summary eq engg

7182019 Summary Eq Engg

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Seismic design principlesBasic principles of conceptual design ndash

1 Structural simplicity Principle Clear and direct load paths so that seismicforces can be transferred from top to bottom Direct load paths from top tobottom can only be assured by regularly arranged structural elements

2

Uniformity symmetry and redundancy Principle The greater the departure from regularity or symmetry the greater theperil to suffer serious damage

During the design process care should be taken with respect to the following rarrsymmetrical arrangement of mass and stiffness in ground plan and elevation rarr

irregularities in the buildings layout will result in eccentricities of mass andstiffness center which will lead to unfavorable torsional effects that are coupledwith large displacements rarr interruptions of lateral stiffness over the buildingsheight will cause weak locations in the structural system

Redundancy means that the building should have more than one single loadpath in order to transfer the forces from top to bottom In case that one load pathis affected by the shaking another load path is taking over Redundant systemsare generally less simple

REGULARITY OF STRUCTURECriteria for structural regularity

A Regularity in plan

Figure 22 opposes the most common regular with irregular plan shapesHowever a regular plan shape cannot always be realized Consequently

measures have to be taken in order to provide that an irregular plan shapelsquobehavesrsquo like a regular one This can be either done by - subdividing the building into separate independent units using seismic joints

(Figure 23)- strengthening those parts of the building that experience largest

displacementsforces (Figure 23)





Plan regularity-symmetrical arrangement of lateral stiffness and mass distributions

- two orthogonal axes ie rectangular buildings with two principal buildingaxes that are orthogonally arranged to each other

- plan configuration is compact able to be delimited by a polygonal convex line(compare withFigure 215)

- if set-backs exist plan can be estimated as compact if the differential area isless than 5 of thetotal floor area (compare with Figure 216 ie A 1 lt 005 A tot and A 2lt 005

A tot )

- slenderness ie ratio between length L max and width L minof the building

shall be smaller than 402

(Figure 217)

B Regularity in elevation





Figure 26 compares favorable with unfavorable building solutions withrespect to elevation shape

In general the following principles shall be considered -avoid sensitive zones where concentrations of stress or large ductility

demands increase damage susceptibility

- provide either constant or continuously decreasing stiffness with height- avoid buildings with too slender (chimney-style) or inverted pendulumshape (heavy mass on top)-arrange continuous bracings over total building height- arrange seismic (movement) joints of sufficient thickness1 in order todecouple separate building segments from each other



Bi-directional resistance and stiffnessBuildings which are designed to withstand these loads shall possess bi-directional resistance and stiffnessA special case consists in slender plan shapes Please consider that both generalbuilding axes usually possess equal resistance A lsquoweakrsquo axis (as sometimes

reported) does not exist (compare with Figure 29)

Torsional resistance and stiffness Torsional effects generally occur if the center of mass and the center of stiffnessare located in a certain distance (ie eccentricity e) to each other This is caused by symmetricalregular plan shapes where the structuralelements are irregularly distributed (Figure 210)



- Unsymmetricalirregular plan shapes where masses and stiffnesses aredisorderly arranged

It should be also considered that slender plan shapes tend more easily totorsional effects (Figure 211) It is therefore advisable to limit the length-to-width

ratio (slenderness) of the plan

Diaphragmatic behavior at storey level

The main role of floor diaphragms is to collect and transmit the inertia forces They further have to ensure that all vertical elements act together (aresynchronous) in resisting the seismic forces and should thus have sufficient in-plane stiffness In order to ensure this large openings or interruptions should

be avoided (Figure 212) Floor diaphragms act as horizontal ties which preventexcessive relative deformations between the vertical elements



Adequate foundation The foundation shall ensure that the whole building is subjected to a uniform

seismic excitation If the superstructure is likely to differ in width and stiffnessa rigid box-type or cellular foundation should be chosen (Figure 213)

Primary and secondary seismic membersWhile primary elements contribute to the seismic resistance secondary elementsresist gravity loads only and have no contribution to the structurersquos seismicresistance (Figure 214) According to EC8 the contribution of all secondaryseismic members should not exceed 15 of that of all primary seismic membersin order to avoid unintentional stiffening by secondary elements (higherfrequencies and inertial loads) as well as to provide flexible joints



