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Chapter 8
Seismic Design of SteelSteel Structures of Buildings
8.1 Behavior of a steel structures8.2 Design of steel frames for middle and high rise
buildings8.2.1 Structural systems of middle and high rise buildings8.2.2 Requirement for arrangement of steel structural
systems8.2.3 Calculation of earthquake action8.2.4 Seismic check for member and section capacity8.2.5 Detailing requirements for seismic design of members8.2.6 Seismic check and detailing requirements
8.3 Design of steel structures of one storey factories
8.3.1 Requirements of structural system 8.3.2 Calculation of earthquake action8.3.3 Seismic check for members and detailing requirement
n Lessons from earthquakes
n The damage or failure phenomena caused by earthquake can be classified as the following aspects: members, joints, whole systems, and non structural components.
8.1 Behavior of a steel structures 8.1 Failure Modes
8.1 Failure Modes Damage of joints
2
Damage of frameQ1: What have been learned from these Q1: What have been learned from these damage phenomena of steel building damage phenomena of steel building structures during recent earthquakes? structures during recent earthquakes?
(1) Care in the design, detailing, and construction of steel structures is needed to assure satisfactory performance in strong earthquakes.
(2) This should lead to the development of building code regulations that specifically address seismic detailing of steelbuilding structures.
• Performance of Steel Buildings in Past Earthquakes
• Key Philosophy for Seismic Design of Steel Building Structures
• Design Earthquake Forces
• Steel Seismic Load Resisting Systems
• Code Provisions for Seismic Design of Steel Buildings
Design of SeismicDesign of Seismic--Resistant Steel Building Resistant Steel Building Structures: A Brief OverviewStructures: A Brief Overview
To Survive Strong Earthquake without Collapse:
Design for Ductile Behavior !Design for Ductile Behavior !
Q2: WhatQ2: What’’s of the most importance to s of the most importance to design a steel building economically? design a steel building economically?
H
HDuctility = Inelastic Deformation
δyO δu
Fy
Fu
F
A
B
F
δ
δ
The ductility factor of steel beam is defined as δu/δy,δy refers to the displacement corresponding to yield load, whileδu to that corresponding to ultimate.
8.1 Behavior of a steel member
3
F i
F 1
∆
Fn
ΣF i
∆
8.1 Behavior of a steel frame To Survive Strong Earthquake without Collapse:
““FuseFuse”” Concept should be Adopted !Concept should be Adopted !
Q3: How to achieve ductile responses of Q3: How to achieve ductile responses of steel building structures? steel building structures?
““FuseFuse”” concept for developing ductile behaviorconcept for developing ductile behavior
• Choose frame elements ("fuses") that will yield in an earthquake.
• Detail "fuses" to sustain large inelastic deformations prior to the onset of fracture or instability (i.e. , detail fuses for ductility).
• Design all other frame elements to be stronger than the fuses, i.e., design all other frame elements to develop the plastic capacity of the fuses.
In general, the common building steel
structures are designed to keep elastic when subjected to the influence of frequently occurred earthquake, while when subjected to expected rare earthquake the structure should keep their capacity to prevent from collapse.
8.1 Behavior of a steel structures
8.2 Design of steel frames
8.2.1 Structural systems of middle and high rise buildings
8.2.2 Requirement for arrangement of steel structural systems
8.2.3 Calculation of earthquake action
8.2.4 Check for the seismic capacity of members and connections
8.2.5 Detailing requirements for seismic design of members
8.2.6 Seismic check and detailing requirements of joint and connection
Profile
Plan
1. Moment resisting frame Moment resisting frame is the structure that is composed of beams and columns. The horizontal forces caused by earthquake are transferred to foundation through beams and columns.
H<110m, 30 stories.
8.2.1 Structural systems
4
2.Braced frame structure and Frame with shear wall
支撑
支撑
支撑
支撑
支撑
The main components to resist horizontal earthquake action are the bracing or shear wall. The braced frame structure system can generally divided as concentrically braced frame (CBF) and eccentrically braced frame (EBF).
Plan Profile
Concentrically braced frame (CBF)：The central lines of the members: column, beam and brace, which are connected in one joint, are through the same point .
