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IRC 6 Provisions on Seismic Design of BridgesAlabcJ/JC. Talldoll,S. K Tbakkar, 5/1dbirK Jaill, alld C.V.RA1/1rty

DEFINITIONS AND SYMBOLS0.1 Definitions

For the purpose of this standard, the following terms are defined:lbse: The level at which inertia forces generated ill the substructure and supcrstructure ne tf:lnsferreJ ro

the foundation.

Bridge Flexibiliry Factor <.: .-\ r.:l.:t~l to 0b~,.> .~A plastic acceleration specu"Um depending on tlexibilityof the structure; it depends on natural period of vibranull of the bridge.

Ce.:nter of ~lass: Ti1e.:poinr through ,,"hich the resultant of the masses of a syste.:m acts. This po liltcorresponds ro the center of gr:l\oity of the system. .

Critical Damping: The minimum damping above which free vibration motion :s f)or mcillator::.Damping: Th~ ,.ffeC! of internal friction, imperfect elasticity of material, slipping, slid!::;, :::"...in reducing

the amplitUde of vibration and is expressed as a percentage of critical damping.Design Seismic force: The seism:L :0::ce prescribed by this standard for each bridge com!'onent th:lt

shall be used i:-, i..0; design. It ~:. obtained as the maxinmm elastic seismic force divided by theapproprl:lte response.: reduction facrar spl'cified in this srandard for each component.

I )l.'sigIl Seismic Force Resultant 1 ': The force resultant (namely axial force, slH.:arforce, bending momenr;;l' rorsional moment) at a cross-secti(>n of the bridge due to d(',rZ~1I.rt:i.l/llil)om'for shaking :llong ::considered direcrion applicd on thc stn!crun:.

Uuctiliry: Dt:ctiliry of :l strucrun.:. or IrS tn;.:mbcrs, is rhe Glpacity to undergo large incbstic dcformatlon~wirhour significant lo~~ of strcngth or stiffness.

Ducrile Detailing: The prefern:d choice of location and amount of rdnforct.:menr in re.:inforceli cnnCrl.!lstruC!1lrcs to prm'idt.: for adt.:ljuate ductility in them. In steel structurt.:s, it is the de.:sign of me.:mbe:rsand their conncctions to makc thcm adcl)uarely ductile.

Elastic Seismic .-\cccleration Coefficient /1: J\ plot of horizontal acceleration value, asaccelerauon due to gra,'ity, lJeHlI,rnatur:rl period of vibration T that shall be used iI,structures.

Imporrance Factor I: :\ facror used ro obtain the design spectrum depending on the importance of thestructure. '

Line:lr Elastic l\nal:,"sis: _Analysis of the strucrure cunsiJering linear properries of the material and of theload-,'ersus deformauon of the different components of the strucrure.

Liquefaction: Liquefaction is the state in saturated cohesion less soil wherein the effectivc shear strcngth. is reduced to negligible value for all engineering purposes due ro pore pressures caused by

,"ibratinnc: durin~ an earthquake when they approach the rotal confining pressurc. In this conditionthe soil tends to beha,"e like a fluid m~:;s.

a fraction of

the desi,l.'11of

J\laximum Elastic Force Resultant [J': The force resultant (namely axial force, shear force, bending

moment or torsional momenr) at a cross-section of the bridge due to maXilJllI1IIe/m"/i..J'('irlllit'fo/i't' for

shaking along a considered dircction applied on the ~tructure, .

J\laximum Elastic Scismic I'orce.:: Thl' maximum force: in the bridgc componenr duc to the cXPl'unl

seismic shaking in rhc considered seismic zone,[\Iodes of Vibr:lIi(Ji1: (see !\onnal ,\lode)

f\lodes Participation I:actor })/:.: How much modc k. of vibration of thc briJgc partICIpates whcn

subjected to base excitation.

Natural Period T Narural period of a structure is its time period of undampeu vibration.

(:'i) Fundamental Na!1lral Period '1',: I t is the highest modal time period of ,..ibration along tht.:

considcred direction of earth()uakt.: motion.

(b) I\lodal l"atur:11 PcrioJ 'Ii: The (nodal narur:l] period of modc l. is rhe rimc pt.:riod of ",ibration

in me Illc k..

Normal :\Iode: !\1'Jdc of ,'ibrarion at \\'hich all ils masst.:s a!l:lill m:l~imllm \':dues (If displaccmcl1ls anJ

Draft IRC6 ProzliJioNSforSeiJlllil'DesigNq(Blidge/ Page2 ,!{:::1~'

rorations simulraneously, and they also pass through eqtIilibrium positions simultaneously. -'-Overstrength: Strength considering all factors that may cause an increase, e.g., steel strength heing higher

than the specified characteristic strength, effect of strain hardening in steel with large strains, andconcrete strength being higher than specified characteristic value.

Principal Axes: Principal axes of a structure are two mutually perpendicular horizontal directions in pbn. . of a structure along which the geometry of the structun.: is orienred.Response Reduction Factor R: The factor by which the acruallateral force, that would be gener:m:d if

the structure were to remain elastic during the most severe shaking that is likely at that site, shall bereduced to obtain the design lateral force.

Response Spectnun: It is a representation of the maximum response of iJealized single degree offreedom systems of different periods for a fL""edvalue of damping, during that earthquake. Themaximum response is plotted agairist the undamped narural period and for various dampip.g values,and can be expressed in terms of maximum absolute acceleration, maximum relati\"e \-elociry ormaximum relative displacemenr.

Seismic ~Iass: Seismic weight divided by acceleration due to gravity.::;...i."mc\X/eight IF': Total dead load plus part of live load as per clause 3.2.3.Soil Profile Factor 5: A fact0!: used to obtain the elastic acceleration spectrum depending on the s(,~l

protlle urdemeath the structure at the site.StIength: The usable capaciry of a strucrure or its n:l.~})'~rs to resist the applied loads.Stiffness of Piers: The force requiIed ro produce U11l~a..:formation in the pier under a bteralload applieu

at its top.Zone Factor Z: .-\ factor to obtain the design sp~ctrum depending on the percei\"ed seismic risk of rht.:

zone in which the strucrure is located.

0.2 Symbols I

The symbols and norations given be\o-,y apply to provisions of this standard. The unirs ust.:d r"rrhe items covered by these symbols shall be consistent throughout, unless specifically nott.:d otht.:rwisc./1 Elastic seismic acceleration coefficient

7r 7./1. ~\rea of the concrete core =- Dk:

, 4Ak Elastic seismic acceleration coeHicient of mode k

./1,;. Area of the bar cross section

C Bridge flexibility factOr

C I? Hydrodynamic force coe fficient

Ck Bridge flexibility factor of mode k of \-ibration

Cj Fraction of missing mass for /h mode.

e I' C 2 ' Pressure coefficients to estim::.te flow load due to stream on the substrucrure

C3,C+

D Dead load reaction of the bridge; dead load reaction at the supportDJ. Diameter of core measured to the outside of the spiral or hoopst( Thick ness of any layer iE, :.!odulus of elasticity of concrett'E, :.lodulus of elasticity of steelF . HyJrodynamic force or:. SUb:;tfucrure; Horizonral corce in K.J\' applied at cenn.r of 111:1:,So!'

superstructUn.: for one IIlIIIhorizontal deflection of bridgt.: along considered directionof horizonral force

F" Inertia force due to mass of a bridge component under carthyuake shaking along a Jirecf1<)(1FlIli<sin g I I e' d

.I

... _atera Loree assOcIate Wit 1 011SSll1gmass

.I.~ Characteristic strength of concrete :H 28 days in !\[Pa

.l Yield stress of steel

I II cJ I I (.

('

cJ cJI J I..

N N S / S 1"'./' 1/11" ~i/1 "av.: .:<:l':l . -I.wk,<; an a ll.:l C,l,jd..I!~III" , ,."g.(.".g.J!.II(S). r(S) ().<!Ik, , ,>

Draft IRC6 Pro",:riol1s/orSeiJlJ/it'DeJZ~lI'-!lB,id..~eJ P",§!l3r;./:::Hi=

Ft. Inertia force vecror due to mass of bridge under earthquake shaking along a direccion in modt: k

I.~;~f !\1aximum elascic force resultants at a cross-seccion dut: to all modes considered

.~ Acceleration due to gra,'ity .

H Height of water surface from lcn~l of deepest scour; height of substructure as per clausc 8.2.2f Importance FactorL Length of bridge deck as per clause 8.2.2/1/ Number of modes of ,'ibration considered

IIIj Total mass of the l\ mode,

[IllI Seismic mass matrL\: of the bridg~ structure

N A,'erage SPT value of the soil prof1le]\.1, Standard penetration resistance of layer i

1>k i\lodal participation facror of mode k. of ,'ibrarion

Pb Pressure due to fluid on submerged superstructures

R Response ReJuccioli Factor'i,'2 ,rJ Force resultants due to full design seismic force along two principal horizontal direccions and

along the "ercical direccion, respeccivclyS Soil Profile Factor

.r pirch of spiral or spacing of hoops511

Bridge flexibility factor along the considered direccion(>

'0

(~

]Bridge flexibility factor of mode k. of ,'ibration

g k

'ii hll1damenral natural period of ,'ibrarion of bridge in considered direction

Tk Natural Period of \-ibracion of mode k

( ; \' errical force at support uue ro seismic force11(5)= Displacement at posicion s causl:u in the accing uireccion of inerrial force wht:n rhe forcc

corresponding to the weight of the superstructure and substrucrure above the ground surface forseismic desif,>11is assumed to act in the accing direccion of inercial force

[ " Lateral Shear Force .

