Norway Steel 1

8/6/2019 Norway Steel 1

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www.argenco.ulg.ac.be Oslo, November 8th, 2007

Design

of

Earthquake Resistant Steel StructuresIn the context of

Eurocode 8

Prof.Dr.ir. Andr PLUMIER


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Earthquake Action


3/55

Whats an earthquake?

Physical phenomenomTectonic plates: tendancy to move, but friction

=> Stresses at edges

If stresses > friction => sudden move => wave propagation= EQ

EQ also existInside plates

intraplate EQ


4/55

Slippage at San Andreas fault(California)

Components of seismic action

Action applied to a structure by an earthquake

A move Horizontal & vertical components


5/55

Caracterisation of earthquakes.

Magnitude: Richter scale, total liberated energy

Intensity I: Mercalli scale, effects on buildings at a given location

Acceleration ag(t) accelerogram at a given location

Displacement of ground dg(t) physical&measured data

Sub-products: usual design data in earthquake engineering agR: maximum ofag(t) , at bedrock

= Peak Ground Acceleration PGA

0,4 g 0,6g highly seismic zones Japan, Turkey0 0,1g low seismic zones

acceleration response spectrum

Duration => cumulation of damage


6/55

Maps of PGA agR

the world

Europe


7/55

Peculiar effects of earthquakes.

Soil settlement Dry sand Tsunami

Soil Liquefaction Fine sand saturated by waterShear resistance of soil: R= ( u ) tg : friction angle ( u): effective pressure

: applied pressure u: internal water pressureMove of soil => u => R max : R= ( u ) tg = 0= > a liquid => soil liquefaction

Effects Inclination & tilt of structures

Landslides if liquefiable soil layerclayConditions high vibration energy required => u


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Earthquake hazard in a given zone Caracterisation : PGA agR

=> maps

Design value ofagR?

Define a probability of the earthquake against which the design is madeChoice : a balance safety cost a less probable EQ => highera

gRand cost

Eurocode 8, EN1998-1 :2004 Design Earthquake

No collapse Earthquake: probability of exceedance in 50 yearsPNCR = 10%

ULS Equivalent: return period TNCR

= 475 years

Damage Limitation to non structural elements Earthquake: SLS

probabilityPDLRof exceedance in 10 years =10% return period TDLR= 95 years

Frequency of earthquakes

(cumulated number/year)

As function of magnitude Ms

For Belgium


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UncertaintiesOn seismic hazard :

- Poor data base: measurement since 1950

return period NCR = 475 years

- Major Eq not frequent

- Hypothesis : history repeat itself- Partial knowledge of geology of sites

recent earthquakes unvail new faults

Northridge (1994), Kobe (1995), Kocaeli (1999)

after earthquake : zone seismic level increasedEx:Istanbul from 0,2g to 0,4g after Kocaeli (1999)

Effective mitigation of uncertainties on seismic action ?

Design with ductilityResistance maintained independently of displacement

=> STEEL STRUCTURES


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du du

Concept a Concept b

V

d

Concept a: low-dissipativestructure

Concept b: dissipativestructure

Velastic responseStructure designed to remain

elastic under design earthquake

VreducedStructure designed to yield under

design earthquake

du

Ultimate displacement

Dissipative and non dissipative global behavior of structures.

The non dissipative structure fails in a single storey mechanism.

large sections

only elastic stresses

smaller sections

a design

for numerous plastic zones


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Steel structures are good at providing energy dissipation capability

Due to:

Ductility of steel material Many possible ductile mechanisms in steel elements

in connections

Effective duplication of plastic mechanisms at a local level

Reliable geometrical properties

Relatively low sensitivity of bending resistance M

to coincident axial force N

Guaranteed material strength, as result a of controlled production Designs and constructions made by professionals


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A tool to evaluate the effects of earthquakes:

Response Spectra


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u (t)

u0Vibrations

non amortiesc = 0

t

T = 2/ T = 2/u(t)

Figure 2.2. Types de vibrations libres (p(t))=0) ralises par cartement u0 de la position dquilibre

Vibrations

amorties

c 0

0

Amortissement critique

= 1

u t

d

M

H

Undamped

Vibrations

= 0

Undamped

Vibrations

0

Critical Damping

= 1

SDOF

Single Degree of Freedom

OscillatorFundamental vibration period

EI

MHT

32

3

1 =

: damping relative to critical damping

Elastic behaviour

Steel structures 1%

Concrete 2%Seismic behaviour

Cracks in RC

Friction in steel connections

partitions-structures=> all structures = 5%

k

MT 21 =


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dFmax=M.(T1)

d

MH

dg(t)

