken from the Eurostandards and their objective is to increase the redundancy ( see4.1.3.b).
Provision 8.3.4 about the crossing point for x diagonals, not included in the Americancodes, has been used successfully in Chile, permanently and was originally based, onAustrian specifications.
In the Chilean practice, taken from the North American (22), it is usual to fix as theminimum height of diagonal profiles 1/90 of its horizontal projection in order to avoiddeformations due to their own weight that reduce resistance to buckling.
C.8.4 Rigid frames
Seismic structures based solely on rigid frames, habitually used in the United States,exhibited many failures in the beam joints and columns in the earthquakes of LomasPrietas and Northridge, therefore many investigations were carried out that originatedsevere design requirements that were included in the main seismic codes (5, 15) andsummarized in the recommendations of AISC (20). In Chile there were no failures in saidjoints mainly due to the lower seismic deformation and not using very thick laminatedprofiles (Jumbo) that has a dangerous methalography. Due to this the proposedprovisions are based on our experience with very few of the recommendations from AISC.
In 8.4.1 it is specified that column to beam moment joints be TR, fully rigid. PR joints,partially rigid, allowed in the United States, are not accepted for two reasons, lack of localexperience and requirements for testing and surveying not available in the country.
In 8.4.3 and Table 8.1 width-thickness ratios are specified taken from the AISCrecommendations for seismic stresses (l0, 11 and 20), with some recommendations basedon local practice. In 8.4.5 and Attachment B provisions are included for panel design ofcolumns in the rigid beam joints, based on the non seismic recommendations of AISC (l0)with very few modifications taken from the seismic recommendations (20). In Chile nofailures of the panel zone are known of. In 8.4.6. provisions are recommended for columnbases, detailed in 8.6.2, that are intended to facilitate inspection and repair of anchorbolts after earthquakes.
C.8.5 Connections
The provisions are based on local practice and the recommendations from AISC. In 8.5.2.and 8.5.3 a design is specified for seismic connections so that they have a resistancegreater than or equal to the connected members. In 8.5.8 requirements are included forthe execution of reliable field welded joints.
In column junctions (see 8.5.9) it is recommended that the junction be designed for ahorizontal stress of 5 kN located at the upper free end, during erection.
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C.8.6 Anchors
Anchors to foundations exhibit failures, generally minor, in all earthquakes. They are, ina certain manner, a seismic fuse.
The provisions of 8.6.2, the object of which is to allow inspection and fast repair after anearthquake, are based on local experience, that mainly takes into consideration thefailures observed in 1960 and avoided in subsequent earthquakes.
The use of shear plates or seismic stops indicated in 8.6.3 to 8.6.7, like the previous case,are based on failures detected in 1960 and the successful subsequent performance of theabove recommendations.
In 8.6.5 the consideration of friction between the base plate and the foundation isexcluded, mainly due to curing contraction of the leveling mortars. In special cases,mainly for major units with many anchors, friction may be taken into consideration, byspecification of non contractible mortars and pretension of the bolts, it being customaryto consider only pretension for the friction.
The recommendation from 8.6.8, in order to avoid failure of the anchors due to theconcrete, it is a customary practice for protection against the difficulties to obtain reliableconcrete mixtures and the incertitudes of the calculation theories for the resistance. Ingeneral it is recommended for the application for design of the Prestessed ConcreteInstitute PCI (23).
C.9 Special provisions for concrete structures
C.9.! Reinforced concrete structures
The specifications are mainly based on national experience in the earthquakes from 1960to 1985, on the provisions of NCh433 and the recommendations of the American ConcreteInstitute ACI-318.99, chapter 21 (12). Consideration has also been given to investigationsmade subsequent to the Lorna Prieta and Northridge earthquakes, published by theEarthquake Engineering Research Institute (24), mainly in what is related toprefabricated members in which local experience is limited.
In 9.1.6 it is specified that it is not necessary to design seismic walls according to thecomplex provisions of ACI. Our designs, that do not apply them, have been successful inearthquakes since 1960, an internationally recognized fact.
The provisions in 9.1.7 for rigid frames that eliminate the ACI requirements are justifiedby the lower seismic deformation of this standard and have been proven in severalnumerical studies.