EARTHQUAKE-RESISTANT DESIGN CONCEPTS

1 Seismic Design Categories

Factors that affect a structurersquos seismic risk include

bull The intensity of ground shaking and other earthquake effects the structureis likely to experience and

bull The structurersquos use including consideration of the number of people who wouldbe affected by the structurersquos failure and the need to use the struc ture for its

intended purpose after an earthquake





The intensity of earthquake shaking and other effects used to assign structures

to a Seismic Design Category is determined using the national seismic maps

In general sites that have deep deposits of soft soils will have larger values of

the design acceleration parameters than sites with shallow deposits of firm soils

or near-surface rock

2 Site Class

Site soil conditions are important in determining Seismic Design Category Hard

competent rock materials efficiently transmit shaking with high-frequency

(short-period) energy content but tend to attenuate (filter out) shaking with low-

frequency (long-period) energy content Deep deposits of soft soil transmit high-

frequency motion less efficiently but tend to amplify the low-frequency energy

content

3 Design Ground Motion

In order to determine the Seismic Design Category for a structure it is first

necessary to determine the design ground motion which is one of the primary

factors used to determine the required seismic resistance (strength) of structures

and supported nonstructural components

4 Structural System Selection

selecting an appropriate seismic-force-resisting system (SFRS) the seismic-

force-resisting systems for building structures and nonbuilding structures with

structural systems like buildings are categorized by construction material (eg

concrete masonry steel or wood) type of system (bearing wall braced frame

moment frame dual or cantilever column) and level of seismic detailing (special

intermediate ordinary or not detailed for seismic resistance)

three design coefficients used to determine the required strength and

stiffness of a structurersquos seismic-force-resisting system

a R is a response modification factor that accounts for the ability of some

seismic-force-resisting systems to respond to earthquake shaking in aductile manner without loss of load-carrying capacity R values generally

range from 1 for systems that have no ability to provide ductile response

to 8 for systems that are capable of highly ductile response The R factor

is used to reduce the required design strength for a structure



5 Configuration and Regularity

structures have nonuniform distribution of strength or stiffness and discontinu-

ous structural systems are termed ldquoirregular structures

two basic categories of irregularity horizontal or plan irregularity and vertical

irregularity

a Horizontal irregularities include

Torsional irregularity- when the distribution of vertical elements of

the seismic-force-resisting system within a story including braced

frames moment frames and walls such that when the building is

pushed to the side by earthquake forces it will tend to twist as well

as deflect horizontally

Extreme torsional irregularity-twisting that occurs as the structure

is displaced laterally becomes very large

Re-entrant corner irregularity

Diaphragm discontinuity irregularity ndash This occurs when a

structurersquos floor or roof has a large open area

Out-of-plane offset irregularity-when braced frames or shear walls

are not aligned vertically from story to story

Nonparallel systems irregularity

Vertical irregularities include the following

Stiffness soft-story irregularity ndash This occurs when the stiffness of

one story is substantially less than that of the stories above

Extreme stiffness soft-story irregularity

Weightmass irregularity ndash This exists when the weight of the

structure at one level is substantially in excess of that at the levels

immediately above or below it

In-plane discontinuity irregularity-when walls or braced frames do

not align vertically within a given line of framing

Weak-story irregularity ndash This occurs when the strength of the

walls or frames that provide lateral resistance in one story is

substantially less than that of the walls or frames in the adjacent

stories

Extreme weak-story irregularity



Seismic action and performance requirements

EC8 provides for a two-level seismic design

Protection of life under a rare seismic action by prevention of collapse of the

structure

Reduction of property loss due to a frequent even

For structures of ordinary importance the recommendation of EC8 is for bull A 10

exceedance probability in 50 years (ldquodesignrdquo) seismic action for collapse

prevention (mean return period 475 years) The ldquodesignrdquo seismic action for

structures of ordinary importance over rock is termed ldquoreferencerdquo seismic action

bull A 10 in 10 years ldquoserviceabilityrdquo action for damage limitation (mean return

period 95 years)