Consider 1 SDOF oscillator having a fundamental period T1Soil move dg(t) => CM Center of Mass is displaced d(t)

Equation of dynamics M u(t) + c u(t) + ku(t) = - M dg(t)Resolved => d(t) and dmax

=> define: a forceF equivalent [gives same dmax]

=> write Fmax as being Fmax=M(T1)=> put in a graph(T1) as function of T1=> do it for a family of oscillators of periods T1i=> a curve: the elastic response spectrum (T1)

If you know T1 of a structure=> you know Fmax=> M, V, N => design

(T1)

T1(s)T1i

ag

i

0


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Code elastic response spectrum Se(T1) A zone average

Parameters

Soil & site : S TB TC TDDamping, if 5%

Importance class of building: I

I agRS

2,5 I agRS

Se(T)

T

( ) 55,05/10 +=

Se(T)

T(s)TB0 TC

Computed spectrum 1

Elastic accelerationspectrum "average"

Computed spectrum 2

Table 1. Importance classes for buildings and recommended values ofI(EN1998-1:2004).

Importance

class

Buildings I

I Buildings of minor importance for public safety,

for example agricultural buildings.

0,8

II Ordinary buildings not belonging in the other categories. 1,0

III Buildings whose seismic resistance is of importance

in view of the consequences associated with a collapse,

for example schools, assembly halls, cultural institutions.

1,2

IV Buildings whose integrity during earthquakes

is of vital importance for civil protection,

for example hospitals, fire stations, power plants, etc.

1,4


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Spectrum Type 1

Remote Earthquake

MS 5,5

Spectrum Type

Nearby Earthquake

MS < 5,5

Table 2. Eurocode 8 values of parameters S , TB , TC and TD defining the elastic

response spectra Type 1 and Type 2.

Type 1 Earthquake Type 2 Earthquake

Soil S TB(s) TC(s) TD(s) S TB(s) TC(s) TD(s)

A Rock or rock-like formation,

including at most 5 m of weaker

material at the surface.

1,0 0,15 0,4 2,0 1,0 0,05 0,25 1,2

B Deposits of very dense sand,

gravel, or very stiff clay, several tens

of metres in thickness,

gradual increase of mechanical

properties with depth.

1,2 0,15 0,5 2,0 1,35 0,05 0,25 1,2

C Deep deposits of dense or

medium-dense sand, gravel

or stiff clay with thickness

from several tens

to many hundreds of metres.

1,15 0,20 0,6 2,0 1,5 0,10 0,25 1,2

D Deposits of loose-to-medium

cohesionless soil or of

predominantly soft-to-firm

cohesive soil.

1,35 0,20 0,8 2,0 1,8 0,10 0,30 1,2

E A surface alluvium layer

of soil similar to C or D

with thickness varying between

about 5 m and 20 m,

underlain by stiffer material

1,4 0,15 0,5 2,0 1,6 0,05 0,25 1,2

S1 Deposits consisting, or

containing a layer at least

10 m thick, of soft

clays/silts with a high

plasticity index (PI > 40)

and high water content

Special studies

S2 Deposits of liquefiable soils,

of sensitive clays, or

any other soil profile

not included in types A E orS1

Special studies


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Code Design Response SpectrumSd(T1) Fmax = M Sd(T1)

Takes into account the capacity of the structure to dissipate energy

by plastic deformations: a behaviour factor q reduces the spectrum

The structure achieves the displacement ds imposed by the

earthquake accepting plastic deformationsin intended places called dissipatives zones

CD

EMFd=M.Sd(T)

Fe=M.Se(T)

Fd=Fe/q

d

ds=q.dyde=dy

Fd = Fe/ q

!!!

ds = q. de


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Summary

The design response spectrumSd(T1) Fmax=M Sd(T1)

Is a function of:

soil & site S TB TC TD

damping

capacity to dissipateenergy in plastic

deformations q

Sd(T1) est fonctionde la priode T1

T1=0,0 sH=5 m

concretebunker

T1=0,7 sH=17 m

T1=1,5 sH=5 m

T1=2,7 sH=100 m

0 1 2 3 4 5

1

2

3

4

T(s)

Soil A - q = 1,5

Soil C - q = 1,5

Soil C - q = 4

Sd(T)(m/s)


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B

Lecture du spectre

1

er

mode 2

e

mode 3

e

mode

D

Calcul des MNV M1bcorrespondant chaque

rponse modaleExemple : EEi = M base, i

E

Calcul de EE = Mbase =2

3

2

2

2

1 bbb MMM ++ (moyenne quadratique)

Sd (T)

Sd (T3)

Sd (T2)

Sd (T1)

Multimodal ResponseSeveral vibration modes

Masses associated to the different mode

Modal Superposition SRSS

Square Root of the Sum of the Squares

E: moments, shear, deformationsmodes: independants

problem: all positive !