Figure 8.8 Concentrically braced frames a) X type b) inverted V type c) V type d) K type
e) and f ) diagonal type
a) b) c) d) e) f)
Eccentrically braced frame (EBF)：The eccentricity can be arranged in different ways, for example, the brace is shifted from the joint center at both ends or one end, or a short cantilever hung from the beam.
a)
a
a
a
a
a
a
a
b)
a
a
a
a
a a
a
a
c)
a
a
a
a
a
d)
a
a
a
a
a
the segment marked ‘a’ is called ‘link’ in EBF, which is the main component for energy dissipation.
Figure 8.10 Types of eccentrically braced frames
3. Tube structures
To the buildings which height exceeds 200 meters, tube structures, mega frame structures are considered more suitable.
Plan ProfilePlan
Figure 8.6 Sections of structures for middle and high rise buildings(a) moment resisting frame (b) braced frame (c) tube structure
(d) braced frame tube (e) mage structure
8.2.1 Structural systems of middle and high rise buildings
a) d)c) e)b)
1. Limitation for building height
2. Limitation for height-width ratio
8.2.2 Requirement for arrangement of steel structural systems (GB50011-2010)
180280300tube and mega structure
120200220Concentrically braced frame5090110frame
IX(0.40g)
VII(0.15g)
VI, VII(0.1g)Structural systems
eccentrically braced frame 240 220 160
VIII
90
180
200
260
(0.20g) (0.30g)
70
150
180
240
5.56.06.5Limitation of the ratio
IXVIIIVI, VIIDesign intensity of the region
5
3. Seismic Grading
8.2.2 Requirement for arrangement of steel structural systems (GB50011-2010)
Height(m)
Intensity
6 7 8 9
≤50 4th 3rd 2nd
>50 4th 3rd 2nd 1st
4. Expansion joint To avoid irregularities of the building frame system, the gaps required to the expansion joint in steel structures should be set 1.5 times of that in RC structures.
5. Layout of structural system
During the expected rare earthquake, the damage to the structurecan be accepted but the structure should keep stable even after the structural damage happens, in order that the gravity loads can be still supported by the structure.
8.2.2 Requirement for arrangement of steel structural systems
a) b)
The plastic hinge mechanism which occurs in most of beam ends is an ideal failure mode, because it is able to dissipate more energy than the column collapse mode .
Figure 8.12 Plastic hinge mechanism
Ideal failure mode
2） In CBF structures, the lateral forces are mainly supported by braces because braces provide most of the lateral stiffness. So the bracing is the first resistant line.
3） In EBF structures, the link will be expected to develop plastic hinge firstly, here links conform the first line.
1） Beams should be considered the first line to defend against earthquake in moment resisting frame, therefore, the ‘strong column and weak beam type’ is usually preferred.
6. Resistant lines
5. Enough rigid floor system
Composite slab and concrete slab cast-in-place are prior to others because these slabs can afford enough stiffness in the floor plane which assures frames work as an entity.
（1）Build a calculation model;
（2）Determine the parameters, including earthquake action factor, periods, modes, damping ratio, etc.
（3）Calculate the earthquake action. Three methods can be used.
（4）Seismic checking for the members, connecters, and bases;
（5）Displacement checking;
（6）Details design.
8.2.3 Computation of earthquake actionSeismic design steps：
6
Seismic designflow chart
8.2.3 Computation of earthquake action1. Damping ration By the experience, the seismic design code
recommends that:n when the action of frequently occurred earthquake is to
be computed, the damping factor can be taken as 0.04 if the building is less than 50m,
n while 0.03 if the building is more than 50m but less than 200m,
n and 0.02 when the building exceeds 200m.n when expected rare earthquake is considered, the
damping factor is taken as 0.05.
2. P-Delta effect : Second order effect To any story, if the second order moment which is computed by the product of gravity load over the story and the storey drift is greater one tenth of the overturning moment which produced by storey shear force timing the storey drift, P-Delta effect should be considered in the computation results.
c) Additional moment due to second order effect of vertical load
b) Storey moment by horizontal load
a) Analsis model
Figure 8.14 Second order momenta)
3. Lateral displacement and the limitation of the storey driftThe storey drift limitations for middle and high rise building steel structures are 1/250 when considering the frequently occurred earthquake; andn 1/50 when considering the rare occurred earthquake respectively, specified by design code.