I d I\Iaximum elascic force resultant at a cross-seccion of a briuge componentI,'I/e/ Oesif,>11seismic force resultant in any componenr of the bridge due ro all modes considered

IF' Seismic weight, which includes full dead load and part live load as discussed in clause 3.2.3,lFb, 1Ft, IF2 Width~ of scating at bearing supports ;It expansion ends of girders.

IF''e \,\feight of water in a hypothecical el1\'eI0fJ~h5 .::-:';nder around a substructure

W(s) =' Weight of the superstrucrure and substructure at posicion sZ Seismic zone factor

8 Oisplacemenr at the acting posirion of int:rrial force of rhe superstrucrures \\'hen rhe force.com.:sponding to ROil" ()f rhe \\'ci~h( of illL' :,ub:,rrucrure aho\'e the ground slIrf:lcL' ("I' ~eisllll<':

tksign and all \veight of rhe supersrructllre portion supported by ir is assumed to ae! 111thL' :1<':(II1.~',

dirt'crio!1 of inerriai forct: (Ill),

f3 Racio of narural freyuencit:s of modes i anu I

{Ipk } l\Iode shape "ector of the briuge in mode k of ,-ibr:lciol1

rPkj l\Ioue shape coefficienr for/', dt:grt:t: of frt:edolll in k-'" mode of vibr:nion

it Net response due ro all moues considereu.}'k. Responst: in mou'e k. of ,'ibration.

};.* I\laximul11 rt:sponse due ro closcly-sp:lccd Ill"lk-S), lIIi~ sin -.: ' I

'f

' .

. ' ,\ aXlIl1um respc ,nse C) 1111SSll1g111:ISS

Draft IRC6 Pro/fiJiolls/or 5 eiJ/JIicDeJ~gllqfB,ir(ges

P ij Coefficient used in combining modal quancitics of modes i and J b:~ C:.:::.:::~.lcrhod

1.0 GENERAL PRINCIPLES2

1.1 Scope

This standard is applicable for the seismic design of ncw bridges an-i ,- r e'-aluacion ofsafety / adequacy of design for the seismic forces on exiscing bridgcs. Bridges and porn.~;~" ,~~<.:n:0fshallbc designed and constructcd, to resist the effccts of design seismic forcc specificd l!1 fa::, '-:.l:iu..uJ as :1mllurnum.

The prO\-isions of this standard o(\rc applicable for normal bridges. Howc\'cr, for a~ ~, ,r ~.~.:special bridge projects, detailed analysis and invescigacions shall be conductcd; specialist literarurc :n.. . ..}.;

consulted for this purpose. In such cases, seismic design shall be based on site-specific critena deycivp-.:.ifor the project.

,.

1.2 The inrencion of dlis standard is to ensure that bridges possess at least a mininml11 strength [.withstand eardIquakes. The intencion is not to prevent damage to them due to the most se"crc shakU1~that they may be subjected to during their lifecimc. /,ctuai [0i:CeSthat appear on porcions of bridgesduring earthquakes may be gre:uer than the dcsib'11seismic forc.e~ specified in .:his standard. However.d/lt'/i/i(~"arismg from material beha\'iour and detailing, and ovm'lrel,,~/harising from the additional resern.'suen:;,th ;]1 them .over and abO\'e dIe design forccs, are relied upon to account for this difference inactual and design lateral loads.

1.3 . The reinforced and prcstrc%t:J concrete componcnts shall bc undt:r-rcinforced so as co cause :1tensile failure. Furrher, they should be suitably designed to ensure that premature failurt: due to "hear orbond does not occur. Stresses induced in the superstructure due to earrhquake induced ~rol1nd m()[i(>11arc usually quitc nominal. Therefore, ductility demand under seismic shaking has not been a majorconcern in bridge superstrucrures during past carrhquakes. Howe,'er, the seismic response of bridges iscricically dependent on dIe duccile characteriscics of thc substructures, foundacions and connectiolb.Provisions for appropriate duccile detailing of reinforced concrete members given in IS: 139:20-199.') shallbe applicable to substrucrures, foundations and conneccions.

1.4 Masonry and plain concrete nch bridges wim spans more than 10 IJ/ shall not be built in thesevere seismic zones IV and V.

1.5 Ground Motion

The characteristics (intensity, duration, eft:,) of seismic ground vibracions cxpected :H an~' locatiO!depends upon the magnirude of earthquake, the' dcpth of focus, distancc from the epicenrer.

, characteristics of the path through which the seismic 'v aves tra,'el, and the soil strata on which thestructure stands. The random earthquake ground mocioi;.5, which cause the struerures tc \-ibrate, can be~esolved in any duee mutually perpendicular direccions.

, .

1.6 Situations arise where earthquake-generated vercical inercia forces need, to be specificall~"considered in design. These siruacions include bridges with large spans, those in which stability(overrurning and/or sliding) is a criterion for design, design of venical hold-do\\'n de,"ict:s at supports ortor overall stability analysis of bridges. Vt:rcical component at ground 11lotions Clil be parricuhr!~dctrinwntaJ in 'jH"estressed concrete girders and. canrileven.:J components such as canrile'"('rs of lkckslabs and cancilever bridges. Hence, special attenrion should be paid to the effect of ,'ertie:!l componl:1Hof the ground motion on them.

1.7 The spatial variation of ground motion shall be considered ,vhen:(i) Geological discontinuicies (e.g. soft soil contiguous to crystalline rock) or m:1rked ropographic:dft".arurc~arc: Dresen r:

.,- -. l...

marked ropographical t<:arures.

2 This section not yd disclissed by the cOlllll1itl<.:e

Drq(t JRC6 ProflisiollS/Or J ei.flJli( DeJi,gll q! B1id.~I'J

Specialist literature may be consulted for this purpose.

1.8 . The response of a strucrure to earthquake shaking is a function of the narure of foundation soil;materials, form, size and mode of constrllction; and characteristics and duration of ground motion. Thisstandard specifics design forces for strucrures standing on soils or rocks which do not settle or slide dueto loss of strength during shaking.

1.9 AssumptionsThe following assumptions are made in the earthquake-resistant design of bridges:

(a) Earthquake causes impulsive ground motions, which are complex and random in character, changingin period and :unplitude, and each lasting for a small duration. Therefore, resonance of tl1e type :>.svisualized under steady-state sinusoidal excitations, will not occur, as it would need time to build upsuch amplitudes.

(b) EarrlH.luake is not likdy to occur simultaneously \\.ith wind or maximum tlood or maximum se:l\vavcs.

(c) The ,'alue df elastic modulus of materials, wherever requireo, l1)a)"b:: raken as for slatic analysisunless a mor~ defmite ,"alue is available for use in seismic conditions.

2.0 DE::;IGN CRITERIA

2.1 Seismic Zone MapFor the purpose of determining design seismic forces, the country is classified into four seismic

zones. .\ seismic zone map of India is shown in Figure D-.1.. .

2.2 Methods of Calculating Design Seismic ForceThe seismic forcc~ for bridges shall be estimated by either onc of the twO methods, namely (a)

the Scismic Coefficient I\lethod described in section 3.0, or (b) the Response Specu"Um t'-.1cthoddescribed in section 4.0.

Response spectrum method will be used for all important bridges in seismic zones IV and \".Further, for seismic zones V, IV and III, if (//!J' 0111'of the following conditions holds, the use ofResponse Specu"Um Method is mandatory:

(a) Irregular bridge as defined in section 2.2.1,(b) Indiyidual span more than 60/1/;(c) Height of top of picr / abutment from the base of foundation is more than 2011/,or(d) Inno,"atin: bridge.

The stmcrural analysis of the bridge to obtain the force resultants (c:.g.,bent..ling moment, shearforce and axial force) at the different locations in the bridge, must appropri:ltely model the stiffnesses ofsuperstmcrure, bearings, 'piers, abutments and foundations.

Detailed investigations and desi!:,'11referred to in clause 1.1 shall be carried out based on sire-sp~cific considerations if any of the following conditions are met with:

(a) Important bridges in seismic zones IV and V where soil conditions ate poor consisting of m:!.rine

clay or loose sant..l (e.g., wherc the soil lip to 30m dcpth has SPT N '"aim' eYlIal to or less than 20),0) ..\11bridgcs wirh span morc rh:1I1 I:!(J Ill; and

(c) All bridgcs with hcighr of rop of Pler/abllul1<:nrs from basc of fOlinJarions (for all rypcs (It

foundations, i.c., open, wcll or piles) is morc rhan i{) III, and(d) Bridgcs locatcd ncar :I known fault or the arca is known for cOI~~plcx scismo-tecronic geological

settIng.

2.2.1 Regular and Irregular Bridge2.2.1.1 Regular Bridge

;\ n:gular bridge has no abrupt or unusual changes in mass, sriffness or gcomcrry along irs spananJ has no brgc diffcrcncl:s in thcsc par:l1l1cu.:rs bcrwc.:en adjac<:nr Sllpp01"lS(abutmenrs excludcd). .\bridg<: shall bc considcred rcgular for th<:purpos<:s of this standard, if(a) It is srraighr or ir describcs :1 s<:Cfor of all arc ~'hich subrcnds :In angle less than lJ{)oal Ih<: CCllllT ,.t'

Drc~ftIRc;6 ProtiJiol1JjorSeiJlI/k DeJ';g1/o./Bn'(!geJ P(~~f6 o/-~!;-.the arc, and

(b) The adjacent piers do not differ in stiffness by more than 25(~~ (percentage diffcn:nce shall bccalculated based on the lesser of the two stiffnesses as reference.).