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A translation exists with comments of the ACI 318 code prepared and published by theReinforced Concrete and Masonry Structural Design Commission and the ChileanConcrete and Cement Institute, which has been proposed by its author as the ReinforcedConcrete Chilean Design Code.
C.9.2 Prefabricated Concrete Structures
The provisions consider the limited Chilean experience in the seismic behavior ofprefabricated structures, the requirements of ACI 318-02 and lEC 2000 and theproposals regarding the issue contained in NEHRP 2000, in order to avid the failures inthese systems observed in the Lorna Prieta, Northridge and Kobe earthquakes (12 and24).
In 9.2.1.1 a) and b) the design is accepted of gravitational systems with humid seismicconnections as equivalent to traditional concrete, because the prefabricated structuremust have a quality higher than site mixed concrete and the joints are equivalent.
Special restrictions have been added to the use of structures with dry connections, due tothe lack of experience with this type of structures in the country. In 9.2.1.1 c) the heightis limited for these structures to 18 m and the number of floors in buildings to 4,maximum values that have been used in local projects.
In 9.2.1.1 c), 9.2.1.4 and 9.2.1.5 it is required that the design be perlormed so that thedry connections fail before the structural members and that the behavior be proven withtests when it is not linear.
In 9.2.1.6 requirements are made for the steel and welding of dry joints equal to thosespecified in 8.2.2 and 8.5.1 in order to avoid fragile failures.
Finally, in 9.2.1.7 conditions are specified for design when the seismic stresses are verylow, similar but more stringent than those specified for non prefabricated structures in5.4.5.
C.9.3 Industrial Bayscomprised by suspended columns
In the design of columns and foundations, including stresses and deformations, it isnecessary to consider the base shear stress assigned to the model, as well as the verticalseismic action. However, if the horizontal bracing system required in 9.3.2 has beendisposed to provide structural redundancy, the design base shear stress must not belower than the value that results from the multiplication of the weight that the columndischarges by the greatest value between C and Cmin..
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C-IO Foundations
C.lO.l The specifications are based on ample Chilean experience, both in foundations forbuildings according to NCh433 as well as several decades of mining and industryprojects.
C.IO.1.3 In this subclaause it must be understood that, the ground stresses, deformationand the stability of the foundation must be confirmed for all the applicable combinationswith the admissible stresses method, that are compatible with soil studies.
C.II Specific structures
C.I1.1 Industrial sheds (Mill Buildings)
C.I1.1.1 The definition is the equivalent to Mill Buildings in English
C.I1.1.2 Sheds in which lateral stresses are resisted by rigid frames of columns andbeams or roof trusses are the most widely used in industries because they allowexpansion of the building.
The continuous roof bracing has the seismic advantages of rigid horizontal diaphragms. Italso makes it possible to, distribute concentrated lateral loads, such as cranes, betweenseveral frames; Chilean practice, taken from the North American, is considered to besufficiently approximate the assumption that the roof bracing transmits 50% of the lateralload to the frames adjoining the loaded frame.
C.I1.1.3 The determination of the magnitude and height of suspended load thatcoincides with the design earthquake is a complex probabilistic problem that it isrecommended should be analyzed jointly among the professional specialists and processengineers. However, if the scarce duration of the seismic stresses is considered incomparison to the life of the structure, the following recommendations can be consideredas safe:
In maintenance cranes, fabrication shops and the like, where the maximum load israrely lifted, and the operation is not continuous, suspended load can be disregardedfor seismic analysis.
In heavy duty cranes in continuous operation with maximum load, such as metallurgyfoundry cranes, it is recommended that this load be used at the highest elevation inthe seismic analysis. This recommendation is based on the dynamic analysis of morethan 600 cases, made in Chile (25), according to which the equivalent load at bridgelevel is equal to the actual for pseudo periods of 1 s or more, at 0,20 of the actual forperiods of up to 0,5 s and varies linearly between both values.
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The following Figures summarize the conditions of the above study
pseudo period
Pl =weight of the building, boom and crane trolley
P2 =weight of the suspended load
mP2=analysis suspended load applied at upper level
K =rigidity
Ts~ 0,5 m= 0,20
Ts= 0,5 - 1,0 m= 1,6 Ts-O,6Ts= ~ 1.0 m= 1,0
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C.II.I.4 The non simultaneousness of dynamic effects of crane operation with theearthquake and the position of several cranes with no load is justified for probabilisticreasons and are a part of the North American practice recommended by the Association ofIron and Steel Engineers AlSE (22).