Mc1
Vb2
Vc1
Mb1
hb
a) Moment and shear in panel zone A
Shear in panel zone Vc=(Mb1+Mb2)/hb-(Vc1+Vc2)/2
Vb=(Mc1+Mc2)/hc-(Vb1+Vb2)/2
A
Vb1Mb2
Vc2
hc
Mc2
column
beam
b) Distorting of panel zone A
Figure 8.15 Distorting of panel zone
4. Proportion of the lateral load shared by bracing and frame
Design code asks the lateral load shared by frame is not less than the minimum value of the next two:
1) 25 percent of the total base shear, and
2) 1.8 times of the shear supported by frame which are computed by elastic analysis.
支撑
支撑
支撑
支撑
支撑
7
5. Provisions about design inner force of beam and column
1） The design end moment of beams can use the moment at the edge of column.
moment at the node by analysis
beam
axis of beam
column
axis of column
Design moment at the beam ends
Figure 8.16 Design moment at the beam ends
2） For prevention of the early damage of the beam segment adjacent to link in EBF structures, the design moment of the beam segment should multiply an amplification factor not less than 1.5 in region VIII, or 1.6 in region IX. Thus the design moment is computed as Equ. (8.1).
（8.1）
c0
( )lcV VM M
V× ×
or ＝(the amplification factor)
In EBF structures, the inner force of column shall be considered to be amplified as Eq. (8.1).
0 is the computed moment of the beam. M
c rc are the shear capacity of linklV V 、 ；
is the shear of linkV ；
6. Inner force of brace member (GB50011-2010)
The boundary condition of brace in analysis is usually supposed as pin connection.
To the brace connecting to link in EBF structure, the design axial force of the brace shall take an amplified factor which is no less than 1.4 for grade 1;1.3 for grade 2 and 1.2 for grade 3 frames.
RE/S R γ≤1、Beam and column
the load combination in which earthquake action shall be combinedS－
；
the design strength of steelR－ ；
RE adjusting coefficient for load bearing To beam and column members, the coefficient is 0.75.
γ － ；
8.2.4 Seismic check for member and section
capacity
Where,
EG G E Eh Ehk v EvkS S S Sγ γ γ= + +
（1）The bending capacity of beam and columnTo I and H shape steel, when bending around its strong section axis ：
y pcwhile / 0.13N N M M≤ = ， （8.2）
y pc y pwhile / 0.13 1.15(1 / )N N M N N M> = −， （8.3）
w web area and the gross area of the whole section. A A、 －
n net area of the sectionA－ ；
p pc ull plastic moment without axial force and the ultimate moment
with axial force existing;
M M、 －f
y y n aydesign axial force and axial yielding capacity, respectivelyN N N A f=、 － ， ；
Wheren net area of the sectionA－ ；
w web area and the gross area of the whole section. A A、 －
p pc ull plastic moment without axial force and the ultimate moment with axial force existing;M M、 －f
When bending about its weak axis：
y w pc/ /N N A A M M≤ =while ，
2y w pc w ay y w ay p/ / {1 [( ) /( )] }N N A A M N A f N A f M> = − − −while ，
（8.4）
（8.5）
y y n aydesign axial force and axial yielding capacity, respectivelyN N N A f=、 － ， ；
8
（2）In order to assure the plastic deformation occurs first at beam end, the following equation shall be checked.
except that: 1) the columns in a storey have the shear capacities 25
percentage greater than those in the above storey, or 2) the design axially compressive force does not exceed 0.4
times of its design axial capacity, or3) the column as an axially compressed member can keep its
global stability under two times of design seismic action
pc yc c pb yb( / )W f N A W fη− ≥∑ ∑pc yc c pb yb( / )W f N A W fη− ≥∑ ∑ （8.6）
Where:
pc pb plastic section modulus of beam and column;W W、 -
design axially compressive force;N -
yc yb yield stress of column and beam respectively;f f、 -
c area of column section; A -
η ——Coefficient,taken as 1.15 for grade 1 frames,1.10 for grade 2 frames and 1.05 for grade 3frames.
（3）In CBF structure, if the inversed V type or V type brace is used, the beam connecting with braces should be fabricated in a continuous member to sustain the force transferred by braces．It is checked as a simply supported beam without middle supporting under gravity load and the unbalanced load due to the buckling of one compressed brace.
（4）In EBF structure, the shear capacity of link shall be checked by Equ. (8.7). This equation considers the effect of axial force in the link.