2.2.1.2 Irregular Bridge. ..\ll bridges not conforming to clause 2.2.1.1 shall be considered irregular. further, arch brit!gcs

of span exceeding 30m, cable stayed bridges, suspension bridges, and other innovative bridgcs shall ;IL'lJbe treated as irregular bridges.

2.3 Vertical Motions'!

The seismic zone factor for verrical motions, when required (see clause 1.6), m:lYbe t:-.kCfl:IStwo-thirds of thatJor horizontal motions given in Table 2.

2.4 Live Load

The desi,gnlive loads shall be as specified in IRC:G.2.4.1 For Calculation of Magnitude of Seismic Forces Only

The !i'"e load shall be ignored while estimating the horiz()Il~:l1se~s~(: forces along rht: din:cril)lIof traffic. ." . :..- . ""; .

The horizontal seismic .force in the direction perpendicular to traffIc .,hall be calculated using

50% of design Jive 10aJ~~ip1pacr). ,,~.., .' --'- ..

"..i.heverrical seism:.-c-fortc"shall be calculared using 50% of design live load ~~~Iudi~tmpact).The abo\-e percentages are applic:lble only f0r calculacing magnitUde of seismic ft)rc<.::lnd :m:

based on rhe assumpcion that only 50% of the design livc load is present in rhe bridge :H rhe (ime nf rhe:carthquake during service condicions.2.4.2 For Calculation of Srresses Due to Live Load, bur to be Combined with Sm:sses due (()Seismic Forces

For c:llculating the stresses due to li\'t' load to be combined with those due ro se:isnllc fOl"CL'S.50% of the design live load (including impact) for road bridges shall be considered to be :lCting at therime of the e:lrrhquake. Table 1 of IRC:6 shall be referred ro for other loads to be: clJnsiJe:re:dsimultaneously with seismic loads.

2.5 Seismic Load Combinations~

2.5.1 The seismic forces shall be assumed to come from any horizontal direccion. For this pUlVnse:.two separate analyses shall be performed for design seismic forces :lCring along tWO orthogonalhorizonral direcrions. The design seismic force resultants (i.~., axial force, bcnding mO\l1enrs, she~lrforces, and torsion) at any cross-section of a bridge componenr resulting from the analyses in (he (\\., I

orthogonal horizontal directions shall be combined as below (figure 1)(a) :t 1'1:t O.3r]

(hi 1:.0.3r, :t r2\vnc1.C l ~ 1.' " 1

( '-", r n- b>'\\ Aa...'-!V= Force resultant due to full design seismic force along~ecri_().?~J.. _ " _ ,' . .. , ... . ! "V'v~ ,v.....--.-(!j)= Force resultant due to full desIgn SelSI111Cforce alon~1rect1on. .2.5.2 \V'hen \'errical seismic forces are also considered, the design seismic force resul(alHs at any c'ross-section of a bridge coinponelH shall be combined :tSbelo\\":

(a) :t 'i ::!:0.31'2 ::!:O.3"J

0)) ::!:O.31i :t r/::!: oj"J

(c) :t 0.3 r, :t 0.3 r2 ::!:,']

where r, and r2 are as defined in clause 2.5.1, and ..5)is the force resultant due co full de:sign "cisnllc

force along the verrical direction.

:;Clauses 1.6.2.3 and 2.5.2 10 be modi lieu in \'i..:\\'01' I:uro cod..:..:\tracls read Ollt by Mr. Tandon: Mr. Tandnn lo h..:l[1dratl.

I Replac..: claus..: 2.5.1 and 2.:\.2 with clause .1.IO.Xor AASIITO 1\1'similar (I'n>r'. (";muon10IIdp)

Orq,'i IRC6 Prol'iiioll.f.!OrJ('i."llIi(D('...~~IIq[Bli({ge.f p('--~r'7 ry:=F9-2.5.3 As an alternative to the proccdurc in 2.5.1 and. 2.5.2, thc forccs duc to thc combined effect oftwO or thrcc componems c:m be obtained on the basis of's<'luare root of sum of square (SRSS)' that is

r-;--, ~7 J .,Vr[ +rj or rr + rj + rj

whcre r,. r2 and I) arc as defincd in clause2.5.1or2.5.2.

2.6. Increase in Pennissible Stresses2.6.1 Increase in Pennissible Stresses in Materials

\\!hen earthquake forccs are considercd along with other normal dcsign forces. the pcnnissiblestresscs in n").aterial,in !oheclastic method of design, shall bc t:l-k"nas stipulated in Table 1 of IRC:6.2.6.2 Increase in Allowable Pressure in Founding Strata

\\lhen earthquake forces are included, the allowable bcaring pressure in soils and rocks shall beincreased as pcr IRC78.

2.7 Material Properties2.7.1 The modulus of elasticity of c:)!"!crete(2,) can be assumed as follows

Eo = 5000y{tJ\\lhere 1.-1; is the characteristic cube compressive strength of concrete.2.7.2 I\[odulus of elasticity of steel may be assumed as 200,000 IvlPa.

2.7.3 For the purpose of this code, a soil profile with a\'crage SPT \":lluc N e:..ceeJing 50 is

considered as rock or hard soil, and N \"aiuc in the range of 15 to 50 as mcdium S()i.l.This Jdinitionapplies to the upper 30m of the soil profile. Profiles comaining disrinctly different $~')ilbyer::: ,:hall hesubJi\.iJcd imo layers, each Jesi,S'11ateJ by a number that ranges from 1 (at the rop) to I: :ar the botrum),where there arc a rotal of II layers in the upper 30 metres, and a wcight:.:d :1\"(:rage\\'~J be obrained :1$follows: .

11

wh<.TC 2: d, is equal to 30 m, .,'.. is the stand:1rd penetration rcsisranc~ of layer i, n(Jr ro cxcccd IlH I. 1=1

blo\~rs per 300 mm as Jirecrly mcasured in rhe field wirhout corrcctioning, and d. is the thicknc,;;:. (,f am"layer i berween 0 and 30m..

3.0 SEISMIC COEFFICIENT METHOD3.1 Elastic Seismic Acceleration Coefficient /1

The I~lastic Scismic .-\cccleration Cncfficienr ".1 due ro de,;ign <.:artlllluake along a cc>nsiden:ddin:crioJ) shall be obrained :1';

1

2.5Z1)-

A = ZISu~jL.o

for single - span bridges

for all other bridges

where

Z = Zone Facror, given in Table 2 for horizontal morion. I.onl' facrors for some: importanr (lwns arc

gi\'et1 in J\pp<.:ndix I).For n:rrical motion, refer to cbuse 2.3.1 =Imporrance I:acror, gin;n in Tal)]C 3,

..

,..

DIi:~ji IRC6 PIVI,,:,',illl.i.!qrSeiJlJlic De.ii...~11rl Blir(~e.f

5a ~ bridge flexibiliry facrar along the considered direction given as .follmvs:g

For IYJCJ:.-)I,or hartl.ioil Jilt'.\"

5

{

2.50tl

g = 1.00/T1rOl'lJIL'dlilll1 .ioil Ji/r!.i

T1 ::;0.40

0.-10::;T1 ::;4.00

5tl

{

2.50

g = 1.36/T1

Fol' J'qjiJail JileJ .

5a _

{

2.50 Tl ::;0.67

g - 1.67/T1 0.67 ::;T1 ::;4.00T, =Fundamenral narural period of rhe bridge (along rhe considered direccion).

A 1 f Sa T. ,p ot 0 - versus ,IS given '.0 rlgcre ~.

g

T1 ::;0.55

0.55::; T1 ::;"4.00

Table 2 : Zone Factor Z for horizontal motion.

I Seismic Zone I IIZ 0.10

Table 3 : Importance Factor I for different bridges.

Note: The imporrance of a bridge shall be decideu' on local conuicion:, cOl1siul:ring rhevarious issues like rhe rype of srrucrure, straregic imporrance, \'iral communicarion linb.ere.

3.1.1 FundamentalNatural Period

3.1.1.1 For simply supported bridges, rhe design vibracion unir consisrs of one substrucrure and :1superstmcrure porcion supporred by ir (Figure 3a). The funuall1enral narural period (T1) shall bc

calculated from the following equacion:

where

8 = Displacemenr at the acting posicion of inercial forcl: vi [he superstmctures when rhe forcecorresponding to 80~/Oof the weight of the substrucrure above the ground surface for seismicdesign and all weight of the superstrucrure porcion supporred by it is assumed to act in rhe actingdireccion of inercial force (m).

i\lrernarin:ly, rhc fundamenral narur:d pcriou T1 (in seconus) uf pieri aburmell[ uf rhe Iniu;..:e .d. '11~

a lwrizonral direction may be esrimated by rhe follO\ving expres:,ion:

W=l\ppropriate dead load of rhe superstrucrures, and live load in kNF= Horizontal force in kN requin.:d to be applied ar the cenrre of mass of rhe :,upe[:,r[ucrun: tor oncmm horizonral deflection ar the rop of the pieri aburmenr along the considered Jin.:crion of 11'J[iZOJ1[:dforce.

