GENERAL CONCEPTS OF EARTHQUAKE RESISTANT …

1 1 GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGNC hapter 3 GENERAL CONCEPTS OF EARTHQUAKERESISTANT INTRODUCTIONE xperience in past earthquakes has dem-onstrated that many common buildingsand typical methods of construction lackbasic resistance to EARTHQUAKE forces. Inmost cases this resistance can be achievedby following simple, inexpensive princi-ples of good building construction prac-tice. Adherence to these simple rules willnot prevent all damage in moderate or largeearthquakes, but life threatening collapsesshould be prevented, and damage limitedto repairable proportions. These principlesfall into several broad categories:(i)Planning and layout of the buildinginvolving consideration of the loca-tion of rooms and walls, openingssuch as doors and windows, thenumber of storeys, etc.

2 At this stage,site and foundation aspects shouldalso be considered.(ii)Lay out and GENERAL design of thestructural framing system with spe-cial attention to furnishing lateralresistance, and(iii)Consideration of highly loaded andcritical sections with provision ofreinforcement as 2 has provided a good overviewof structural action, mechanism of damageand modes of failure of buildings. Fromthese studies, certain GENERAL principleshave emerged:(i)Structures should not be brittle orcollapse suddenly. Rather, theyshould be tough, able to deflect ordeform a considerable amount.(ii)Resisting elements, such as bracingor shear walls, must be providedevenly throughout the building , inboth directions side-to-side, as wellas top to bottom.

3 (iii)All elements, such as walls and theroof, should be tied together so as toact as an integrated unit duringearthquake shaking, transferringforces across connections and pre-venting separation.(iv)The building must be well connectedto a good foundation and the , soft soils should be avoided, andthe foundation must be well tied to-gether, as well as tied to the MANUALW here soft soils cannot be avoided,special strengthening must be pro-vided.(v)Care must be taken that all materialsused are of good quality, and are pro-tected from rain, sun, insects andother weakening actions, so that theirstrength lasts.(vi)Unreinforced earth and masonryhave no reliable strength in tension,and are brittle in compression.

4 Gen-erally, they must be suitably rein-forced by steel or principles will be discussed andillustrated in this CATEGORIES OFBUILDINGSFor categorising the buildings with thepurpose of achieving seismic resistance ateconomical cost, three parameters turn outto be significant:(i)Seismic intensity zone where thebuilding is located,(ii)How important the building is, and(iii)How stiff is the foundation combination of these parameters willdetermine the extent of appropriate seismicstrengthening of the Seismic zonesIn most countries, the macro level seismiczones are defined on the basis of SeismicIntensity Scales. In this guide, we shall re-fer to seismic zones as defined with refer-ence to MSK Intensity Scale as described inAppendix I for A:Risk of Widespread Collapseand Destruction (MSK IX orgreater),Zone B: Risk of Collapse and HeavyDamage (MSK VIII likely),Zone C:Risk of Damage (MSK VII likely),Zone D:Risk of Minor Damage(MSK VI maximum).

5 The extent of special earthquakestrengthening should be greatest in ZoneA and, for reasons of economy, can be de-creased in Zone C, with relatively little spe-cial strengthening in Zone D. However,since the principles stated in , are goodprinciples for building in GENERAL (not justfor EARTHQUAKE ), they should always be Importance of buildingThe importance of the building should be afactor in grading it for strengtheningpurposes,and the following buildings aresuggested as specially important:IMPORTANT Hospitals, clinics, com-munication buildings, fire and police sta-tions, water supply facilities, cinemas, thea-tres and meeting halls, schools, dormito-ries, cultural treasures such as museums,monuments and temples, Housings, hostels, of-fices, warehouses, factories, Bearing capacity offoundation soilThree soil types are considered here:Firm:Those soils which have an allowablebearing capacity of morethan 10 t/m23 GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGNSoft:Those soils, which have allowablebearing capacity less than or equalto 10.

6 Those soils, which are liable to largedifferential settlement, or liquefac-tion during an can be constructed on firmand soft soils but it will be dangerous tobuild them on weak soils. Hence appropri-ate soil investigations should be carried outto establish the allowable bearing capacityand nature of soil. Weak soils must beavoided or compacted to improve them soas to qualify as firm or Combination ofparametersFor defining the categories of buildings forseismic strengthening purposes, four cat-egories I to IV are defined in Table inwhich category I will require maximumstrengthening and category IV the least in-puts. The GENERAL planning and designingprinciples are, however, equally applica-ble to GENERAL PLANNING ANDDESIGN Plan of building (i)Symmetry: The building as a wholeor its various blocks should be keptsymmetrical about both the leads to torsion duringearthquakes and is dangerous,Fig Symmetry is also desirablein the placing and sizing of door andwindow openings, as far as possi-ble.

7 (ii)Regularity: Simple rectangularshapes, Fig (a) behave better inan EARTHQUAKE than shapes withmany projections Fig (b). Tor-sional effects of ground motion arepronounced in long narrow rectan-gular blocks. Therefore, it is desirableto restrict the length of a block tothree times its width. If longerlengths are required two separateblocks with sufficient separation inbetween should be provided,Fig (c).(iii)Separation of Blocks: Separation of alarge building into several blocksmay be required so as to obtain sym-metry and regularity of each Torsion of unsymmetrical plans4 IAEE MANUALFor preventing hammering orpounding damage between blocks aphysical separation of 3 to 4 cmthroughout the height above theplinth level will be adequate as wellas practical for upto 3 storeyedbuildings, Fig (c).

8 The separation section can be treatedjust like expansion joint or it may befilled or covered with a weak mate-rial which would easily crush andcrumble during EARTHQUAKE separation may be consideredin larger buildings since it may notbe convenient in small buildings.(iv)Simplicity: Ornamentationinvo1ving large cornices, vertical orhorizontal cantilever projections, fa-cia stones and the like are danger-ous and undesirable from a seismicviewpoint. Simplicity is the best ornamentation is insistedupon, it must be reinforced withsteel, which should be properly em-Fig Plan of building CONCEPTS OF EARTHQUAKE RESISTANT design bedded or tied into the main struc-ture of the : If designed, a seismic coeffi-cient about 5 times the coefficientused for designing the main struc-ture should be used for cantileverornamentation.

9 (v)Enclosed Area: A small building en-closure with properly intercon-nected walls acts like a rigid boxsince the EARTHQUAKE strength whichlong walls derive from transversewalls increases as their length structurally it will be ad-visable to have separately enclosedrooms rather than one long room,Fig For unframed walls of thick-ness t and wall spacing of a, a ratioof a/t = 40 should be the upper limitbetween the cross walls for mortarsof cement sand 1:6 or richer, and lessfor poor mortars. For larger panelsor thinner walls, framing elementsshould be introduced as shown atFig (c).(vi)Separate Buildings for DifferentFunctions: In view of the differencein importance of hospitals, schools,assembly halls, residences, commu-nication and security buildings, etc.

10 ,it may be economical to plan sepa-rate blocks for different functions soas to affect economy in strengthen-ing Choice of siteThe choice of site for a building from theseismic point of view is mainly concernedwith the stability of the ground. The fol-lowing are important:(i)Stability of Slope: Hillside slopes li-able to slide during an earthquakeshould be avoided and only stableslopes should be chosen to locate thebuilding. Also it will be preferableFig Enclosed area forming box units6 IAEE Fire resistanceIt is not unusual during earthquakes thatdue to snapping of electrical fittings shortcircuiting takes place, or gas pipes maydevelop leaks and catch fire.