REwhile 0.15 /lN Af V Vϕ γ≤ ≤， （8.7a）
w ay p
w f w
p p
min{0.58 ,2 / }
( 2 )l l
l
V A f M aA h t tM W f
=
= −
=
c RE0.15 /lN Af V Vϕ γ> ≤while ，
2c w ay pmin{0.58 1 [ /( ) ], 2.4 [1 /( )] / }l lV A f N Af M N Af a= − −
（8.7b）
Where, coefficient, taken as 0.9;ϕ -
design shear force and design axial force of the link;V N、 -
c design shear capacity of the link and its modified capacity considering the influence of axial force;
l lV N、 -
p full plastic moment of the link; lM -
w the length, section height, depth, thickness of web and flange of the link;a h t t、 、 、 －
w web area and the gross area of the whole section of the link; A A、 －
p plastic section modulus of the link;W －
ay design strength and yield stress of the link;f f、 －
RE adjusting coefficient for load bearing of the link, taken as 0.85.γ －
3、 Brace
The compressed brace in CBF structure shall be checked by Equ. (8.8a) through (8.8c)
（8.8a）br RE
n
n ay
/( ) /1/(1 0.35 )
( / ) /
N A f
f E
ϕ ψ γψ λ
λ λ π
≤= +
=（8.8b）
Where, design axial force of the brace;N －
br the area of the brace; A －
stability coefficient for axially compressed steel members;ϕ －
reduced factor for strength considering the influence of cyclic load; ψ －
n slenderness ratio; λ －
（8.8c）
ay yield stress of the brace;f －
RE adjusting coefficient for load bearing of the brace, taken as 0.80.γ －
Elastic modulus of the brace; E －
1、Slenderness ratio of columnThe slenderness ratio of columns shall not be greater than the limitation listed in Table 8.3. The table is made according to Q235 steel. For other steel, the limitation should time the factor of .
ay235/ f
8.2.5 Detailing requirements of members
606080120Maximum slenderness ratio to the buildingover 12 stories
100120120120Maximum slenderness ratio to the buildingnot exceeding 12 stories
IXVIIIVIIVIPlate element
Table 8.3 Limitation for slenderness ratio of columns
9
2、Width-thickness ratio of plates in beam and column For the sake of prevent from early local buckling which decrease
the deformability of members, the width-thickness ratio shall be limited according to Table 8.4 and Table 8.5. Both two tables are based on Q235 steel and it should be times the factor of to the steel with different yield stress.
ay235/ f
85-120Nb/(Af）
<=75
80-110Nb/(Af )<
=70
72-100Nb/(Af )<
=65
72-120Nb/(Af)<=60
webs of H and box section
36323030plates of box sectionbetween two flanges
111099flange of H shaped sectionand box sectionBeam
40383633plates in box section
52484543web of H shaped section
13121110flange of H shaped sectionColumn
Grade IVGrade IIIGrade IIGrade IPlate element
Table 8.5 Limitation of width-thickness ratio for beams and columns in the building(GB50011-2010)
3、Slenderness ratio and width-thickness ratio of bracesAt the section where plastic hinge is expected the brace should be
set at both flanges of the member. The slenderness ratio of the laterally supported members shall meet the requirements decided by the following equations.
1 1y
px px y
235While 1 0.5 (60 40 )M MW f W f f
λ− ≤ ≤ ≤ −，
1 1y
px px y
235W 0.5 1.0 (45 10 )M MW f W f f
λ< ≤ ≤ −hile ，
（8.9a）
（8.9b）
Where， 11
yy
y
is the distance between the lateral support points
is the radius of gyration about the out-of-plane axis of the section;
lli
i
λ = ， －
1
1
the moment of the section where the lateral brace is connected, the value taken as positive if the bending moment in same direction among the range of ,or taken as nega
M
l
-
tive.
ay235 / f
The slenderness ratio of the brace members in CBF structure shall not exceed the limitation listed in Table 8.6. And their width-thickness ratio shall not exceed the limitation listed in Table 8.7. Both two tables are based on Q235 steel and it should be times the factor of to the steel with different yield stress.