3.1.1.2 1-'<>1'multi-span inrcgral briJgcs, the design \'ibcHi,)() unir L()nsi:-.h IIt.1 numher III sub..;rrucrures

and superstructure purtions supported h~' ir (hgure .)1». The lund:ul1etHal n:dur:d peJ'1' ,d (T}) ,hall he

Use 1J

Importanr Bridges 1..5

Other Bridges 1.0

Drqft IRC6 Prol1isiollsforSeismicDeJigllq(Brid..geJ Page9~calculated by any suitable method. For example, Rayleigh's method may be used as follows:

T1= 218

8 = fW(S)//(s)2ds

fW(s)lI(s)ds

W(s) =\XIeight of the superstructure and substructure at position s (kJ\~

I/(s)= Displacement at. position J caused in the actin~ direction of inertial force ,vhe~ the force .

corresponding to the weight of the superstructure and substructure above the ground surface forseismic design is assumed to act in the acting direction of inertial force (m)

3.1.1.3 For calculating the natural period, rhe stiffness for rhe deformation caused in rhe srmcruralmember shall be used and, in principle, the int1uence of deformaaon of foundation ground shall betaken into accOllnt.

3.2 Maximum Elastic Forces and Defonnations

The inertia forces due to mass of each component or portion of the bridge as obtained fromclause 3.2.1 shall be applied at the center of mass of the corresponding component or portion of the

. bridge. A linear static analysis of d1e bridge shall be performed for these applied inertia forces to obtainthe force resultants (e.~~..bending moment, shear force and axial force) and deformations (('.~~.,

displacements and rotations) at different locations in the bridge. The stress resultants 1/" anddeformations so obtained are the maximum elastic force resultants (ar the chosen cross-section of thebridge component) and the maximum elastic deformations (at the chosen !lodes ir.. the bridge strucrure):respectively.3.2.1 Inertia Force Due to Mass of Each Bridge Component

The inertia force due to the mass of each bridge component (e.g.,supersrrucrure, subsu-ucrureand foundation) under eard1quake ground shaking along any direction shall be obtained from

Fe =AW ,where

A = Elastic Seismic Acceleration Coefficient along the considered direction of shaking obtained as perclause 3.1, and

I~?= Seismic weight as discussed in clause 3.2.3.3.2.2 Elastic Seismic Acceleration Coefficient for Portions of Foundations below Scour Depth

For portions of foundations at depths exceeding 30m from the scour depth (as defined in clause6.2), the inertia force as defmed in clause 3.2.1 due to that portion of the foundation mass may becomputed using the elastic seismic acceleration coefficient taken as O.5A, where A is as obtained fromclause 3.1.

For portions of foundations placed between the scour depth and 30m depth below the scourdepth, the inertia force as defmed in clause 3.2.1 due to that portion of the foundation mass may becomputed using the elastic seismic acceleration coefficient value obtained by linearly interpolatingbetween ./1 at scour depth and 0.5..1 at a depth 30/J/ below the scour depth, where ./1 is as spccifieu inclause 3.1.

3.2.3 Seismic WeightThe seismic weight of the superstructure shall be taken as its full dead load plus appropriate

amount of live load specified in clause 2.4.1. The seismic weight of the substructure and of thefoundation shall be their respective full dead load. Buoyancy and uplift shall be ignored in the calculationof seismic weight.3.2.3.1 Seismic Mass

The seismic mass of a bridge component is its seismic weight obtained as per clause 3.2.3,divided by the acceleration due to gravity.

Drt~ji IRC6 P/TJlliJioJ/"-jorj'(iJ/l/i,' De"-{gJ/of Bli((~(j: p{{~( 10 11:"-""

3.3 Design Seismic Force Resultants for Bridge Components ~

The design seismic force resultant Vat a cross-section of a bridge component duc ro carrhyuakeshaking along a considered dircction shall be gin:n by. .

\\'here

I ,,' =~Iaximum elasac force resultant at the chosen cross-section of that bridge component due roearthquake shaking along the considered direction as obtained from clause 3.2, and

R =Response Reducaon Facror for the component as given in Table 5.Response Reducaorl Factor shall not be applied for calculation of design displacements.

3.4 Multi-directional Shaking\,\'hen carthyuakc ground shaking is considercd along more than one direcriol1. rhe desi~11

seismic force resultants obtained from 3.3 at a cross-secaon of a bridge component duc ro earrhyuakeshaking in each considered direcaon, shall be combined as per 2.5.

3.5 Combination of Seismic Design Forces with Design Forces Due to Other EffectsThe design seismic force resultant at a cross-secaon of a bridge compon.:ni: gi\'en by rhis code

shall be appFopriately combined with those due to other forces as per Table 1 of IRC:6.

Table 5 : R Red F R for Brid C dC

No/eJ:' 1. &sponse redm1ioll Jf:z,.tor is Ilot to be applied Jor the t"ok-lilation ofdisplot"ements.

2. IF'hell wing Ilollle! ofR Jor.f0llndotioIIS ond t"oll//ediol/J",pleoie refer to ,'Iollie 7.2 olld 8.1.2. mpedille!y.

4.0 RESPONSE SPECTRUM METHOD

The Response Specrrum Method requires the evaluation of natural periods and modc shapes ofscyeral modes of vibration of the structure. This method will require usage of a suitable dynamic analysi:;

4.1 Elastic Seismic Acceleration Coefficienr /lk in Mode k

The elastic seismic acceleration coefficient Ak for mode k shall be determined by:

Ak = z; (Sa

),- g k

whe'e Z aod J a,e as defined in 3.1, aod (5; )k is the bcidge flexibility facror for mode k giveo by rhe

5 To discuss in the committeeb Need R value for abutments too

-- - . - --- r - -- ---

/{ IComponent.

Supersrructure 3.0Subsrructure6

(a) Reinforced Concrete with specid duccile detailingfultiple Column Frames

'} -_.JSingle Column, Wall or Pier 2.0

Reinforced Concrete with ordinary derailing 1.5

(b) Masonry 1.l)Foundation 1.0

Connection i

Bcarings (L-t !Supersrructure ro abUtment (1'-+

Expansion joints within a span of the super:;rructure 0'-+

Column, piers, or pile bents ro cap beam or superstructure 0.5

Columns or piers to foundation 0.5

Dr41 /RC6 PIV1,i.,ioI/Jfor 5 eiJmi( De.rigl/ of BlidgeJ

following expression:

ror !YJck:;',or hard Joil .dlu

(

S

) {

2.S0

; k = 1.00jTk

Tk 5: OAO

0.40 5: Tk 5: 4.00

(Sd

) {

2.S0

g k = 1.36jTk.For j°f!/i Joil JileJ

(

Sa

) {

2.50

g k = 1.67/Tk

Tk 5: 0.55

0.555: Tk 5: 4.00

Tk 5: 0.67

0.67 5: Tk 5: 4.00

where Tk is the narural period of vibration of mode k of the bridge. For modes orher than the

fundamental mod~, the bridge flexibiliry factor(~

Jfor Tk 5: 0.1 sec may be taken as:

g k

( S; J=1+ 15Tk .

.:> k

A plot of(~

J\"ersus Tk is gi\'en in Figure 4.

g k

4.2 Inertia Force due to Mass of Bridge at Node i in Mode kThe effect of seismic shaking can be quantified as concentrated seif.mic inertia forces and

.moment corresponding to the translacional and rotational degrees of freedom, respectively, at each nodeof the discretised of the bridge strucrure. Each mod~ of vibration k contributes to these seismic inerria

forces and moments. The vector {Fk} of maximum elastic inertia forces to be applied at different nodes

in mode k of yibration due to earthquake shaking along a considered direction shall be obtained as:

. ° {r1}= [m] {CPk}Pk Ak g,\vhere

[Ill] =Seismic mass matri..x of the bridge structure, as defined in clause 4.2.1,{cpk} = :Mode shape vector of-vibration mode k of the bridge structure obtained from free vibration

analysis,Pk = I\"Iodalparticipacion factor of vibration mode k of the bridge structure for a given direction of

earthquake shaking

{cpk }T [111]{1}{cpk }T [Ill ]{cpk} ,

./lk = Elastic seismic acceleration coefficient for moue k as uefineu in clause 4.1,

g = Acceleration <;:\ueto gravity, and

{I} = Vector consisting of unity (one) associated with translational degrees of freedom in the

considered direccion of shaking, and zero associated with all other degrees of freedom.

4.2.1 Seismic Mass Matrix

The seismic mass matrix of the bridge structure shall be constructed by considering its seismicmass lumped at the nodes of discretisation. The seismic mass of each bridge component shall beestimated as per clause 3.2.3.1, and shall be proportionally distributed to the nodes of discretisation ofthat bridge component. ~

DrqJi IRC6 ProP1JiollJfOrSeirlJlit"DeJ~gl1q(Bl7fl,gp PtJ.ge/2 jJl4<r4.3 Maximum Elastic Forces and Deformations ~

The maximum elastic seismic forces in mode k obtained trom clause 4.2 shall be applied on rhe

bridge and a linear sratic analysis of the bridge shall be performed to evaluate the maximum clastic force

resultants 1-1 (e.~~..bending moment, shear force and axial force) and the maximum elastic deformations

(e.,g..displacements and rorations) in mode k at different locations in the briugc for a considereudirection of earthquake shaking.

The maximum elastic force resultants r~:e! and the maximum clastic deformations, due to all

modes considered, for the considered dire,tion of earthquake shaking, shall be obrained by combiningthose due [0 the individual modes as follows:

a) If the structure does not have closely-spaced modes, then the maximum response It due [0 all modesconsidered may be estimated by the J"qllareIvaI of J"llmofJ"qllareJ"(SRSS) methodas:

Where

Itk=Absolute value of response in mode k, and

m =Number of modes being consideredClosely-spaced modes of a strUcrure are those of its natUral modes of \'ibration whose natUralfrequencies differ from each other by 10 percent or less of the lower frequency.

b) If the structUre has a few closely-spaced modes, then the maximum response (It) due to these modesshall be obtained by the absolute sum method as:

r

X =~AL.. <"c=1

where the summation is fo~ the closely-spaced modes o~ly. This maximum response due to closely-

spa~ed modes (It) is then combined with those of the remaining well-separated modes by the squareroot of sum of square (SRSS) method in a) abm'e.