CQC: idem, more general

Addition

2EiE EE =

global flexure storey in shear floor vibration


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Seismic massPeriods Tare functions of the masses M=> correct evaluation of real M necessary

=> seismic mass based on weight W =

E,i to estimate a likely value of service loads, computed as: can be


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Lateral force method of analysisIf structure response 1st vibration mode

the case for structure regular in elevation Approximate method: seismic base shearFb Advantage: signs of action effects are correct

( ) = mTSF 1db

Fb

Estimation of fundamental period T1 ?

Statistical relationships function of: structural type

height H

The lateral force method is the Approximate Method for Seismic Design

i,ki,Ej,k Q""G + m=

= 0,85


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.

Caution!

Units have to be consistent MassesM KgHeightH m

Acceleration Response SpectrumSd(T) m/s2

=>Forces N

Formulae for the estimation of the fundamental period T1 of a building.

Period T1 Reference structure

EI3MH2T

3

1 =

Exact formula for Single Degree of Freedom Oscillator.

Mass Mlumped at top of a vertical cantilever of heightH.Cantilever mass MB = 0

EI3

HM24,02T

3

B1 =


Vertical cantilever of heightHand of total mass MB

EI3

H)M24,0M(

2T

3

B

1

+

=


Mass Mlumped at top of a vertical cantilever of heightHand of total mass MB.

4/3t1 HCT =

H building height in mmeasured from foundation

or top of rigid basement.

Approximate Relationship (Eurocode 8).

Ct = 0,085 for moment resisting steel space framesCt = 0,075 for eccentrically braced steel frames

Ct = 0,050 for all other structures

d2T =1 Approximate Relationship (Eurocode 8).

d: elastic horizontal displacement of top of buildingin m under gravity loads applied horizontally.

( ) = mTSF 1db


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jj

iibi

ms

msFF

=

jj

iibi

mzmzFF

=

j

ibi

z

zFF

=

Storey Forces

If deformes shape simplified into linear

If storey masses are all equal

s: deformed shape

W4

W3

W2

W1

F4

h2

F3

F2

F1

Fb

z2


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Caracterisation of Structures

Specific to Seismic Design


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CD

EMFd=M.Sd(T)

Fe=M.Se(T)

Fd=Fe/q

d

ds=q.dyde=dy

Energy dissipation capacity

To increase safety and reduce costs:

dissipate permanently energy in plastic deformationsm u(t)u(t) + c [u(t)]2dt + F(u) u(t)dt = - m dg(t) u(t) dt

Ekintic + Eviscous + Edeformation = Etotal input

Edeformation = EEL +EEP

Fd = Fe/ q

!!!

ds = q. de

M


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Ductilit = max / y

MEP = 1/2 MELenergy of elastic deformation (triangle) at dmax : EELenergy absorbed plastically after one cycle (+ dmax, - dmax) : EEP = 2EEL

after 4 cycles (+ dmax, - dmax ): EEP = 8EELCondition: 2

Lower cost, because: MRd MEL in the non-dissipative structure

MRd = 0,5 MEL = MEP in the dissipative structureCost reduction : max 50%

Definition ofq ? Based on ductility: q = max / y = 2

dmax

H

A

max MEP

MEL

EP

EL

MA

max+ max

y

Currently: = 6 to 8 in steel elements


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2 // Universe

The real behaviour

The elasto-plastic behaviour

The designer sees onlythe elastic part of

the real behaviour


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Values of the behaviour factor q ?Reflect the potential for energy dissipation of the structural type

the design

Code values ofq: safe side estimates

In reality q varies depending on: seismicity redundancy influence of design conditions other than EQ

F

P

F

P

F

P

4 plastic hinges 1 plastic diagonal no plastic mechanism

q=6 q=4 q=1 (1,5)

* Stability of a K bracing depends on slender diagonal in compression, which fails in a brittle way.