C.II.I.5 In the May 1960 earthquakes there were systematic failures in the joints ofvertical plates between crane support beams and the columns due to the superimposingof the seismic stresses with fatigue tensions. Cases were also observed of wheels falling offfrom the rail to the upper wing of the crane supports. The recommendations are intendedto avoid these failures (I, 26, 27).
C.II.I.6 The object of the provision is to avoid the formation of rigid towers in extremefacades, that have failed in Chilean earthquakes because they bear seismic stresses thatthey were not designed for (27).
C.II.I. 7 The recommendation is self explanatory. The suggested detail has producedgood results in Chilean practice (I, 27).
C.II.2 Light steel bays
C.II.2.1 The characteristics of light steel bays are defined (sheds), of limited clearanceand height, and light duty cranes and equipment, in which the wind stresses aregenerally higher than seismic stresses. In the country a great number of these bays havebeen built over the years, that do not meet all the requirements of this standard and thathave resisted earthquakes with no damage.
C.II.2.2 Defines the parameters to determine the design seismic stresses. In general,transverse and longitudinal stresses in the extreme panels due to wind are greater thatseismic stresses, but in intermediate panels the longitudinal earthquake can take control.
C.II.2.3 to C.II.2.7 Provisions are specified for bracing. If there are no cranes orequipment of an equivalent weight, traction only diagonals are accepted.
C.ll.3 Industrial buildings with several floors
C.II.3.1 Industrial buildings with several floors, process, energy generation or similar,generally have heavy loads and valuable equipment. In Chilean practice the best resultshave been obtained with dual buildings, with braced or concrete shear walls, combinedwith rigid ductile frames as a second line of resistance (1, 26, 16). These buildings, wherethe seismic deformation is much lower than that of buildings with North American ductileframes, have not suffered the generalized failures in welded joints observed in theearthquake of Northridge in 1994 (28, 29, 30, 31).
C.II.3.2. The recommendations are based on Chilean practice proven in earthquakesfrom 1960 to 1985 (1, 16,26,27,28).
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C.I1.4 Major suspended equipment
C.I1.4.I Figure A. 7 in Attachment A shows a typical boiler, suspended with stringers inthe upper portion. To control the seismic oscillations and avoid impacting the structure itis necessary to install connectors that allow thermal expansion, both vertical as well ashorizontal, some of which are illustrated in the figure. The same figure shows anchorbolts of the hammer head type, with ample ductility, easily repairable and replaceable,that are recommended for major units.
The units are generally projected by foreign suppliers that frequently are not providedwith seismic experience. Due to this it is necessary to establish systems for early advisoryservices and approval of the design by professional specialists approved to practice inChile.
The above recommendations have been successfully proven in a great number ofearthquakes in Chile since 1960 (1, 16,32).
C.I1.4.2 In electrostatic precipitators there are suspended electrode cages of isolators, ofvery high voltage, that it is not possible to secure laterally and can strike the casing in theevent of an earthquake. Chilean practice has shown that these knocks are not important,but that electrical problems occur and fragile breakage of the porcelain isolators. Due tothese reasons it is frequently necessary to specifY special isolators and be provided withpower cut-off devices.
C.I1.5. Pipes and ducts
C.I1.5.I The arrangements of supports and joints must be performed jointly by thepiping specialists and professional specialists.
C.I1.5.2 In general, it is considered necessary to take seismic action into considerationin pipes or ducts with dimensions over 200 mm. In the great majority of cases the weightof the pipes is minor in comparison to buildings and structures and it is only necessary tointroduce seismic deformations in the piping system analysis and in joints design.
C.I1.6 Major mobile equipment
C.I1.6.I Major mobile units of equipment are especially important in an industry, bothbecause the cost is very high as well as because a failure can mean extended shutdowns.They are frequently large and have eccentric loads. Due to this the seismic design iscritical and adequate coordination and approval systems must be established betweensuppliers and seismic specialists during the entire project.