(c)The number of modes to be considered in the analysis shall be such that at leasr 90°'0 of the [Otalseismic mass of the structUre is included in the calculations of response for earrhquake shaking alongeach principal direction. If modes with natUral frequcncy beyond 33 Hz arc [0 be considered, modalcombination (Clause 4.3 (a) and 4.3 (b» shall be carried out only for modcs with natUral frequency lessthan 33 Hz. Modes with natUral frequency exceeding 33 Hz shall be treated as rigid modes andaccounted for through missing mass correction discussed below.

d) At degree of freedom}: the missing mass is given by

" e}11;j =(1- f Pk4>kj

]m j

k=1

when~

Pk = Modal participation factor for mode k,

4>kj =Mode shape coefficient for/', degree of freedom in k'h mode-of vibration

mj = Total mass of the/h m~de,

Cj =Fraction of missing mass for /' mode.

Lateral force associated with missing mass is

F missillg - (ZI). -c.m.) } }

the structUre will be statically analysed for this set of inertial forces and response It",issingwill be

obtained. The response A,lnlssmgwill be combined with regponse It for fl~xiblc modes by the squarcroot of sum of square (SRSS) method in a) abm'c.

.::

Draft JRC6 ProlJisiollJ.forSri.fllli£"De.ri,gllo.fB1ir(~eJ Pa..~r13 ~4.4 Design Seismic Force Resultants in Bridge Compo.nents

The design seismic force resultant r /"el at any cross-section in a bridge componcnt for a

considered direction of earthquake shaking shall be determined asFe

Jf =~II,'{ R.'

where the maximum elastic force resultant r~:el due to all modes considered is as obrained in clause 4.3,

and Response Reduction Factor R' of that component of bridge is as per Table 5. Howc\'er, ResponseReduction Factor shall not be applied for calculation of design displacemenrs.

4.5 Multi-directional Shaking

\\lhen earthquake ground shaking is considered along more than one direction, the designseismic force resulrants obtained from clause 4.4 at a cro%-sectjon of a bridge compo!1L'1H due (()earrhquake shaking in each consiuercd direction, shall be combine.d as per clause 2.5.

4.6 Combination of Seismic Design Forces with Design rOrl.!:'s Due to Other EffectsThe de~ign seismjc force resulrant at a cross-section of a b::idge componer.~ given by this code,

shall then be appropriately combined with those due to other forces a:.>per Table 1 of IRC:6.

4.7 Site-Svccific Spectrum

In case design spectrum is specifiCilly prepared for a structure at a particular site, rh:: same m:l~'be used for dcsit,'11subjecr ro rhc minimum requiremenrs spccitied in this srandard.

5.0SUPERSTRUCTURE

5.1 The superstructure shall be designcd for the design seismic forces sp~cified in clauses 3. or 4..plus the orher appropriate loads.

5.2 Cnder simultaneous accion of hOl1.zontal and vertical accelerations, the supersrructlue shall han.:a facror of safet)" of at least 1.5 against overturning. In this calculation, the forces to be considen.:d onthe superstructure shall be the maximum elastic forces generated in the superstructure, as ca1cubtcdusing clauses 3.2 and 4.3.

5.3 The superstrucmre shall be secured, whcn necessary, ro the substmcturc, parcicularly in seismic70nes 1\- and V, through \'ertical hold-down de\.ices and/or anti-dislodging elemenrs in horizonraldirection. as specified in clauses 5.3.1 and 5.3.2, respecci\'ely. These \'ercical hold-down de\.ices and/oranci-dislodging elements may also be used to secure the suspended spans, if any, with the resrraincdportions of the superstrucrure. Howc\'er, the friccional fOiLes shall not be relied upon in rlw design ofthese hold-down d~\'ices or anci-dislodging elements.;;

5.3.1 Vertical Hold-Down Devices

\\:rtical hold-down devices shall be prm'ided at all supports (or hinges in continuous srructurcs),where resulting n:rrical force () due ro the maximum clastic horizontal and yerrical

scismic furct.:s (combint.:d as pcr clausc 2.5) opposcs and cxcel:Js '50"" of rhc ul:au luau n.:actiu!l D.5.3.1.1 \Vhere n:rtical force l', due ro the combined effect of maximum clastic hori7.onral anu ,'crrjcalscismic forces, opp()~es and cxct.:cus 50" 0, bur is less than 100" ", of the ut.:ad loau reaction f), rhe ,'crrical

hold-uo\\"n de\.ice shall be designed for a minimum net up\\'ard force of 1(y~;;,of the do\\'n\\'aru ucadload reaccion that would bc exerted if the span were sinlply supported.5.3.1.2 If the vercieal force U, due to the combjned effect of maximum horizonral and vertical seismic

forces, opposes and exceeds 1O()<~:oof [he ucad load reaccion D, thcn tht.: de\.icc shall be designcd for anet upward force ()f 1.2(L'-D); ho\\'c\'er, it shall not be less than IOI!" of the UO\"11\vard dead k)au

reacrion thar would be exerred if the span wcre simply s~pp()rrL'd.

7 Sample sketches to be given in appendix !v!r. -j'andoll.'lI1dothers to 1h.'lp.

...

D,,!{lIRC6 P~V/.'i.fiolls.lo"5 "i.wlic Dt'.fZ~1Iq( Bli{(~(,J - PI~~('1-1i!!' ...5.3.2 Anti-Dislodging Elements in the Horizontal Direction

Anti-dislodgement clemems shall be prO\'idcd between adjacenr sections of rhe superstructure atsupporrs and at expansion joints within ;:lspan.5.3.2.1 Thc linkages shall be designed' for, at least, elastic seismic acceleration coefficicnr ./1 times the

\v.eight of thc lighter of the two adjoining spans or parrs of the structure as in the case of suspendedspans.S.).~ ~ l( tI", linkl'.'" IS .\1 \,\,'.\11"1\' \\ h,'\\' \\'\.\lIY,' ,kt, 11\\1\lll'lh If\' ,11";\1'11\'\\II' ,\I-Cur, 11\\'\\ ';UfCI,'i,'1\1" .

slack must be allowcd in the linkage so that Imkages start function1l1g only when the design relaun:displaccmenr at the linkage is exceeded and linkage becomes effective, after overcoming the designedslack in the linkage.5.3.2.3 \'V'hen linkages arc provided ~t columns or piers, the linkage of c:lch span m:l)' be connected rothe column or pier instead of to d1e adjacent span.

6.0 SUBSTRUCTURE

6.2 Scour DepthThe scour to be considered for design shall be based on mean design flood. In the abscncc of

detailed data th~ ~(;0Ur'lObe considered for design shall be 0.9 times the m:1ximum design scour depth.N._,t~: The desigr1'.:r is cautioned d1at the maximum seismic scour casc may not ahvays be gO"-:::ll;!1~design condition.

6.3 Design Seismic ForceThe dcsign seismic forccs for the substructUre :,hall bc obtaincd as the maximum ela,;ric force on

it (:ISdefmed in clause 6.3.1) di,'ided by thc appropriatc responsc reduction factor gi\'cn in T:1bk ~,.6.3.1 Maximum Elastic Seismic Forces

The maximum elastic seismic force rcsultants at any cross-~ection of thc substructure shall becalculatcd considcring all of the following forces on it:

(a) t-.Iaximum ebstic scismic forces transferred from thc superstrucrure to the top of the substructure(b) l\b.ximum ebstic seismic forces applied at its center of mass due co the substrucrure's own iner:l:l

forces, Reduction due co buoyancy shall be ignored in thc calculation of scismic \veighr.(c) Hydrodynamic forces acting on piers as pcr clausc 6A, and(d) t-.Iodification in earth-pressure due to eardlquake acting 00 abutmcnrs as per f\ppendi.\: E

6.4 Hydrodynamic Force

For the submerged portion of the pier, dle total horizonral hydrodynamic force along rhedirection of ground motion is giyen by

F=CeAWc,

whcre C e is a coefficient given by Table 6, depending on dlC hcight of submcrgcnce of the picr rebti,'c

to dlat of the radius of a hypothetical em.eloping cylinder (Figure 5); and /1 is the clastic seismicacceleration coefficicnr as per clause 3.1; and {We is rhc wcight of the water in the hypothetical

enn.:]oping cylinder. The prcssure distribution due to hydrod)'namic effect on pier is gi\'en in l-'igllre (,;

the coefticient.s C" C 2, C ; and C.,t in figure 6 arc gi,'cn in Table 7. ,

Table 6 : Values of Ce.

( Heigill oj Submerged Portiou oj Pier H J

1.0 2.0 3.0 4.0

RadiliS of Ellveloping Cylillder

Ce (J.3? O.5S O.6S 0.7]

1

Drqji IRC6 PIVI.i.iioIlJfOr SeiJ"!)licDe.rig" o/Bri{!~e.r

Table 7: Pressure Distribution Coefficients (:" C2, C3 and Col.

PaOl' /5 Jff=/:9:'.

6.4.1.Analysis for ycrtical accelerarion:\,;hilc carrying our rhe analysis for n.:rtical acceleration, rhc added'mass of warer for hydrodynamic effecrshall nor be considered.

6.5 Substructures. of Continuous Girder Superstructure6.5.1 'W'hen the superstructure of a multi-span bridge:: consists of a single continuous girder re:.ting ona restrained bearing (in longitudinal direction) over one of the piers and on sliding bearings O\"er theother piers, the design seismic force at the top of the substructures along the longitudinal direction ofrhe bridge shall be taken as follows:(a) For the pier supporting the restrained bearing, ir shall be the full elastic seismic force transmirrcd

from the superstrucrure to the top of the pier in the longitudinal direction di,-ided by the appropriareresponse reduction factor, assuming no friction between the other sliding bearings and rhecorresponding piers.