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With a given local plastic rotation capacity ,a global mechanisme allows greater ultimate displacements

Eurocode 8 Definition of Ductility Classes or DC for design


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Ductility classes

The designer can choose to design structures: as usual, non dissipative

dissipative

Modern seismic design codes leave the choice open

define several Ductility Classes.

Eurocode 8: 3 Ductility Classes DCL Low Ductility, non dissipative structures

DCM Medium Ductility

DCH High DuctilityDCL: highest seismic design forces, only usual static design checks (Eurocode 3)

DCH: smallest possible design earthquake actions & seismic action effects

bending moments, etc are reduced in comparison to DCL[note: not displacements]

But other requirements to fulfil:

Example: class of sections related to q for dissipative elements

DCM: less requiring


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Behaviour factors q (maximum values).

Ductility ClassSTRUCTURAL TYPE

DCL DCM DCHMoment resisting frames (MRF) 1,5 (2*) 4 5u/1

Concentric diagonal bracings

Concentric V-bracings

1,5 (2*) 4

2

4

2,5

Eccentric bracings 1,5 (2*) 4 5u/1

Inverted pendulum 1,5 (2*) 2 2u/1MRF with concentric bracing 1,5 (2*) 4 4u/1

MRF with unconnected concrete or masonry

infills in contact with the frame

MRF with infills isolated from the frame

1,5 (2*) 2

4

2

5u/1

*the National Annex can allow q = 2 in class DCL

Behaviour factors in Eurocode 8.


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Conditions required for a very dissipative behaviour

Reliable dissipative zones

Local plastic deformation capacity with constant resistance arelimited. Ex: plastic rotation of a classe A section: 50 mrad

Numerous dissipative zones

To avoid excessive local plastic deformation as a result

of a concentration of deformations in few points

Dissipative zones in a decided scheme refering to a global plastic mechanism

Because it is not thinkable to have all zones of the structure with ideal

characterisics for plastic deformations

No to partial failure mechanism Fig: concept b concept a

concept a = du/ h1 tage

concept b

= du

/ h4 tages

concept b = concept a/4


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Design aiming at a global plastic failure mechanism

1. Define the objective: a global mechanism

2. Pay a price at local zones: criteria for local ductility

3. Pay a price for the mechanism to be global:

criteria to form numerous dissipative zones


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2. Pay a price at local zones:

Capacity Design

of resistances of all elements other than the plastic zones

overstrength of all zones adjacent to the intended plastic zones

=> stronger, elastic and stable

during the plastic deformations of the fuse zone

ductile link i other links j

fuse fragile

Computed action effect Edi = P Edj = P

Required resistance R di > Edi (Rdi / Edi) Edj >> P !!!

Capacity design Examples


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Capacity design . Examples.Beam in portal frames: dimensions results from several criteria:

resistance under 1,35 G + 1,5 Q Deflection limits Earthquake Mpl,Rd MEd

Checks of beam and connection adjacent to the plastic hinge in bending

For shear resistance

VEd,G due to non seismic actions

VEd,Mdue to Mpl,Rd,A et Mpl,Rd,Bsigns at ends A et B

VEd, can be >> VEd, analysis

Design bending moments

of connections

> MEd

, , , , ,( ) /Ed M pl Rd A pl Rd BV M M L= +, ,Ed Ed G Ed M

V V V= +

Non-seismic design Seismic design: t2 >> t1 a2 > a1

+1,1 ovMpl,Rd,beam

-1,1 ovMpl,Rd,beam


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3. Pay a price for the mechanism to be global: Example

Criteria to form numerous dissipative zones

Moment resisting frames:

rule of hierarchy of formation of plastic hinges

RbRc

MM 3,1

MRb beams resisting moments = fusesMRc columns resisting moments

At each node

S h i d i i


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Synthesis on design options

VEd

dSDe (T)

DCL

DCM

DCH

a)

b)

c)

Behaviour of frames with equal period Tsubmitted to pushover up to a design displacementSDe(T) :

a) Elastic design DCL

b) Medium ductility project DCM

c) Very dissipative project DCH

[Note : the design displacement is approximately independent ofq ]

Base Shear

Seismic action effects

are smaller in DCH.

This includes Base Shear

and Base Moments.


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General rules for the design of buildings in

Eurocode 8.

EN1998-1:2004 . Chapter 4


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General rules for the design of buildings in Eurocode 8.