Enhanced performance of essential or large occupancy facilities is achieved not

by upgrading the performance level for given earthquake level as US codes do

but by modifying the hazard level (the mean return period) for which collapseprevention or damage limitation is pursued

Behaviour factor

The majority of structures designed with EC8 are expected to be designed for

ldquoenergy dissipationrdquo Medium (M) and High (H) ductility DC M and H buildings

are entitled to values of the force reduction or behaviour factor q well above the

minimum value of q=15 attributed to overstrength

Availability of the global energy dissipation and ductility capacity needed for

values of q (much) higher than 15 is ensured throughbull Measures to control the inelastic response mechanism so that concentration

of inelastic deformations in a part of the structure (mainly a soft storey

mechanism) and brittle failure modes are avoided

bull Detailing of the plastic hinge regions for inelastic deformations expected to

develop there under the design seismic action

Concentration of inelastic deformations and soft storey mechanisms are avoided

by configuring and dimensioning the lateral-force resisting system so that vertical

members( column walls) remain practically straight ndash ie elastic ndash above their

base and designed to be stronger than the beams

Analysis procedures and modelsEC8 provides the following analysis options for design and for evaluation of the

performance of buildings

bull Linear static (termed ldquolateral forcerdquo method)

bull Linear modal response spectrum analysis

bull Nonlinear static analysis (ldquopushoverrdquo)



bull Nonlinear dynamic (response time-history) Linear time-history analysis is not

explicitly mentioned In US codes linear static analysis is the reference in EC8 the linear

modal response spectrum method is the standard procedure applicable

to all types of buildings

The lateral force procedure(US code) may be applied if the effects of

higher modes are not significant ie only when

bull In both horizontal directions the fundamental period is less than 2sec

and 4 times the transition period Tc between the constant-acceleration

and the constant-velocity regions of the spectrum

bull There are no significant irregularities in elevation

In the response spectrum analysis(EC8 code) modal contributions are

combined by rigorous application of the SRSS or CQC rules ie at the

level of the final seismic action effects of interest (internal forces

displacements etc)

Behaviour factor q for reduction of elastic

forces

For structures designed for energy dissipation the behaviour factor q

by which the elastic spectrum for use in linear analysis is reduced is

linked directly or indirectly to the ductility and deformation demands

the type of lateral-force-resisting-system and on the ductility classselected for the design System overstrength is explicitly included in the value of the q-factor

through the ratio αuα1 (denoted here for convenience αR) of the seismic

action that causes development of a full plastic mechanism (ie for fully

yielded structure) q=3αR for DC M and 45αR for DC H

Soil-structure interaction

Two types of SSI are commonly referred to in the literature

1

ldquoKinematicrdquo interaction is caused by inability of a foundation to follow

ground motion due to greater foundation stiffness in comparison with

ground stiffness

2 ldquoInertialrdquo interaction is caused by the existence of structural and

foundation masses Seismic energy transferred into a structure is

dissipated by material damping and radiation back into ground

causing superposition of incoming and outgoing ground waves As a



result the ground motion around a foundation can be attenuated or

amplified depending on a variety of factors

The most important factor in determining the response is the ratio between the

fundamental period of a foundation and the fundamental period of the adjacent ground

in the free-field The ratio of unity indicates resonance condition between foundation

and its adjacent ground which is to be avoided

Section 6 of EN1998-12004 states that the effects of dynamic soil-structure interaction

shall be taken into account in the case of

bull structures where Pndash effects play a significant role

structures with massive or deep seated foundations

bull slender tall structures

bull structures supported on very soft soils with average shear wave velocity less than 100

ms

bull The effects of soil-structure interaction on piles shall be assessed

EN1998-12004 is the only code which recognizes the importance of kinematic

interaction for piled foundations as it is stated in clause 542(6) of EN1998-52004

Bending moments developing due to kinematic interaction shall be computed only

when two or more of the following conditions occur simultaneously

bull the subsoil profile is of class C (soft soil) or worse and contains consecutive layers

with sharply differing stiffness

bull the zone is of moderate or high seismicity S ag gt 01 g

bull the supported structure is of importance category III or IV

SSI is motivated not only by the need to satisfy geotechnical requirement related to

foundation response to earthquake loading (eg bearing capacity assessment

settlement calculation) but also by the necessity of computing the ldquoeffectiverdquo earthquake

excitation to a structure with respect to the free-field ground motion (which is also called