(b) For the other piers supporting the sliding bearings, it shall be the horizontal friction force genera redon the pier due ro the superstructure resting on the pier considering the maximum possible frictionbetWeen the sliding bearings and rhe top of the pier.

6.5.2 In trans':erse direction, the seismic force from superstruc::rure is to be transmirred ro rhesubstructures in proportion ro their lareral stiffness.

7.0 FOUNDA TIONS

7.1 In loose sands or poorly graded sands with lirtle or no fInes, vibrations due ro earrhquake maycause liquefaction or excessive total and differential settlements. For the sites of imporranr briJge~ inseismic zone III and of all bridges in seismic zones IV and V, liquefaction porential anal~'sis shall beconducted. If found necessary, remedial measures may be undertaken ro mitigare liquefaction porentialor the structural design of bridge shall take inro account the liquefaction effects.

Appendi..'( B provides a method for liquefaction analysis.

7.2 The foundations of all bridges in seismic zones IV and V and important bridges in zone III shallbe designed ro resist smaller of the following:(a) Design seismic forces obrained from clauses 3.3 or 4.4 using R value for foundations, and(b) Forces de,~eloped when o\Tersrrength plastic moment hinges are formed in rhe subsrrucrurc, as

described in secrion 9.

7.3 While considering the srabiliry of rhe substrucrure against overturning, rhe minimum facror ofsafery shall be 1.5 under simultaneous action of maximum elastic seismic forces in both horizontal and~'erricaldirections during the earthquake.K

8.0 CONNECTIONS

Elastomeric bearings (as referred to in IRC:83 (part II) - 1987) shall not be used for transferring in-plane horizontal seismic forces for all bridgt:s in seismic zones IV and V and for important bridgesin seismic zone III. 9

8 To be discussed by the comminee.9 Todiscussand fonnulatea revision

- C, C2 C] C-J .

0.1 0.-110 0.026 0.93./50.2 0.673 0.093 0.87120.1 0.832 0.18<1- 0.80130.4 0.922 0.289 0.75150.5- 0.970 0../03 0.69./50.6 0.990 0.521 0.63900.8 0.999 0.760 0.53201.0 ;.000 1.000 0../286

Drc~ji IRC6 ProIJiJ"ioll.r.!or 5 (i.lIIli,' De.r{~11~/BI7f(~eJ

8.1 Design Force for" Connections withinSubs tructure

8.1.1 The connections between adjacent sections of the superstructUre or between the supcrstructUreand the substructurc shall be designed to resist at least a horizontal seismic force in the restraineddirections cqual to 0.20 times the \'crtical dead load reaction at the bearing, irrcspccti\'C of thc number ofspans. .

8.1.2 Seismic Zones IV and V

The connections between the superstructUre and substrucrure, anJ the subsrrucrure andfoundation shall be designed to resist the smaller of the following:(a) I\faximum eL'\stic horizontal seismic force obtained from analysis and transferred through it in therestrained directions divided by the appropriate Response Reduction Factor R applic:lble to connections,which are giyen in Table 5, and(b) i\faximum horizont:ll forcc that develops \vhen o\'erstrength plastic moment hinges :lre formcd inthc substrucrure. The o\'crstrcngth plastic momcnt c:lpacity at thc critical scctions sh:lll bc c:llcubtcd asper clause C-5.2 in Appendix C.

P(~~e/6.oA

Superstructure and between Superstructure' a~d

8.2 Provisions to Account for Displacements at Connections where Motions are Permitted8.2.1 Separation Between Adjacent Units

When relative movement between two adjacent units of a bridge are desiglled to occur :It aseparation joint: sufficient clearance shall be provided between them, to permit the calculated relatiycmovement under design earthquake conditions to freely occur without inducing damage. Where the tWOunits may be out of phase, the clearance to be provided may be estimated as the square root of the sumof squares of the calculated displacements of the two units under maximum elastic seismic forccs gi\'cnb}' clauses 3.2 or 4.3.8.2.2 The detailing of non-critical srructlJral elemc!1i.s (e.g., deck movement joints) expected to be

damaged during the design seismic momenr should cater as far as possible for a predicrable modcof damage and provide for the possibility of permanent repairs.

8.3 Minimum Width of Seating at Supports of Superstructure on Substructure, or of theSuspended Span Portion on the Restrained Portion of the Superstructure

The widths of seating W (in 111m) at supports measured normal to the face of thcabutment/pier/pedestal of bearings/restrained portion of superstructure from the closest end of thegirder shall be the larger of the following:(a) 1.4 times the calculated displacement under the maximum elastic seismic forces estin1atcd as pcr

clauses 3.2 or 4.3, to account for uncerrainry in deflection calculation; and(b) the value specified below:

{

500 + 1.5L + 6H

W = 800 + 2.5L + 10H

for seismic zones II and III

for seismic zones IV and V

where .

L = Length (in meters) of the superstructure to the adjacent expansion joint or to the end ofsuperstructure, In case of bearings under suspended spans, it is sum of the lengths of the twoadjacent portions of the supersaucrure. In case of single span bridges, it is equal to thc length ofthc supcrstructure.

For b~arings at abutments, H is the average height (in 11Ietm) of all columns sL:pporring thcsuperstructure to "the next expansion joint. It is equal to zero for single span bridges. For bcarings atcolumns or piers, H is the height (in meten) of column or pier. For bearings under suspended spans, H isthe average height (in meters)of the two adjacent columns or piers.

Graphical representation of seating widths is shown in Figure 7.

9.0 SPECIAL DUCTILE DETAILING RE UIEMENTS FOR BRIDGE COMPONENTS

The design seismic force for bridgcs is lower than the maximum expected seismic force on them.However, to ensure good performance at low cost, the difference in the design seismic force and the

..

,,-

,

"

Dr4f IRC6 PIVI'lJ"iOIlJfor J('i.f//lil'D(,J(~11q! Bli{(~t'J P(~~e/7 oj:.l!/:::

maximum expected seismic force shall be accounteu for through additional safet)' prO\-isions. Thecapacity design provisions gi\'en under clause 9.0 shall be applicable to important bridges in seismic zoneIII and to all bridges in seismic zones IV and V. (fhese provisions are meant for briuges ha\'ingreinforced concrete subsU\.1ctUres; however, if steel substructUres are used in high seismic zones,reference should be made to specialist literatUre.) Appendix C uescribes the detailing procedure.

10.0 SPECIAL DEVICES

Special devices may be employed to improve the seismic performance of bridges.

10.1 Seismic Isolation Devices

In stiff bridge systems (generally, with funuamental natural. period less than 1.0sec), seismicisolatioG devices may be provided between superstructures and substructures, ~nd thereby thc seismicinertia force transmitted from the former to the latter may be rcuuccd. Howc\'er, specialist literatUreshall be referred ro for the design of such bridge systems anu of the de\-ices themsekes.

10.2 Shock Transxp.ission Units

!\fula-sp~n bridgcs with continuous sur.J~rsW:l.1cmremay bc prO\-idcd with rcstrained bcaringsover only one picrl abuanenr ro sl1:>r~~he seismic rClce generated at the supersuLlcrure to more than onepiers. Shock Transmission Units (STUs) may be introduced bctween the supersu'ucrun: and the otherpier(s)1 aburmenrs(s) where freel guided bearings arc used, to transfer some of the lateral seismic ineniaforce generated ar the supt:rstructUre level to those subsaucrurcs also. This may make the seismic designor the substrucrures economical. H()\\"c\'c~, specialist literature shall be consulrcu for the uetails of such::::TUs and their design in briJg<.:s subjcct<:u TOs<:ismic effects. STl's also facilitate the breathing of thl.:bridge due to rhermal anu shrinkage diccts.+-

10.3 Restrainers

To control excessi\'e dispbcements from causin.~ collapse of thl.: superstructUre spans, rcstrainl.:rs ma\" bl.:prm'ided as discussed under clause 5.0. Se<:.-\nncxurc B for typical Jctails.

II)Mr. Tanuon 10 provide alt~rnall.: Fig. X.

"

J)Jl~(tIRC6 PIVliJ"iOIlJfor J('i.f/J/i('D('.I~~11q! Bli{(~t'.r P((~e/7 0]:1.2:::

maximum expected seismic force shall be accounreu for through additional safet)' prm-isions. Thecapacity design provisions gi,-en under clause 9.0 shall be applicable to important bridges in seismic zoneIII and to all bridges in seismic zones IV and V. (fhese provisions are meant for briuges ha,-ingreinforced concrete subsu"Ucrures; howe,-er, if steel substrucrures are used in high seismic zones,reference should be made to specialist literarure.) Appendix C describes the detailing procedure.

10.0 SPECIAL DEVICES

Special devices may be employed to improve the seismic performance of bridges.

10.1 Seismic Isolation Devices

In stiff bridge systems (generally, with fundamental narural_ period less than 1.0sec), seismicisolatioG devices may be provided between superstrUcrures and substrucrures, ~nd thereby the seismicinertia force transrnitted from the former to the latter may be reduced. Howe,-er, specialist literatureshall be referred ro for the design of such bridge systems and of the de,-ices themseh-es.