Table 8.6 Limitation of slenderness ratio of brace in CBF
6090120Building exceeding 12 stories
tensile member150150200
designed as
compressive member120120150
designed as
Buildingnot exceeding 12 stories
IXVIIIVI, VIIPlate element
The width-thickness ratio of the brace members in CBF structure shall not exceed the limitation listed in Table 8.7. Both two tables are based on Q235 steel and it should be times the factor of to the steel with different yield stress.
ay235/ f
Table 8.7 Limitation of width-thickness ratio for brace in CBF
of pipe section38404042
diameter-thickness ratio
19212123252831web of box section
21232325273033web of H section
788991113outstanding flange
IXVIIIVIIVIIXVIIIVII
Building exceeding 12 storiesBuilding not exceeding 12 storiesPlate element
10
To the brace in EBF, the slenderness ratio shall not be greater than 120 , and the width-thickness ratio shall follow the provision of the code for design of steel structures.
ay235 / f
4、 Link（1） Due to high dissipating ability is desired, the high
strength steel which does not possess good yielding deformation shall not be used as the link. By the experience until now, the link had better use the steel which yield stress is not greater than 345
MPa.ay235 / f
（2）The width-thickness ratio of the link as well as the beam segments in the same bay of the link shall not greater thanthe limitation of Table 8.8, while when the steel other than Q235 is used the factor of should be multiplied .
while / 0.14N Af ≤
while / 0.14N Af >
[ ]90 1 1.65 /N Af−
[ ]33 2.3 /N Af−
Table 8.8 Width-thickness ratio of links and the beams in the same bay
web
8outstanding flangelimitationPlates
（3）Since the brace produces axial force in the link, the length of the link shall be limited by Equ. (8.10) to keep the necessary energy consuming ability if the axial force in the link is greater than 0.16 times of the design axial capacity.
w pwhile ( / ) 0.3 1.6 /l lA A a M Vρ < <，
w w pwhile ( / ) 0.3 [1.15 0.5 ( / )]1.6 /l lA A a A A M Vρ ρ≥ ≤ −，
(8.10a)
(8.10b)
Where， a－the length of the link, referred to Figure 8.10;
the proportion of the design axial force and design shear of the link.ρ －
（4）The web of the link can neither be strengthened by welding a parallel plate nor be holed. The former will be obstacle to develop plastic deformation while the latter will weaken the dissipatingability.
（5）The necessary rib should be welded to the link to transfer the shear force and prevent from local buckling. Furthermore, at the both ends of the link the braces should be set on both of the upper and lower flanges to keep the twist of the link. The detailing requirement can refer to the corresponding provisions in the code.
the cross point of brace and the link shall be at the end or inside of the link
ribs
beam segments in the same floor of the link
brace gusset the middle ribs in the web of link
the length of the link ,a
profiled bar with wide flange
the cross point of brace and the link shall be at the end or inside of the link
beam segments in the same floor of the link
the length of the link ,a
ribs
the middle ribs in the web of link
Figure 8.17 The detailing of link
1、Beam-to-column rigid jointEqu. (8.11) shall be used for the seismic check of the joint to make the yield capacity keeping reasonable.
pb1 pb2 p v( ) / (4 / 3)M M V fψ + ≤
To H shaped column,p b c wV h h t=
To box column, p b c w1.8V h h t=
（8.11）
（8.12a）
（8.12b）
Equ. (8.13) shall also be checkedw b c( ) / 90t h h≥ +
b1 b2 v RE( ) / (4 / 3) /pM M V f γ+ ≤
（8.13a）
（8.13b）
8.2.6 Seismic check and detailing requirements of joint and connection
Volume
11
reduced factor, for region VI with site soil type IV and region VII taken as 0.6, region VIII and IX taken 0.7;ψ －
；
b c web depth of beam and column, respectively; h h、 －
w thickness of panel; t －
b1 b2 design moment of the beam ends at the joint;M M、 －
RE adjusting coefficient for joint, taken as 0.85.γ －
design shear strength of the steel;vf －
pb1 pb2 full plastic moment of the beam ends at the joint;M M、 －
p the volume of joint zone; V －
Where，2、Detailing requirement In beam-column joint, column through type is commonly
used and recommended by seismic design code. In practice, especially for box column, two through type diaphragms are adopted to shape a special joint, as shown in Figure 8.18(b), which will be convenient for fabrication. In the case of H shaped column, if the joint connects beams rigidly only in one direction, the strong bending axis had better to be chosen as the rigid way.
(a) Column through type (b) beam through typeFigure 8.18 Beam-to-column joint
Referring to Figure 8.19, to column through type joint, the next points should be paid attention to.