EN1998-1:2004 . Chapter 4

Seismic action - definition of objective

No collapse Design Earthquake ULS

No damage Service Earthquake SLS

Regularity in plan geometrical criteria

in elevation criteria on stiffness distribution

torsional stiffness

Methods of analysis and regularity


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Linear Methods of Analysis.

Modal response using a design spectrum.

Standard method applicable to all types of buildings

regular or irregular in plan and/or elevation

Lateral force method

A static analysis for regular structures

which respond essentially in one single mode of vibration.

Can be applied to planar models of the structure depending on regularity criteria.

Structural regularity and permissible simplifications in seismic analysis in Eurocode 8.

Regularity Permissible Simplification Behaviour factor

Plan Elevation Model Linear-elastic

Analysis

q

YesYes

Limited

No

No

YesNo

Yes

Yes

No

2 planar2 planar

2 planar

1 model 3D

1 model 3D

Lateral forceModal response

Lateral force

Lateral force

Modal response

Reference valueReference value /1,2

Reference value

Reference value

Reference value /1,2 &

reduced u/1


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Non Linear Methods of analysis.

Pushover analysis

Non-linear static analysis carried out under constant gravity loads G

and monotonically increasing horizontal loads

Applied to verify or revise u/1 to estimate the expected plastic mechanisms

the distribution of damage

to assess the structural performance of existing/retrofitted buildings

Non-linear time-history analysis

Dynamic analysis direct numerical integration of differential equations of motion

Earthquake action: accelerograms (min 3)

For research and code background studies

Torsion


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3 reasons: eccentricity at every storey between the storeys resultant force

passing at mass centre CM and centre of rigidity CR.

rotation aspects of ground movement for very long structures uncertainty on exact location ofCM

=> accidental eccentricity = 5% of building length ppd to earthquake direction

CR: point where an applied force generates only a translation parallel to that force

Effects of torsion are determined based on CM-CR distance + the accidental eccentricity in + or -

Irregular structures

Computation of torsional effects from non coincidence CM/ CR => a 3-D model

Effects of accidental eccentricity: found applying at every level a torquecomputed as: (storey force) x (CM-CR distance)

Structures symmetrical in plan with CM= CR

Effects of accidental eccentricity approximated by amplifyingthe translational action effects by

x distance in plan between the seismic resisting structure considered and CM

Le the distance between two extreme seismic resisting structures

Symmetrical buildings with peripheral resisting structures: 1,3

e

6,01L

x+=

Di l t i di i ti t t


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Displacements in dissipative structures.

ds = q de

Combination of the effects of the components of the seismic action

Horizontal components of the seismic action are taken as acting simultaneously

Action effects evaluated separately

Maximum of each action effect due to the two horizontal componentsestimated by: SRSS of the action effect due to each horizontal component

Alternative

a) EEdx "+" 0,30EEdyb) 0,30EEdx "+" EEdy"+" implies "to be combined with'';

EEdx: action effects due to seismic action along horizontal axis xEEdy: due to seismic action along horizontal axis y

Behaviour factorq may be different in x and y

Vertical component of the seismic action


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Vertical component of the seismic action

If avg > 0,25 g the vertical component of the seismic action is taken into account: for horizontal structural members span 20 m

for horizontal cantilever 5 m

for beams supporting columns

in base-isolated structures

Analysis for determining the effects of the vertical componentmay be based on a partial model of the structure

If the horizontal components are also relevant for these elements: SRSSOr combinations:

a) EEdx ''+" 0,30 EEdy "+" 0,30 EEdzb) 0,30 EEdx "+" EEdy "+" 0,30 EEdzc) 0,30 EEdx "+" 0,30 EEdy "+" EEdz

Ultimate limit state No collapse requirement


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Ultimate limit state. No-collapse requirement .Conditions on resistance, ductility, equilibrium, foundation stability and seismic joints

Resistance condition

Rd is the design resistance of the element

Ed is the design value of the action effect due to the seismic design situation:

Ed = Gk,j + P + 2i.Qki + 1AEdIf necessary, second order effects are taken into account in the value ofEd

Limitation of second order effects mitigation of soft storey

2nd order momentsPtot

dr 1st order moments V

toth at every storey

Ptot total G at and above the storey considering seismic mass h: storey height

dr : drift based on ds = q de Vtot : total seismic shear at considered storey

0,1 => P- effects negligible

0,1 < 0,2 => multiply action effects

by 1/(1 - )Always: 0,3

dd RE

N

V

N

V

Ptot

dr= q.dre

h

Vtot


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Global and local ductility condition

