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STRUCTURAL PERFORMANCE DURING EARTHQUAKES

Chapter 2

STRUCTURAL PERFORMANCE DURINGEARTHQUAKES

2.1 INTRODUCTIONEarthquakes are natural hazards underwhich disasters are mainly caused by dam-age to or collapse of buildings and otherman-made structures. Experience hasshown that for new constructions, estab-lishing earthquake resistant regulationsand their implementation is the criticalsafeguard against earthquake-induceddamage. As regards existing structures, itis necessary to evaluate and strengthenthem based on evaluation criteria before anearthquake.

Earthquake damage depends on manyparameters, including intensity, durationand frequency content of ground motion,geologic and soil condition, quality of con-struction, etc. Building design must be suchas to ensure that the building has adequatestrength, high ductility, and will remain asone unit, even while subjected to very largedeformation.

Sociologic factors are also important,such as density of population, time of dayof the earthquake occurrence and commu-

nity preparedness for the possibility of suchan event.

Up to now we can do little to diminishdirect earthquake effects. However we cando much to reduce risks and thereby reducedisasters provided we design and build orstrengthen the buildings so as to minimizethe losses based on the knowledge of theearthquake performance of different build-ing types during an earthquake.

Observation of structural performanceof buildings during an earthquake canclearly identify the strong and weak aspectsof the design, as well as the desirable quali-ties of materials and techniques of construc-tion, and site selection. The study of dam-age therefore provides an important step inthe evolution of strengthening measures fordifferent types of buildings.

This Chapter discusses earthquake per-formance of structures during earthquakeintensity, ground shaking effects on struc-tures, site condition effects on buildingdamage, other factors affecting damage,

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failure mechanisms of structures, earth-quake damage and damage categories.

Typical patterns of damage for specifictypes of construction are discussed in therespective chapters.

2.2 EARTHQUAKE EFFECTSThere are four basic causes of earthquake-induced damage: ground shaking, groundfailure, tsunamis and fire.

2.2.1 Ground shakingThe principal cause of earthquake-induceddamage is ground shaking. As the earthvibrates, all buildings on the ground sur-face will respond to that vibration in vary-ing degrees. Earthquake inducedaccelerations, velocities and displacementscan damage or destroy a building unless ithas been designed and constructed orstrengthened to be earthquake resistant.Therefore, the effect of ground shaking onbuildings is a principal area of considera-tion in the design of earthquake resistantbuildings. Seismic design loads are ex-tremely difficult to determine due to the ran-dom nature of earthquake motions. How-ever, experiences from past strong earth-quakes have shown that reasonable andprudent practices can keep a building safeduring an earthquake.

2.2.2 Ground failureEarthquake-induced ground failure hasbeen observed in the form of ground rup-ture along the fault zone, landslides, settle-ment and soil liquefaction.

Ground rupture along a fault zone maybe very limited or may extend over hun-dreds of kilometers. Ground displacementalong the fault may be horizontal, vertical

or both, and can be measured in centimetersor even metres. Obviously, a building di-rectly astride such a rupture will be severelydamaged or collapsed.

While landslide can destroy a building,the settlement may only damage the build-ing.

Soil liquefaction can occur in low den-sity saturated sands of relatively uniformsize. The phenomenon of liquefaction isparticularly important for dams, bridges,underground pipelines, and buildingsstanding on such ground.

2.2.3 TsunamisTsunamis or seismic sea waves are gener-ally produced by a sudden movement ofthe ocean floor. As the water waves ap-proach land, their velocity decreases andtheir height increases from 5 to 8 m, or evenmore. Obviously, tsunamis can be devas-tating for buildings built in coastal areas.

2.2.4 FireWhen the fire following an earthquakestarts, it becomes difficult to extinguish it,since a strong earthquake is accompaniedby the loss of water supply and traffic jams.Therefore, the earthquake damage increaseswith the earthquake-induced fire in addi-tion to the damage to buildings directly dueto earthquakes. In the case of the 1923 Kantoearthquake 50% of Tokyo and 70% of thetotal number of houses were burnt and morethan 100,000 people were killed by the fire.

2.3 GROUND SHAKING EFFECTON STRUCTURES2.3.1 Inertia forcesBuildings are fixed to the ground as shownin Fig 2.1(a). As the base of a building moves

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the superstructure including its contentstends to shake and vibrate from the posi-tion of rest, in a very irregular manner dueto the inertia of the masses.