(1) The beam flange must be welded in butt welding and the toughness should be assured strictly.
(2) Horizontal stiffener corresponding to the beam flange has been set and the thickness shall not be thinner than the corresponding beam flange. If the stiffener is the inner diaphragm inside the box column, the rims of the stiffener shall be butt welded. The stiffeners to the H shaped column flange shall be welded in butt welding while to web can use fillet welding.
(3) The beam web can be connected with high strength bolts tothe column. By scallop hole beneath the flange the butt welding is apart from the web to prevent the concentration of the welding.
5∼10
35°6
r=20
Detailing A
35°
r=35
r=10~15∼10
6
A
B
hw＝6，the length equal to the flange width
Detailing B
Figure 8.19 Detailing of the joint for site connection
Figure 8.20 shows another type of joint. Short cantilever is welded to the joint in shop and the free end will connect to themiddle beam segment by high strength bolts or welding and high strength bolts. It makes the construction easily in site.
Figure 8.20 Joint with short cantilever for site construction
High-strength bolts of slip-critical connections
a) b)
High-strength bolts of slip-critical connections
8.3 Design of steel structures of
one storey factories
8.3.1 Requirements of structural system
8.3.2 Calculation of earthquake action
8.3.3 Seismic check for members and detailing requirement
12
1、 The structural systems for one storey factory can be generally divided as transverse frame and longitudinal frame. The transverse frame is composed of column and beam or column and truss, while the longitudinal frame is composed of column, linking member (truss) and braces.In the large span workshop, the grid structure is also used
nowadays. Here, only the points of frame structures are explained. Figure 8.21 and 8.22 shows examples of frame structures.
8.3.1 Requirements of structural system
(a) framed bent (b) rigid frame
(c) multi-bay frame with same height (d) multi-bay frame with different heights
Figure 8.21 Transverse frames
Figure 8.22 Longitudinal frames
trusssupport truss vertical brace
brace between lower columns
brace between upper columns
crane beam
tie bar
2、Earlier than 1990s, most of the industrial workshop adopted purlinless roof structures, by using the pre-cast concrete roof slab, and in many cases with the skylight truss out of roof. Nowthe roof structures commonly use purlin system in which steel deck replaces the concrete slab that the roof weight becomes light. Instead of skylight truss small ventilator are widely used. These changes are advantageous to seismic design due to the decrease of roof mass therefore the inertial load caused by ground motion.
3、 In the view of seismic design the arrangement of structural system for industry workshop, the following should be paid attention.
(1) The height of multi-bay frame had better been designed same. If this requirement can not be satisfied, the lateral force at the column due to the lower roof mass should be fully considered.
. (2) The layout of the plan shall be arranged regular as possible. For example, two bays have different longitudinal sizes, thus the displacement, vibration velocity and acceleration of transverse frame in axis 4 will the different from those of frame in axis 5. The connection will be easily damaged.
(3) If the layout is complicated and the structural engineers have no way to avoid the irregularity, expansion joint should be set referring to Figure 8.24
5
Figure 8.23 Irregular plan
Location of expansion point
Figure 8.24 Workshops have perpendicular longitudinal layout
13
(4) The climb ladder of crane shall not arranged near the same transverse axis, because the crane mass will concentrate after work.
(5) The rigidness of longitudinal frames shall be designed without great difference.
(6) The horizontal transverse bracing in the roof plan is necessary for transferring horizontal earthquake action to the column and reduce the effective length of the roof truss chord or roof beams. The vertical bracing and horizontal longitudinal bracing are conditionally needed, referring to the provisions of the design code.
(7) In the longitudinal direction, the most efficient way to resist earthquake and provide with necessary stiffness is to arrange cross braces or other type braces. If limited by the technical condition the bracing is not able to arrange, the rigid frame is the option.
Figure 8.25 Single freedom model Figure 8.26 Two freedom model
1. Analysis model
8.3.2 Calculation of earthquake action
Horizontal action is necessary to be consideration in earthquakefortification.
Vertical earthquake action should be consideration if the span of roof exceeds 24 meters
2、 The consideration for the weight and stiffness of enclosure material
If profiled steel sheet or the pre-cast concrete panels with flexible connector are taken as enclosure only their weight shall be considered in analysis model.