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The design, considering the actual support condition between the wheels and the rails,with possibilities of impact or uplift, posses incertitudes that in practice make it hard toachieve. Due to this, it is generally assumed that said supports are swivels for analysispurposes and precautions such as counterweights and self centering wheels are taken toavoid impacts.
The above provisions have provided satisfactory results in Chilean practice. Most of theunits -thus protected have not experienced failure, excepting some cases of successiveimpacts (hammering) that have caused repairable damage to wheels and trolleys (26, 27).For the dimensioning of the counter weights it is normal practice to a pseudostatic safetyfactor of around 1,0 to 1,2 in a static analysis.
In dynamic analysis it is necessary to consider vertical and horizontal accelerations anddetermine, together with the Operators, the probable overloads during the occurrence ofan earthquake.
The total overturning due to seismic action is not a real possibility due to alternation ofthe loads and it is not necessary to consider them in the design (33, 34). In the Chileanearthquakes of May 1960 and March 1985 there were cases where cranes overturned inthe ports of Puerto Montt and San Antonio, due to major settlements of the soil and notbecause of the horizontal seismic stresses (29, 35).
C.Il. 7 Elevated tanks, process vessels and stacks
C.Il.7.1 Elevated tanks must be designed as inverted pendulums with R = 3. Water ingeneral can be considered as a solid of 0,48 times its own weight (35). If X diagonals areused of traction only, it is necessary to apply a pretension equal to half the maximumtraction of the tensed diagonal.
C.Il. 7.2 The dynamic analysis of process vessels must be performed with R = 3. Theconnection between the columns and shell can be direct when the plate is thick, or bymeans of a circular support beam. The design Of these connections is complex and canbe done with the methods developed by Brownel and Young (37).
C.Il.7.3 Stacks may be self supporting or not self supporting, with an external metal orconcrete structure. The latter are used in stacks that are very tall in thermoelectricplants. Chilean experience, up to heights of 53 m has been successful with dynamicdesigns according to paragraph 5.4 of this standard and R = 3. Very tall non selfsupporting stacks, up to 100 m have been designed with the more conservative methodrecommended by the North American Civil Engineers Society (27, 38). Therecommendation to use the inner concrete coating projected for calculation of the rigiditybut not of the resistance is based on studies carried out by Blume on the effects of the1960 earthquakes in the Huachipato Steel Plant (14). Blume recommends for the coatinga value of E 1/20 of the steel.
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C.I1.7.4 The formula (11-1) is based on the expressions of Timoshenko, corrected byBlume according to his observations of the behavior of 12 chimney stacks from 33 m to52 m in height in the Huachipato plant, 3 of which exhibited failure due to local bucklingin the earthquakes of May 1960. The recommended failure stress, that considers defectsin fabrication and erection, is the following:
See equation on page III of the original document.
If admissible stresses are applied, to the acceptable value is 0,6 X 1,33 Fu = 0,8 Fu, whichis equivalent to formula (11-1).
See equation on page 111 of the original document.
If ultimate loads are applied, according to articles 4.5 and 8.1 b), the seismicrequirements must be multiplied by 1,1 and accept Fa = 0,9 =Fu = 153 Fy ejD.
C.I1.8 Vertical tanks supported on the ground
C.I1.8.I Scope
Major tanks directly supported on the ground are widely used in industries. Most of theseare circular steel tanks, but a few are reinforced concrete or rectangular in shape, Theliquids most commonly used are petroleum, water and other special liquids such assulfuric acid, liquid oxygen, alcohol, etc.
C.I1.8.2.General principles and standards
In general North American design and construction codes are applied of the AmericanPetroleum Institute API for hydrocarbon products and of the American WaterworksAssociation AWWA and the American Concrete Institute ACI for water (39, 40, 41 & 42).Recommendations of the New Zealand National Society for Earthquake Engineering NZhave also been used, applicable to any liquid and material; these specifications, originallypublished in 1986, are very comprehensive, but were modified in the year 2000 becausethey were too conservative (43, 44).
All the standards have two important sections, the seismic section which determines theseismic stresses considering the required location, risk and safety, and the Design sectionthat allows the dimensioning of the tanks and the foundations.