When the base of the building suddenlymoves to the right, the building moves tothe left relative the base, Fig 2.1(b), as if itwas being pushed to the left by an unseenforce which we call �Inertia Force�. Actu-ally, there is no push at all but, because ofits mass, the building resists any motion.The process is much more complex becausethe ground moves simultaneously in threemutually perpendicular directions duringan earthquake as shown in Fig 2.1 (b), (c),and (d).

2.3.2 Seismic loadThe resultant lateral force or seismic loadis represented by the force F as shown in

Fig 2.1(e). The force F is distinctly differentfrom the dead, live, snow, wind, and im-pact loads. The horizontal ground motionaction is similar to the effect of a horizontalforce acting on the building, hence the term�Seismic Load�. As the base of the build-ing moves in an extremely complicatedmanner, inertia forces are created through-out the mass of the building and its con-tents. It is these reversible forces that causethe building to move and sustain damageor collapse.

Additional vertical load effect is causedon beams and columns due to vertical vi-brations. Being reversible, at certain in-stants of time the effective load is increased,at others it is decreased.

The earthquake loads are dynamic andimpossible to predict precisely in advance,

Fig 2.1 Seismic vibrations of a building and resultant earthquake force

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since every earthquake exhibits differentcharacteristics. The following equivalentminimum total lateral force is, used for seis-mic design:

F=S.Fs.I.C.W

Where S, Fs, I, C and W are the factorsaffecting seismic load, which will be ex-plained in the following section.

2.3.3 Factors affecting seismic loadThe earthquake zone factor S dependsupon the ground intensity of the earth-quake. The value of S usually is plotted onmaps in terms of seismic intensity isolinesor maximum acceleration isolines. Obvi-ously, the higher the intensity or accelera-tion, the larger will be the seismic force.

The soil-foundation factor Fs dependsupon the ratio of fundamental elastic pe-riod of vibration of a building in the direc-tion under consideration and the charac-teristic site period. Therefore, Fs is a numeri-cal coefficient for site-building resonance.

The occupancy importance or hazardfactor I depends upon the usage of thebuilding. The higher the importance orlarger the hazard caused by the failure ofthe building, the greater the value of thefactor I.

The C is a factor depending on the stiff-ness and damping of the structure. Largerthe stiffness for given mass, shorter the fun-damental period of vibration of the struc-ture and larger the value of C. Damping isthe energy dissipation property of the build-ing; larger the damping, smaller the valueof C.

The W is the total weight of the super-structure of a building including its con-tents. The inertia forces are proportional tothe mass of the building and only that partof the loading action that possesses masswill give rise to seismic force on the build-ing. Therefore, the lighter the material, thesmaller will be the seismic force.

2.3.4 Nature of seismic stressesThe horizontal seismic forces are reversiblein direction. The structural elements suchas walls, beams and columns that werebearing only vertical loads before the earth-quake, have now to carry horizontal bend-ing and shearing effects as well. When thebending tension due to earthquake exceedsthe vertical compression, net tensile stresswill occur. If the building material is weakin tension such as brick or stone masonry,cracking occurs which reduces the effec-tive area for resisting bending moment, asshown in Fig 2.2. It follows that the strengthin tension and shear is important for earth-quake resistance.

2.3.5 Important parameters inseismic designIt follows that the following properties andparameters are most important from thepoint of view of the seismic design.

(i) Building material properties

� Strength in compression, tensionand shear, including dynamic ef-fects

� Unit weight

� Modulus of elasticity

(ii) Dynamic characteristics of the build-ing system, including periods,modes and dampings.

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STRUCTURAL PERFORMANCE DURING EARTHQUAKES

(iii) Load-deflection characteristics ofbuilding components.

2.4 Effect of site conditions onbuilding damagePast earthquakes show that site conditionsignificantly affects the building damage.Earthquake studies have almost invariablyshown that the intensity of a shock is di-rectly related to the type of soil layers sup-porting the building. Structures built onsolid rock and firm soil frequently fares bet-ter than buildings on soft ground. This wasdramatically demonstrated in the 1985Mexico City earthquake, where the damageon soft soils in Mexico City, at an epicentraldistance of 400 km, was substantiallyhigher than at closer locations.