10.2 Shock Transmission Units

l\Iult1-sp~n bridges with continuous sU,JCrSLll1cruremay be prO\-ided with restrained bearingso\'er only one pier/abutment to sh"r~ :he seismic rCLcegenerated at the supersu"Ucrure to more than onepiers. Shock Transmission Units (STUs) may be introduced between the supersu'ucrun: and the otherpier(s)/ aburmenrs(s) where free/guided bearings are used, to transfer some of the lateral seismic incniaforce generated ar the :mperstructure h~\'cl to those substructures also. This may make the seismic designof rhe substructures economical. H()\\'c\'e~, spccialisr lirerature shall be consulred for rhc details oi such~TUs and their design in briJgcs subjectcd to scismic efiecrs. STl's abo facilirate the breathing of rhcbridge due to thennal and shrinkage effecrs.+-

10.3 Restrainers

To control excessi\'e dispbcemenrs from causing colbpsc of rhe supersrructure spans, n:srrainers mar beprm'ided as discussed under clause 5.0. Sec .-\nnexure B ior typical derails.

'" Mr. Tandon 10provide alL~rnale Fig. X.

'I

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maximum expected seismic force shall be accounted for through additional safet)' prm'isions. Thecapacity design provisions gi\'en under clause 9.0 shall be applicable to important bridgl:s in seismic zoneIII and to all bridges in seismic zones IV and V. (fhese provisions are meant for bridges lUI\-ingreinforced concrete subsU"l.lctures; ho\\'e\-er, if steel substructures arc used in high seismic zones,reference should be made to specialist literature.) r\ppendix C describes the detailing procedure.

10.0 SPECIAL DEVICES

Special devices may be employed to improve the seismic performance of bridges.

10.1 Seismic Isolation Devices

In stiff bridge systems (generally, with fundamental natural. period less than 1.0sec), selsnllCisolatior.. devices may be provided between superstructures and substrUcrures, and thereby the seismicinertia force transmitted from the former to the latter may be reduced. Howe\'er, specialist literatureshall be referred to for the design of such bridge systems antI of the de\'ices themselves.

10.2 Shock Transmission Units

l\fu!Q-sp~n bridges \vith continuous surJ(~rs.:rl.lcruremay be pro\'ided \vith restrained bearingsoyer only one pieri abuQnenr to sh:;.r:c~he seismic rcrce generated at the supersuLlcrure to more than onepiers. Shock Transmission Units (STUs) may be introduced between the superstructure and the otherpier(s)/ abutlnents(s) where free/guided bearings are used, to transfer some of the lateral seismic inertiaforce generated ar the superstructure le\'c\ to those substructures also. This may make the seismic designot the substructures economical. Howe\'c::, spL'cialisr literature shall be consulted for rhe details of stich~Tl:s and their design in briJgcs subjecTcd to scismic diects. S'lTs also facilitate the breathing of rhebridge due to thenn:!l and shrinkage dtccts.+-

10.3 Restrainers

To control excessi\'e dispbccments from C:1Usin~collapse of the stlperstrucrure spans, n;srrainers n1:l~'beprm'ided as discussed under cl:!use 5.0. See .-\nnexure B for typic:!l details.

IIIMr. Tandon to provide alt~rnatl: Fig. X.

zI

r

I;--

.I/,

~'- -/11 '-.

.~... /

Bridge Plan Global X-Z axes zAirllv/{ ;f lvf~'( ,\;1;

16'//,~ ~/

~~ x' "...x I ' /

~ I :: l I/ ~~: /

, i";:;~ (Local x-x and ,-= axes) ~

..

:.,lomenrsfor ground motionalong X-axis

fv{oments 1'01'ground motionalong Z-axis

D~signMoments

Mx = .\I{ +O.3.\f.f,.\' Z

XIx =O..J,\Ix + '\/x

.\" I .... ,Z,\/_ =J/. -r-().~.!1-- - .

, '.r zAf- =().~,\.f- +:\/-

,,'here, AJyand AI;are absolute moments about local axes.

Figure 1: Combination of orthogonal seismic forces (clause 2.5.1).

-Type I (Rock, or H ;tru Soil

- Type II (i\[CUi:Ull Soil)

- Type III (SoftSoil)

-,

1.0 Fundamental Natural Period (Tt> 3.5 -t.O

F'

2 PI-B

.d FI

.b

'I'

F fi~Ni(rd (s), I ". ) ) '. J'

I'

1 J.

Igllre: or ot n ge . eXl I Ity actor \g) \'crsus l'lInd:tmcnra l'1:Hura Crill , ro )l.: LIse In

the ~cismic Cocfficient !\fcthod (claLls~3.1).

3.0

') -- _.J-2:!-v:::--

2.0.§-

Z:>1.5

,£;.;..: I.()

-.

:;L 0.5

..,..

0.0

0,0 n.s

Dr,!/i IRC6 Prol1isiollJfor S eisnllf DeJ~~11 f!f 13n'd..~eJ.,./

I.

Tr;Aunenc [)Irrcliun

Wu~

,n

~a.8W," , §o.8w,.!8

Positionat which theinertialforceof lhe

= superstructureacts~ ," i'" '

\i

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liE! "I \ ~1\

\I '-' 0'1~i I'

\ I.1-.t...

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Page 19"f1j1!L

LoDlihldl.al Dinctioo

Position at which theinertial force of the

~_ 'superstructureacts

n . i 1

~

'__ I

d1Direction perpendicular to bridge axis Bridge axis direction

Figure 3a: NatUral period calculation for simply supported bridges

..-~---_..-

(:,; Fi1clIsin; (In t:1C dif'~c(ion

:\.:[p:(}dicui~r to bridge a.\i~

--------------

w(s) ~, ( )p : u\s. I

;:) "~t;..J'...,;.J

H .

[j ,..

H,D.~-! 1

'-u

Figure 3b: NatUral period calculation model for multi-span integral bridges (clause 3.1.1)..

To be used for k =1 -1' pc I (Rock, or Hard Soil- T pc II (Mcdiam Soil)- T pc III (Soft Soil)

To be llsed for k > 1

,I\

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Natural Period (Tk)

Figure 4: Plot of Bridge Flexibility Factor for mode kl :J~, )

versus NatUral Period Tf to be used in() k

the Response SpeCITUni Method. (clause 4.1).

Figure 5: Hypothetical Enveloping Cylinders to Estimate Hydrodynamic Forces on SubstructUres dueto Seismic Shaking (clause 6.4). -

,,-...

3.0

V::

':::;: "5-.Q)

02.0

,...c

..2

.§ 1.5u'"ti.

q 1.0..t=-.;:::

0.5':Jt.:..."):I.

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0.0

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:C,H ~~ C}F .. I

(ReSt/llalll ~!pres.lllre OJ!

shaded area liP 10 depth C,H)I

IHIII

I

Figure 6: Hydrodynamic Pressure Distribution on the Subsrrucrun: due tl, :::~2::11.!:]O\\" (cbust: (>.4).

L

,..------I S!:Jb/Girderrt

Abutment

\V~

(a) Abutment

- L '.: 1.---L ;;, /1

Slab/Girder Slab/Girder

I Top of Pier

~\'\'2\'\'1 ~

(b) Column or Pier

L,

SuspendedRestr;lined Portion

\,' ~

(c) Suspended Span on Restrained Portion of Superst.uctureFigure 7: ~rinimum \,\:idth of Scaring of Spans on Supports (clausl: 83)

...

(a) Discominuous sUi>~~rstructurcs

~b) Cununu()u:> ;,upt.:r~tructurL:

Figure 8: Components of bridge partici~):H-jn~In carrying the lateral load for shaking in the longirudinaldirection with and without STUs in multi-span bridges. II

.1 Drt!fi JRC6 PIVI,iJioIlJ.!OrJeiJflliL"DeJ(gll 0/ 13li((~fJ

Appendix C

Ductile Detailing Specifications

C-O GeneralThe derailing rules giv'en haye been chosen with the intention that reliable plastic hinges should

form at the top and bottom of each pier column, or at the bottom only of a single stem pier underhorizontal loading and that the bridge should remain elastic between the hinges (Figure C-l). The aim isto achieve a reliable ductile strucrure. Repair 0f plastic hinges is relatively easy.

Design strategy to be used is based on assumption that the plastic rc~ponse will occur in thl:substrucrure. However, in case of a wall type substrucrure, the nonlinear beha,'iour may occur in thefoundation-ground system (Figure C-2).

C-l Specification ,C-l.l Minimum grade of concrete should be M25 (/;J:=25 MPa).

C-l.l Steel reinforcement of grade Fe 415 (see IS 1786: 1985) or less only shall be used. However, highstrength deformed steel bars of grades :fe 500, having elongation more than 14.5 percenr andconforming to other requiiemenrs of IS 1786 : 1985 may also be used for the reinforcement.

C-2 Layout(a) The use of circular column is preferred for better plastic hinge perfonnal.1ce and ease of

construction.

(b)The bridge must be proportioned and detailcd by the designer so that plastic hinges occuronly at the controlled locations (e.g., pier column ends) and not in other uncomrolled places.

C-3 Longitudinal ReinforcementThe area of the longirudinal reinforcemcnt shall not be less than Q._Bperccnt nor more than 6

percenr, of the gross cross section area A,(, Splicing of flexural region is not permitted in the plastic hingeregion.' Lap shall not be located \vithin a distance of 2 times the maximum column cross-sectional

dimension from the end at which hinging can occur. The splices should be proportioned as a 'tcnsionsplice.

C-4 Transverse ReinforcementThe transverse reinforcement for circular columns shall consist of spiral or circular hoops.

Continuity of these reinforcements should be provided by either (Figure C3.a and C3.b):(a) Welding, where the minimum length of weld should be 12 bar diameter, and the minimum weldthroat thickness should be 0.4 times the bar diameter.