In this standard the seismic action is specified according to our conditions, which aredifferent to those n API, AWWA or NZ. Knowing the seismic stresses the design isperformed according to the code adopted among those listed above. This philosophy isalso applicable in the Unites States (45).
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C.1I.8.3 to C.1I.8.5 Massesand periods
In design it is necessary to consider, for the liquid mass, two portions, the impulsive thatvibrates along with the structure and the convective, over it, that has waves. The 3standards listed in 11.8.2 have formulas to determine the masses and the periods of eachone of them, that are practically coincidental.
C.1I.8.G to C.1I.8.13 Analysis and design
To determine the seismic stresses and the structural R parameters and damping S acomparative study was made of eight steel tanks and two concrete tanks, of sufficient sizeto cover practical requirements, and the results were compared with the values of thestandards listed in 11.8.2. The relations between the seismic coefficients, for the 10tanks, were the following:
NCh2369 j API 1,01 to 1,17
NChjAWWA 0,80 to 0,90
NChjNZ 0,96 to 1,00
C.l1.8.14 Anchor bolts
The provisions about anchor bolts have been successfully applied in national projectsover the past decades.
C.l1.8.15 to C.l1.8.18 Methods are specified to avoid tanks without anchor bolts fromsliding off the foundations, top avoid damage due to compression of air or convectiveliquid hammer or suffer secondary problems in the structure and piping.
The recommendations are based on damages observed in the 1964 Alaska earthquakesand Chile in 1960 and 1985 and the recommendations made on each occasion (1, 27, 28,46,47.48,49 and 50).
C.l1.9 Rotating furnaces and driers
C.l1.9.1 Rotating furnaces and driers are units that can have large diameters andlengths and that operate at high temperatures and low rotational speed. The foundationsare massive and the own period is low, which justifies the use of the static method.
They have appreciable thermal expansions, both longitudinal as well as radial. If theseunits stop rotating for a period of around 20 minutes thermal distortions can occur thatcause considerable damage. These limitations affect design, that requires early andcontinuous coordination between the suppliers and the professional specialists.
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The recommendations of the standard have been successfully proven in a great number offurnaces and driers installed in Chile, since the 1940's (26).
Experience indicates that the impact when the free space closes in the longitudinalearthquake can duplicate the seismic stress (51) and that this can be several timeshigher than normal operating stress. Due to this, sometimes it is necessary to accept thefailure of the roller and their mechanisms, under the condition that these can be replacedover a short period with a controlled rotations procedure of the furnace in order to avoidmajor thermal deformations.
In order to allow this operation it is necessary to install an emergency motor to rotate thefurnace if electrical power supply to the furnace is interrupted in an earthquake.
The indications in Figure All detail 1 are intended to standardize the seismic resistancewith operational conditions.
In the seismic thrust H calculation on support 3 the friction can be subtracted insupports I, 2 and 4 with a coefficient of 0,1.
C.l1.9.2 Detail 2 of Figure All summarizes the design provisions for the lateralearthquake. The overturning calculation is not intended to avoid this occurrence, which isnot an actual possibility, but rather to avoid uplifts and alternative impacts on both sides,a phenomenon known as hammering.
When the longitudinal thrust rollers are missing, major displacements can occur (51). Inorder to avoid falls it is necessary to increase the width of the wheel rims as shown inFigure All, detail 2.
C.l1.10 Refractory brick (masonry) structures
C.l1.10.1 Very rarely are the resistant properties of refractory bricks at hightemperatures known. Mortar disappears or is transformed by high temperature andfrequently the resistance depends on thermal compressions. Generally the brickworks donot behave elastically and do not have reliable resistance to traction. Due to this, it isnecessary to avoid considering brickworks as structural elements or earthquake resistant.Figure A 12 shows two foundry furnaces, one with an arc roof that resists vertical andhorizontal stresses, and the other with a non structural roof, hanging from a steelstructure, in Chilean earthquakes the former and not the latter have failed (I, 27, 32).
In industrial brickworks continued cooperation is required between process engineers andthe professional specialists form the very start of the project.
C.l1.10.2 In furnaces of the type shown in Figure AI2 b), the static method is generallysufficient. In more complex furnaces, with hanging reactors or coolers such as the flashtype of the copper industry, it is necessary to perform spectral dynamic analyses.