From studies of the July 28, 1957 earth-quake in Mexico City, it was already knownfor example that the damage on the soft soils

in the center of the city could be 5 to 50 timeshigher than on firmer soils in the surround-ing area. Another example occurred in the1976 Tangshan, China earthquake, inwhich 50% of the buildings on thick soilsites were razed to the ground, while only12% of the buildings on the rock subsoilnear the mountain areas totally collapsed.Rigid masonry buildings resting on rockmay on the contrary show more severe dam-age than when built on soil during a nearearthquake as in Koyna (India) earthquakeof 1967 and North Yemen earthquake of1980.

Lessons learned from recent earthquakeshow that the topography of a building sitecan also have an effect on damage. Build-ings built on sites with open and even to-pography are usually less damaged in anearthquake than buildings on strip-shapedhill ridges, separated high hills, and steep

Fig 2.2 Stress condition in a wall element

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slopes.

2.5 OTHER FACTORSAFFECTING DAMAGEThe extent of damage to a building dependsmuch on the strength, ductility, and integ-rity of a building and the stiffness of groundbeneath it in a given intensity of the earth-quake motions.

Almost any building can be designed tobe earthquake resistant provided its site issuitable. Buildings suffer during an earth-quake primarily because horizontal forcesare exerted on a structure that often meantto contend only with vertical stresses. Theprincipal factors that influence damage tobuildings and other man-made structuresare listed below:

2.5.1 Building configurationAn important feature is regularity and sym-metry in the overall shape of a building. Abuilding shaped like a box, as rectangularboth in plan and elevation, is inherentlystronger than one that is L-shaped or U-shaped, such as a building with wings. Anirregularly shaped building will twist as itshakes, increasing the damage.

2.5.2 Opening sizeIn general, openings in walls of a buildingtend to weaken the walls, and fewer theopenings less the damage it will suffer dur-ing an earthquake. If it is necessary to havelarge openings through a building, or if anopen first floor is desired, then special pro-visions should be made to ensure structuralintegrity.

2.5.3 Rigidity distributionThe rigidity of a building along the verticaldirection should be distributed uniformly.

Therefore, changes in the structural systemof a building from one floor to the next willincrease the potential for damage, andshould be avoided. Columns or shear wallsshould run continuously from foundationto the roof, without interruptions or changesin material.

2.5.4 DuctilityBy ductility is meant the ability of the build-ing to bend, sway, and deform by largeamounts without collapse. The oppositecondition in a building is called brittlenessarising both from the use of materials thatare inherently brittle and from the wrongdesign of structures using otherwise duc-tile materials. Brittle materials crack underload; some examples are adobe, brick andconcrete blocks. It is not surprising thatmost of the damage during the past earth-quakes was to unreinforced masonry struc-tures constructed of brittle materials, poorlytied together. The addition of steel reinforce-ments can add ductility to brittle materials.Reinforced concrete, for example, can bemade ductile by proper use of reinforcingsteel and closely spaced steel ties.

2.5.5 FoundationBuildings, which are structurally strong towithstand earthquakes sometimes fail dueto inadequate foundation design. Tilting,cracking and failure of superstructuresmay result from soil liquefaction and dif-ferential settlement of footing.

Certain types of foundations are moresusceptible to damage than others. For ex-ample, isolated footings of columns arelikely to be subjected to differential settle-ment particularly where the supportingground consists of different or soft types ofsoil. Mixed types of foundations within the

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STRUCTURAL PERFORMANCE DURING EARTHQUAKES

same building may also lead to damage dueto differential settlement.

Very shallow foundations deterioratebecause of weathering, particularly whenexposed to freezing and thawing in the re-gions of cold climate.

2.5.6 Construction qualityIn many instances the failure of buildingsin an earthquake has been attributed topoor quality of construction, substandardmaterials, poor workmanship, e. g., inad-equate skill in bonding, absence of�through stones� or bonding units, andimproper and inadequate construction.

2.6 FAILURE MECHANISMS OFEARTHQUAKES2.6.1 Free standing masonrywallConsider the free standing masonry wallsshown in Fig 2.3. In Fig 2.3(a), the groundmotion is acting transverse to a free stand-ing wall A. The force acting on the mass ofthe wall tends to overturn it. The seismicresistance of the wall is by virtue of itsweight and tensile strength of mortar andit is obviously very small. This wall willcollapse by overturning under the groundmotion.

The free standing wall B fixed on theground in Fig 2.3(b) is subjected to groundmotion in its own plane. In this case, thewall will offer much greater resistance be-cause of its large depth in the plane of bend-ing. Such a wall is termed a shear wall.The damage modes of an unreinforcedshear wall depend on the length-to-widthratio of the wall. A wall with small length-to-depth ratio will generally develop a hori-zontal crack due to bending tension and

then slide due to shearing. A wall withmoderate length-to-width ratio and bound-ing frame diagonally cracks due to shear-ing as shown at Fig 2.3 (c).