In the case that the brick block is used and necessary anchorage is embedded, the stiffness of the wall can be taken into account. Usually the effective stiffness factor is taken as 0.4.
3、Space work effective
Though the structure is supposed to behave as independent plane frames, if the roof structure uses concrete slab which has great horizontal stiffness, the space work effective shall be taken into account in analysis.
4、 Distribution of earthquake action in longitudinal direction
To the structure where light weight wall panel or concretepanel with flexible connector is used as enclosure, the distribution of earthquake load in different longitudinal frame shall refer to the following guidance:
Purlinless concrete slab roof: the stiffness of roof structure had better be considered and the earthquake load is distributed as the stiffness of longitudinal frames.
Roof material without enough stiffness: earthquake load is distributed according the ratio of the mass that each longitudinal frame supports.
Concrete slab roof with purlin: the distribution method may adopt the average value of the results of above two.
There is no special requirement for the strength and stability check for structural members for the low rise factory structuresis commonly not higher statically indeterminate structures.
Based on the point, the seismic design code asks the engineers to pay more attention to the detailing requirements.
1、The limitation of slenderness ratio of steel column
The code limits the maximum slenderness ratio of steel column in factory structure not exceed 120 , where is the yield stress of steel.
8.3.3 detailing requirement
ay235 / fayf
14
2、The limitation for width-thickness ratio of plate element
The design code requires strict limitation comparing with the provisions for static design. The requirements can be found in Table 8.9.
Table 8.9 Width-thickness ratio limitation for structural members
353940Nb / Af 0.37
72-100ρ80-110ρ85－120ρWeb of box section Nb / Af < 0.37
303236Flange of box section
91011outstanding flange of H section
Beam
505560Diameter-thickness ratio of pipe
485258Nc / Af 0.25
606570Web of box section Nc / Af < 0.25
363638Flange of box section
101113outstanding flange of H section
Column
IXVIIIVIIplateMember≥
≥
.
It should be noticed that the limitation is for Q235 steel and should be timed the factor if any other steel is used In table, the symbols Nc, Nb, refer to axial force of column and beam respectively, A to the area of corresponding members, f the design strength, and ρ= Nb / Af.
ay235 / f
3、Column brace The shaped steel is desirable for the brace under the level of
crane beam, while above the level ether steel bar or shaped steel can be chosen. X type brace is prior to be adopted and the declined angle of the brace member to the horizontal plane shall not be greater than 55 degrees. If the lower brace is difficult to use X type due to technical procedure, the portal bracing or inversed brace will be the options.
The maximum slenderness ratio of X type brace shall follow Table 8.10.
Table 8.10 Maximum slenderness ratio of X type brace
150150200200Lower Brace
150200250250Upper Brace
region IX soil type III,IV
region VIII & soil type III, IV region IX & soil
type I, II
region VII & soil type III, IV
region VIII & soil type I, II
region VI, VII & Soil type I,II
Design intensity of the region
Location
4、Roof brace
Section design of the roof brace shall be according to the inner force of members. The strength of the connection shall be stronger than the inner force of members.
5. Requirement for the column base
There are three basic types for column bases: embedded column base, concrete-clad column base, exposed column base as shown in Figure 8.27. The former two are belonging to the rigid base. Theexposed column base can be dealt with either rigid or pinned. d) rigid exposed base
b) concrete-clad base
c) pinned exposed base
a) embedded base
Figure 8.27 The column base type
15
In region VI and VII, the exposed base can be adopted, but had better design it as rigid base. The design moment which will be resisted by anchor bolt shall multiply an amplified factor of 1.2. A shear connector is usually needed. In the past earthquake damage, the pull out of the exposed base was one of the type. Sothe engineers should be care when planning to use this type of base especially in the region where high intensity of earthquakewill probably occur.
The deepness in the concrete foundation to the embedded baseis not be less than two times of the section depth of the solid web column, and shall check the deepness, d, by the Equ. (8.14)
6 / f cd M b f≥ (8.14)
The deepness in the concrete foundation to the embedded base is not be less than two times of the section depth of the solid web column, and shall check the deepness, d, by the Equ. (8.14)
Where，M－full plastic moment of column at the base section;bf－width of column flange in the bending direction;fc －design compressive strength of foundation concrete
6 / f cd M b f≥
Calculation book of a frame under a rolling rock
attack
Wuhan University,2012.10
Case Study