(b) Lapping, where the minimum length of L"lpshould be 30 bar diameters and each end of the baranchored with 135° hooks with a 10 diameter cxtcnsion inra tht: confined core.

Splicing of the spiral reinforcemem in the plastic hinge region should be avoided.In rectangular columns, rectangular hoops may be used. .A recmngular hoop is a closed stirrup,

having a 135° hook with a 10 diameter extension at ea.ch end that is embedded in the confined core(Figure C3.c). When hoop ties are joined in any place other than a corner the hoop ties shall overlapeach other by a length 40 bar diameter of the reinforcing bar which makes the hoop ties with hooks asspecified above .

Joint portion of hoop ties for both circularand rectangularhoops should be staggered.

I S. K. Thakkar to give a clause on curtailment restrictions.

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C-S Design of Plastic Hinge Regions

C-5.1 Seismic Design Force for SubstructurePrO\'isions gi,'en in ,-\ppendix C for the ducrile derailing of RC members subjected ro seismic

forces shall be adopred for supporring componenrs of the bridge. Furrhcr, the design shear force ar thecrirical.secrion(s) of substrucrures shall be the higher of the following:(a) Maximum elasric shear force at rhe crirical secrion of the bridge componenr di,'ided by the response

reducrion facror for that componenrs as per Table 5, and(b) ~Iaximum shear force that deyelops wben the substrUcrure has maximum momenr that it can sustain

(i.e., the overstrength plasric momenr capacity as per clause C-5.2) in single-column or single-piertype substructure, or maximum shear force that is developed when plastic momenr hinges areformed in the substructure so as to form a collapse mechanism in multiple-column frame type ormultiple-pier type subsrmcrures. in which the plasric momenr capaciry shall be the ()\'ersrrengrhplastic momenr capacity as per clause C-5.2.

In a single-column type or pier type substructUre, the critical secrion is at the bottom of the column orpier as shown in Figure 7a. And, in multi-column frame-type substmctures or mulri-pier subsrmcrures,the critical sections are at the bottom and/or top of the columns/piers as shown in Figure 7b.

II,II

C~5.2 Overstrength Plastic Moment CapacityC.5.2.1 Limit State 'Method of Design

The overstrength plastic moment capacity at a reinforced concrete secrion shall be taken as 1.-1-times the ultimate momenr capacity based on the usual partial safety facrors recommended by relc\'anrdesign codes for materials and loads, and on the actUal dimensions of members and the acrualreinfcrcemenr detailing adopted.C.5.2.2 Working Stress Method of Design

The overstrength pl:1scic moment capacity at a section mar be taken as 2.1 times the designmomenr capaciry obtained using the permissible stresses for materials given in the relevant Indian codesof practice, and on the actual dimensions of the members and the actual reinforcement detailingadopted. The increase in permissible stresses given in clause 2.6.1 need not be considered for calculationof overstrength plastic momenr capacity.

C-5.3 Special Confining Reinforcement:

Special confIning reinforcement shall be provided at the ends of pier columns where plasrichinge can occur. This transyerse reinforcemenr should extend for a distance from the point of maximummomenr over the plastic hinge region over a length fr"The length 1"shall not be less than,

(a) 1.5 times the column diameter or 1.5 times the larger cross sectional di.tr1ension where yieldil1goccurs

(b) 1/6 of clear height of the column for frame pier (i.e when hinging can occur at both ends of rhe-~column) .

(c) 1/4 of clear height ofd1e column for cantilever pier (i.e when hinging can occur at only one endof the column)(d) 600 mm

C-5.4 Spacing of Trans\'erse Reinforcement

The spacing of hoops used as special confIning reinforcemenr shall not exceed(i) 1/5 times the least lateral dimension of the cross section of column,(ii) 6 times the diameter of the longitudinal bar,(iii)200 mm

The parallel legs of rectangular stirmps shall be spaced not more (han 1/,) of (he small/.'srdimension of the concrete core nor more than 350 mm centre ro cenrre. If the length of any side of thestirrups exceeds 350 mm,' across tic shall be proyided. i\lternari,-cly, O\'erlapping stirrups may beprovided within the column.

/

, '

\vhere

I MO =rhe sum of rhe oversrrengrh momenr capacicies of the hinges resiscing bter:1lloads, :1:;

derailed. In case of rwin pier this would be the sum of rhe overstrength momenr capacicies ar thc rop andbortom of the column. For single stem piers the oversrrength momenr capaciry at the bottom onlyshould be used (Figure C.4).

h =clear height of the column in the case of a"column in double CUl"varure;heighr to calcubtcd poinr ofconrra-flexure in the case of a column in single curvature.

Outside the hinge regions, the spacing of hoops shall not exc(~ed half the lea~t brer~! dimensionof the column, nor 300 mm.

C-6.2 Longitudinal reinforcement

The area of the longirudinal reinforcemenr shall not be less than U.01 or more than O.U(>,runc:;

the gross cross seccion area ./1,Lap shall not be located within a distance of :2 rimes the maximum column cross-secril>n:d

dimension from the end at which hinging can occur. The splices should be proporrioned as :1 rcn:;ionsplice.

C-7 Design of Joints:

Beam-column joinrs should be designed properly to resist rhe forces caused b~' axial load:;,bending and shear forces in the joining members. Forces in the joint should be determined b,'considering a free body of the joinr wirh the -forces on rhe joinr member boundanes rrorerl~'represented.

The joint shear srrengdi should be entirely prm'ided by rransverse reinforcemcnr. \\'herc rht.:joint is nor confined adequately (i.e. \vhere minimum pier and pile cap width is lcss (han (hrcc columndiameters) the special confU1ement requirement should be sacisfied.

C-5.5 Amount of Transverse Steel to Be Provided

1) The area of cross section, A,h' of the bar forming circular hoops or spiral, to be used as specialconfining reinforcement, shall not be less than

or,

whichever is the greaterwhere

/1.1, =area of the bar cross section,

S =pitch of spiral or spacing of hoops,Dk =Diameter of core measured to the outside of the spiral or hoops

11. =characteristic compressive strength of concrete.!r= yield stress of steel (of circular hoops or spiral)/1~= gross area of the column cross section

- TC0

'/1 =;\rea at the concrete core =- ;:

, 4 ~

2) The ((>tal area of cross-section of the bar forming n:ctangular hoop and cross ties, ./1d. to be used asspecial confining reinforcement shall not be less than

[

A

]

.

_? -L _ j ckA,,], - 0._4511 1.0Ac Iy

.(.kOf, As], = 0.096511-

III

Ii = longer dimen'sion of the rectangular confining hoop measured to its ourer face. I t should notexceed 300 mm

.' 1t- = ..\rea of confined core concrete in the rectangular hoop measure to its ourer sidedimensions.

Note: Cross ties where used should be of the same diameter as the peripheral hoop bar and .-1; shall bemeasured as the overall core. area, regardless the hoop area. The hooks of cross ties shall engageperipheral longitudinal bars

C-6 Design of Components between the Hinges

Once the position of the plastic hinges has been determined and these regions detailed ro ensurea ductile performance, the structure between the plastic hinges is designed considering the clp:H.:ity ofthe plastic hinges. The intention her~ is:

(i) To reliably protect the bl~dge against collapse so that it will be a,'ailable for sef\'ice after amajor shaking.

(ii) To localize structural damage to the plastic hinge regions where it can be controlled andrepaired.

The process of designing the structure between the plastic hinges is known as "capacity design".

C-6.1 Column Shear and Transverse Reinforcement

To avoid a brittle shear failure design shear force for pier shall be based on O\'erstrcngth momentcapacities of the plastic hinges and given by:

-.--.-.

Drcifi IRC6 ProlJi.riollJjor Seismic Design of Bridges'1..1--

PageJ.9~

EarthquakeForce

-+1

EarthquakeForce

Column Cap

Potential P

Hinge Reg.

A~Elevation Section AA

(a) Single column or pier type substructures

A~

CJ .EarthquakeForce

Potential Plastic

Hinge Regions

-.. - ..-

::m~~~~~':-:".::::::;:::~,,:::

:~~:::::::;:::::::::..............

~mf:~;

~~~~~~ID

A~Elef-'ation SectionAA

(b) multi-column or frame type substructuresFigure C-l: Potential location of plastic hinges in substructures (clause C-O).

-I

JJ

'ClJ'tic'ons

}i........... -'-

MI I

'-- '--

Drcift IRC6 Provisiol/J)or Seisllli" DeJig1/ q[Bridges Page -!fD..,.

Horizontal capacity

Subsidiary nonlinearity

Subsidiarynonlinearity-- ~

Bending damage at base of pier

- - Principal plastic hinge

- Subsidiary nonlinearity

(a) When a principal plastic hinge is formed at the base of a pier

Horizontal capacity

Principal nonlinearity

(b) When seismic isolation design is used

Horizontal capacity

Subsidiary nonlinearicy

Nonlinearicy' of foundation-ground system

(princi pallinc:arity)

(c) When principal nonlinearity OCCUISin the foundation-ground system

Figure C-2: Nonlinearity assumed in the ductility design method

.... . .-- _..,-

Draft IRC6 ProviJiowfor S eislllic DeJZgll of BridgeJ

'1/1Page~

(a) Welding in Circular hoops (b) Lapping in circular hoops

(c) Rectangular hoopsFigure C-3: Transverse reinforcement in column (clause C-4)

CDKwlo,W1" 0/ t;/CI1nt~ltr.UI''' ~._ - ~ 'I"

In CQ'","""__ ~ . /

Figure C-4: Shear in columns (clause C-6.1)