A wall with large length-to-width ratio,on the other hand, may develop diagonaltension cracks at both sides and horizontalcracks at the middle as shown at Fig 2.3 (d).

2.6.2 Wall enclosure without roofNow consider the combination of walls Aand B as an enclosure shown in Fig 2.4. Forthe X direction of force as shown, walls Bact as shear walls and, besides taking theirown inertia, they offer resistance againstthe collapse of wall A as well. As a resultwalls A now act as vertical slabs supportedon two vertical sides and the bottom plinth.The walls A are subjected to the inertia forceon their own mass. Near the vertical edges,the wall will carry reversible bending mo-

Fig 2.3 Failure mechanism of free standing walls

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ments in the horizontal plane for which themasonry has little strength. Consequentlycracking and separation of the walls mayoccur along these edges shown in the fig-ure.

It can be seen that in the action of wallsB as shear walls, the walls A will act asflanges connected to the walls B acting asweb. Thus if the connection between wallsA and B is not lost due to their bonding ac-

tion as plates, the building will tend to actas a box and its resistance to horizontalloads will be much larger than that of wallsB acting separately. Most unreinforced ma-sonry enclosures, however, have very weakvertical joints between walls meeting atright angles due to the construction proce-dure involving toothed joint that is gener-ally not properly filled with mortar. Conse-quently the corners fail and lead to collapseof the walls. It may also be easily imaginedthat the longer the walls in plan, the smallerwill be the support to them from the crosswalls and the lesser will be the box effect.

2.6.3 Roof on two wallsIn Fig 2.5 (a) roof slab is shown to be restingon two parallel walls B and the earthquakeforce is acting in the plane of the walls.Assuming that there is enough adhesionbetween the slab and the walls, the slabwill transfer its inertia force at the top ofwalls B, causing shearing and overturningaction in them. To be able to transfer its in-ertia force to the two end walls, the slabmust have enough strength in bending inthe horizontal plane. This action of slab isknown as diaphragm action. Reinforcedconcrete or reinforced brick slabs have suchstrength inherently and act as rigid dia-phragms. However, other types of roofs orfloors such as timber or reinforced concretejoists with brick tile covering will be veryflexible. The joists will have to be connectedtogether and fixed to the walls suitably sothat they are able to transfer their inertiaforce to the walls. At the same time, the wallsB must have enough strength as shear wallsto withstand the force from the roof and itsown inertia force. Obviously, the structureshown in Fig 2.5, when subjected to groundmotion perpendicular to its plane, will col-

Fig 2.4 Failure mechanism of wall enclosure without roof

Fig 2.5 Roof on two walls

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STRUCTURAL PERFORMANCE DURING EARTHQUAKES

lapse very easily because walls B have lit-tle bending resistance in the plane perpen-dicular to it. In long barrack type buildingswithout intermediate walls, the end wallswill be too far to offer much support to thelong walls and the situation will be simi-lar to the one just mentioned above.

2.6.4 Roof on wall enclosureNow consider a complete wall enclosurewith a roof on the top subjected to earth-quake force acting along X-axis as shownin Fig 2.6. If the roof is rigid and acts as ahorizontal diaphragm, its inertia will bedistributed to the four walls in proportionto their stiffness. The inertia of roof will al-most entirely go to walls B since the stiff-ness of the walls B is much greater than thewalls A in X direction. In this case, the plateaction of walls A will be restrained by theroof at the top and horizontal bending ofwall A will be reduced. On the other hand,if the roof is flexible the roof inertia will goto the wall on which it is supported andthe support provided to plate action ofwalls A will also be little or zero. Again theenclosure will act as a box for resisting thelateral loads, this action decreasing in valueas the plan dimensions of the enclosuresincrease.

2.6.5 Roofs and floorsThe earthquake-induced inertia force canbe distributed to the vertical structural ele-ments in proportion to their stiffness, pro-vided the roofs and floors are rigid to act ashorizontal diaphragms. Otherwise, the roofand floor inertia will only go to the verticalelements on which they are supported.Therefore, the stiffness and integrity of roofsand floors are important for earthquake re-sistance.