Capacity design principle

Resistance of horizontal diaphragmsDiaphragms and bracings in horizontal planes able to transmit with overstrength

the effects of seismic action to the lateral load-resisting systems

resistance verifications:seismic action effects in the diaphragm from the analysis x d = 1,1

Resistance of foundations

Rd overstrength factor EF,G due to the non-seismic actionsEF,E : action effect from the analysis of the design seismic action

: value ofRdi/Edi of the dissipative element i of the structurewith highest influence on effectEF under consideration;

Rdi design resistance of the zone or element i;Edi design value of the action effect

Check always: Rdi/Edi q

RbRc 3,1 MM

EF,RdGF,Fd EEE +=


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Seismic joint condition

SRSS of the maximum horizontal displacements of the two buildings x 0,7 if same owner

Damage limitation

Non-structural elements of brittle materials attached to the structure: Ductile non-structural elements:

Non-structural elements not to interfering with structural deformationsOr no non-structural elements:

dr design interstorey drifth storey height;

reduction factor for lower return period of the seismic actionassociated with the damage limitation requirement.

Recommended : = 0,4 for importance classes III and IV = 0,5 for importance classes I and II

hd 005,0r

hd 0075,0r

hd 010,0r


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Architecture of Earthquake Resistant Buildings

Architecture of Earthquake Resistant Buildings.


50/55

Architecture of Earthquake Resistant Buildings.

Basic features of an earthquake resistant building.Buildings are boxes

Subjected to earthquakes they work in the way sketched

Storey forces are attracted

by the diaphragms

which distribute them

to the vertical resisting structures

which transfer the forces

down to the foundations.

P i t t S d t t


51/55

Primary structure Secondary structure.The vertical load resisting structure may comprise:

a main or primary system designed to carry the total earthquake effects

a secondary structure designed to carry only gravity loadsThe contribution to lateral stiffness and resistance of the secondary structure

should not exceed 15% of that of the primary structure.The members of the secondary structure and their connections must be able:

to accommodate the displacements of the primary structure

responding to an earthquake

to remain capable of carrying the gravity loading

Secondarystructure

Primarystructure

Conceptual design.


52/55

p g

A good conceptual design enables the development of a structural systemto resist earthquakes at low additional costs in comparison to a non-seismic design.

Conceptual design: apply to the primary resisting system

leave freedom in the design of the secondary structure

Principles of conceptual design of earthquake resistant structures

Structural simplicity

Clear and direct paths for the transmission of the seismic forces

Modelling, analysis, designing, detailing and construction of simple structures

less uncertainties

more reliable prediction of seismic behaviour

Uniformity in plan

Even distribution of structural elements


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Even distribution of structural elements

Short and direct transmission of inertia forces created by the masses of the building

Symmetric layout of vertical earthquake resistant structures: achieves uniformity

eliminates large eccentricities between mass and stiffness

minimises the torsional moments

Uniformity may be realised by subdividing a building

by seismic joints into dynamically independent units

Favourable in-plan shapes

action

reactiontorsion

Don't do Do

Symmetrical in-plan shapes reduce torsion.

Structural systems distributed close to the periphery are the most effective at resisting torsion.

joints

Uniformity over the height

Avoids the occurrence of sensitive zones: concentrations of stress


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Avoids the occurrence of sensitive zones: concentrations of stress

large ductility demands

possible premature collapseRequires that non structural elements do not interfere with the structural elements

to localise the plastic deformations

like in soft storey mechanism

infills

soft storey

plastic

hinges

Redundancy

Evenly distributed

structural elements

increases redundancy

facilitateredistribution of action effects

energy dissipation

spread reactions at the foundations

action

d

reactions

Small lever arm of reactions

Don't do

action

d

reactions

Great lever arm of reactions

Do

Small lever arm of reactions Great lever arm of reactions

Torsional resistance and stiffness


55/55

Limit torsional movements.

Structural systems distributed close to the periphery are the most effective.

Diaphragms

Distribute storey seismic force in the earthquake resisting frames.

Particular care with:

very elongated in-plan shape

large floor openings, especially those located near vertical structural elements.

Foundations

Ensure that the whole building is subjected to a uniform seismic excitationDesign should reduce problems with differential settlement under seismic action.

=>Rigid, box-type foundation:

foundation slab+ cover slab

Footings should be tied togetherby a foundation slab or tie-beams

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Norway Steel 1