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C.I1.IO.3 Before heating up the structure has a different condition than nonnal, becausespaces have been provided for expansion as shown in Figure 12 b). This conditiongenerally takes hours or days and it is not necessary to consider it as coincidental withthe design earthquake.
C.II.II Electrical equipment
C.I1.11.1 Electrical equipment is essential in an industry, due to the need to be providedwith electrical power and communications after the earthquake, for seismic design specialspecifications exist or international standards, with accepted and proven use, that exceedthe scope of this standard. The best known in Chile are those of the National ElectricityCompany, ENDESA. General Technical Specifications 1.015 prepared by professor ArturoArias (52).
C.I1.11.2 The ENDESA standard defines as robust units those in which, due to theirfunction, are designed for greater requirements than seismic ones and that do not havefragile components, and as rigid units those that have a fundamental frequency of 30 Hzor more. Typical examples are generators, motors, valves, pumps and similar. Therecommended fonnulas for static design are based on the ones from ENDESA (52).
C.I1.11.3 The recommendations about isolators are taken from the ENDESAspecification (52).
In units where the conditions of robust and rigid are not complied with dynamic orempiric analyses may be required. For dynamic analysis the ENDESA specificationprescribes spectrums, dampening and R values that depend on the unit; these are, ingeneral, more severe than this standard. Empirical qualification tests consist inoscillation tests to detennine frequencies and dampening, tests under static stresses andin vibrating masses or similar. These are required in major units, such as (encapsuladas)substations.
C.I1.12 Minor structure and equipment
There are a great number of minor units in industries such as pumps, motors, compactboilers, panels, shelves and similar, that in general have good structural resistance, butthat can fail at the anchors, connections and other details, sometimes causing prolongedshutdowns. It is essential to verifY all these elements from the seismic viewpoint and addthe necessary reinforcements, that are usually simple and can be added on site. FigureA.13 illustrates some of these cases.
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C.I1.13 Wood structures
The provisions are based on NCh1l98 supplemented by the recommendations of SEAOCand DBC (4 and 5) and New Zealand standards referenced in North Americanpublications (53).
The failure of structures can occur in wood, caused by traction or flection, or in theconnections. The failure in wood is fragile and in the connections can be ductile.
Generally the structures are classified as ductile, non ductile or semi ductile.
Ductile structures are those that have ductile connections of a resistance lower thanwood. Typical ductile structures are those that resist seismic stresses with braced wallsor diaphragms connected with bolts or nails, those that have wood to wood joints withsmall diameter bolts or nails or those that have joints with joiner plates or steel sheets.Non ductile structures have joints with a greater resistance than wood, that fail due totraction or flection. In general they have rigid glued joints or with 20 mm bolts or larger.
Semi ductile structures are an intermediate instance between the above.
The recommended R values are 4 for ductile structures, 1 for non ductile structures, and2,5 for semi ductile structures.
C.B Column to beam joints design in rigid steel frames
C.B.I Overview
AlSC standards (10 and 11) have design provisions of the panel zone, which is the core ofthe beams, that face the beam moment connections, a zone that is projected to resist theshear stresses generated, that can be major.
AlSC as special conditions for the seismic case of rigid frames (20) in order to avoidfailures due to lack of ductility observed in the earthquakes of Lorna Prieta andNorthridge, that make it necessary to perform tests in many cases.
In Chile no similar failures have occurred, because the maximum seismic deformations inour standards are around half on the North American ones. Due to this, in this standardthe special provisions of AlSC are omitted, with some minor exceptions.
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C.B.2 Panel zone design of moment joints
Includes in detail the design provisions. If the thickness of the core is insufficient, it mustbe reinforced with attached plates or stiffeners in diagonals shop welded. Saidreinforcements can be avoided by changing the column frame for a different one with athicker core. The problem is economical and it is advisable to review it for each case.Below, cost information is provided published by AISC (54), with equivalent valuescalculated for Chile.
Costs expressed as kg of structural steel
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U.S.A. ChileOne plate attached 160 70Two stiffeners welded with fillet 140 60Two stiffeners butt welded 450 200
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REFERENCES
See original document
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