The roofs and floors, which are rigidand flat and are bonded or tied to the ma-sonry, have a positive effect on the wall,such as the slab or slab and beam construc-tion be directly cast over the walls or jackarch floors or roofs provided with horizon-tal ties and laid over the masonry wallsthrough good quality mortar. Others thatsimply rest on the masonry walls will offerresistance to relative motion only throughfriction, which may or may not be adequatedepending on the earthquake intensity. Inthe case of a floor consisting of timber joistsplaced at center to center spacing of 20 to25 cm with brick tiles placed in directly overthe joists and covered with clayey earth, thebrick tiles have no binding effect on the

Fig 2.6 Roof on wall enclosure

Fig 2.7 Long building with roof trusses

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joists. Therefore, relative displacement ofthe joists is quite likely to occur during anearthquake, which could easily bring downthe tiles, damaging property and causinginjury to people. Similar behaviour may bevisualized with the floor consisting ofprecast reinforced concrete elements notadequately tied together. In this case, rela-tive displacement of the supporting wallscould bring down the slabs.

2.6.6 Long building with rooftrussesConsider a long building with a single spanand roof trusses as shown in Fig 2.7. Thetrusses rest on the walls A. The walls B aregabled to receive the purlins of the endbays. Assuming that the ground motion isalong the X-axis, the inertia forces will betransmitted from sheeting to purlins totrusses and from trusses to wall A.

Fig 2.8 Deformation of a shear wall with openings.

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STRUCTURAL PERFORMANCE DURING EARTHQUAKES

The end purlins will transmit someforce directly to gable ends. Under the seis-mic force the trusses may slide on the wallsunless anchored into them by bolts. Also,the wall A, which does, not get much sup-port from the walls B in this case, may over-turn unless made strong enough in the ver-tical bending as a cantilever or other suit-able arrangement, such as adding horizon-tal bracings between the trusses, is madeto transmit the force horizontally to endwalls B.

When the ground motion is along Y di-rection, walls A will be in a position to actas shear walls and all forces may be trans-

mitted to them. In this case, the purlins actas ties and struts and transfer the inertiaforce of roof to the gable ends.

As a result the gable ends may fail. Whenthe gable triangles are very weak in stabil-ity, they may fail even in small earthquakes.Also, if there is insufficient bracing in theroof trusses, they may overturn even whenthe walls are intact.

2.6.7 Shear wall with openingsShear walls are the main lateral earthquakeresistant elements in many buildings. Forunderstanding their action, let us considera shear wall with three openings shown in

Table 2.1 Categories of damage

Damage category Extent of damage Suggested post- earthquakein general actions

0 No damage No damage No action required

I Slighty non-structural Thin cracks in plaster, falling of Building need not be vacated.damage plaster bits in limited parts. Only architectural repairs

needed.

II Slight Structural Small cracks in walls, failing of Building need not be vacated.Damage plaster in large bits over large areas; Architectural repairs required

damage to non-structural parts like to achieve durability.chimneys, projecting cornices, etc.The load carrying capacity of thestructure is not reduced appreciably.

III Moderate structural Large and deep cracks in walls; Building needs to be vacated, todamage widespread cracking of walls, be reoccupied after restoration

columns, piers and tilting or failing and strengthening.of chimneys. The load carrying Structural restoration andcapacity of the structure is partially seismic strengthening arereduced. necessary after which architec

tural treatment may be carriedout.

IV Severe structural Gaps occur in walls; inner and outer Building has to be vacated.damage walls collapse; failure of ties to Either the building has to be

separate parts of buildings. Approx. demolished or extensive50 % of the main structural restoration and strengtheningelements fail.The building takes work has to be carried outdangerous state. before reoccupation.

V Collapse A large part or whole of the building Clearing the site andcollapses. reconstruction.

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Fig 2.8. Obviously, the piers between theopenings are more flexible than the portionof wall below (sill masonry) or above(spandrel masonry) the openings. The de-flected form under horizontal seismic forceis also sketched in the figure.

The sections at the level of the top andbottom of opening are found to be the worststressed in tension as well as in compres-sion and those near the mid-height of pierscarry the maximum shears. Under reverseddirection of horizontal loading the sectionscarrying tensile and compressive stresses

are also reversed. Thus it is seen that ten-sion occurs in the jambs of openings and atthe corners of the walls.

2.7 EARTHQUAKE DAMAGECATEGORIESIn this section, an outline of damage cat-egories is simply described in Table 2.1 onthe basis of past earthquake experience.Therein the appropriate post-earthquakeaction for each category of damage is alsosuggested.

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