Foundation Input Motion or FIM)

There are three primary categories of soil-structure interaction (SSI) effects These

include bull filtering of the ground motions transmitted to the structure (kinematic effects)bull introduction of flexibility to the soil-foundation system (flexible foundation effects) bull

dissipation of energy from the soil-structure system through radiation and hysteretic

soil damping (foundation damping effects)



Behavior factor Vs Design Spectrum relationship

The Behavior factor is a reduction factor of the design response spectrum in

relation to the elastic response spectrum You can edit its value after selecting

the Design spectrum option Note The behavior factor q reduces

the design spectrum this way it also decreasesseismic loads and the resulting

internal forces

The starting point is an elastic response spectrum which is reduced with factors

that take into consideration the ability of the structure to absorb seismic energy

through rigid deformations The design acceleration spectrum comes from the

elasticity spectrum with a depreciation of 5 by dividing the spectral

accelerations by the behavior factor q

The elastic acceleration spectrum with a damping of 5 of Eurocode 8is given

graphically below It contains an area of fixed spectral acceleration between the

periods Τ Β and Τ C with a value 25 times the maximum soil acceleration agS that

is followed from an area of fixed spectral velocity between the periods Τ C and Τ D

where the spectral acceleration is proportional to 1Τ and an area of fixed

spectral displacement where the spectral acceleration is proportional to 1Τ 2



Figure 2 Elastic spectrum EC in the horizontal direction for a damping of 5

(Fardis 2009a)

In the areas of fixed spectral acceleration velocity and displacement the design

spectrum originates from an elastic response with a 5 damping divided by q

Exceptionally the increasing part for a vibration period from Τ up to ΤleΤ Β comes

from the linear interpolation between (α) the maximum ground acceleration Sag

divided by 15 that expresses overstrength compared with the design capacity

and the fixed design acceleration for Τ=0 and (β) 25 agq for Τ=Τ Β Moreover

there is a lower limit in the design spectral acceleration equal to the 20 of the

maximum acceleration on the rock ag (Fardis 2009a)

dependence of the importance factor on the mean return period chosen fordesign

Buildings in EN 1998-1 are classified in 4 importance classes depending ono the consequences of collapse for human lifeo their importance for public safety and civil protection in the immediate post-earthquake period ando the social and economic consequences of collapse







INSTRUMENTED BUILDING USING RECORDED

RESPONSE MOTION



Conclusion

Drift is defined as the lateral displacement Storey drift is the drift of one level of

a multistorey building relative to the level below Interstory drift is the difference

between the roof and floor displacements of any given story as the building sways

during the earthquake normalized by the story height For example for a 10-foot high story an interstory drift of 010 indicates that the roof is displaced one

foot in relation to the floor below

The greater the drift the greater the likelihood of damage Peak interstory drift

values larger than 006 indicate severe damage while values larger than 0025

indicate that the damage could be serious enough to pose a serious threat to

human safety Values in excess of 010 indicate probable building collapse

































A tot )



(Figure 217)













































2 Site Class






content


























irregularity



























stories







structure







period 95 years)




Behaviour factor



































displacements etc)


forces










1



ground stiffness



















ms


























internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion













































































2 Site Class






content


























irregularity



























stories







structure







period 95 years)




Behaviour factor



































displacements etc)


forces










1



ground stiffness



















ms


























internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion








































irregularity



























stories







structure







period 95 years)




Behaviour factor



































displacements etc)


forces










1



ground stiffness



















ms


























internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion







































structure







period 95 years)




Behaviour factor



































displacements etc)


forces










1



ground stiffness



















ms


























internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion

















































displacements etc)


forces










1



ground stiffness



















ms


























internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion
















































ms


























internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion









































internal forces















(Fardis 2009a)


















RESPONSE MOTION



Conclusion





































(Fardis 2009a)


















RESPONSE MOTION



Conclusion









































RESPONSE MOTION



Conclusion




































Conclusion


































summary eq engg

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