Methodology for Seismic Vulnerability Assessment of Building Stock in Mega Cities

Transcription

1 A Workshop on Mcrozonaton Interlne Publshng, Bangalore Methodology for Sesmc Vulnerablty Assessment of Buldng Stock n Mega Ctes *Shalesh Kr. Agrawal and **Ajay Chourasa Scentst, Central Buldng Research Insttute, Roorkee , *agrawal_shaleshkr@yahoo.com; **ajayapc@yahoo.com Abstract The damage to bult envronment durng recent earthquake n Inda has demonstrated the need for sesmc rsk assessment that s capable of predctng the consequences of earthquakes. The collapse of man-made engneered and non-engneered buldngs durng an earthquake s the chef contrbutor to the loss of lves and njures to the people. Vulnerablty Atlas of Inda states that there are about 11 mllon sesmcally vulnerable houses n sesmc zone V, whle the correspondng fgure for sesmc zone IV s 50 mllon. In all, there are about 80 mllon buldng unts n Inda, whch are vulnerable, and pose unprecedented rsk, f earthquake strkes. The greatest challenge, therefore, s not only to rehabltate these vulnerable houses so as to reduce consderable loss to human lfe and property but also to evolve an accepted methodology n Indan context to estmate/quantfy the sesmc vulnerablty of the exstng bult envronment, whch wll be provde a useful nformaton for polcy makng. The paper proposes an approach to estmate sesmc vulnerablty of exstng buldngs of a cty n Indan context. The scheme estmates sesmc vulnerablty of exstng buldng stock quanttatvely and qualtatvely. The quanttatve approach covers demand-capacty computaton, whle qualtatve procedure estmates structural scores based on natonal & nternatonal state-of-the-art procedures vz. Rapd Screenng Procedure (RSP). The methodology presented would lead to dentfy buldngs that mght pose rsk n the event of damagng earthquake and would form an ntegral part of mcrozonaton studes beng taken up for Indan vulnerable ctes. Preamble The rapd growth of Indan ctes n the recent past, have accelerated pressure on housng ndustry, especally n hgh sesmc zone.e. Zone-IV & V [16]. The bult envronment n these zones have been sesmcally found vulnerable as most of these constructon are wthout earthquake resstant measures. The Indan ctes are dotted wth all knds of buldngs and nfrastructural facltes comprsng of very good constructon to poorly desgned & constructed ones. The most challengng task s to evaluate sesmc safety of these constructons and take necessary steps for ther retrofttng so as to protect them from future earthquakes. Assessment of sesmc vulnerablty of exstng buldng stock n urban areas would help n dsaster mtgaton and management by plannng mtgaton measures before an earthquake strkes. Sesmc vulnerablty s a measure of the sesmc strength or capacty of a structure [14], hence t s found to be the man component of sesmc rsk assessment. The revew of the bult envronment for sesmc vulnerablty estmaton s normally carred out n the lght of earthquake resstance of buldngs, past earthquake damage hstory & repar thereof, constructon practces beng adopted, buldng typology, sesmc zonng of the area, buldng samples, detaled survey of selected buldngs, and creaton of database and ts quanttatve and qualtatve analyss. The quanttatve approach covers demand-capacty (DCR) computaton, whle qualtatve procedure estmates structural scores for buldngs and s known as Rapd Screenng Procedure (RSP).

2 Methodology for Sesmc Vulnerablty 183 Buldng Typology The exstng buldng stock of Indan ctes s a rch mx of several dfferent buldng types & constructon technologes. The most commonly used buldng typology are (1)renforced concrete frame buldng wth nfll brck walls (Type-C); (2)brck masonry buldngs wth renforced concrete roofs and usng cement mortar n most of the case and mud/lme mortar n few of buldngs (Type-B); (3)buldngs made of GI sheets, thatch and other lght weght and cheaper materals (Type-A). The frst two categores, fall under engneered constructon n whch assstance from qualfed structural engneers are sought at each stage n most of the cases, whle the thrd category s governed wth soco-economc consderaton rather than engneerng one and falls under non-engneered constructon. Sesmc Vulnerablty Assessment In order to estmate sesmc vulnerablty of exstng buldng stocks on a wde spread area, t s mperatve to make several assumptons for selecton of buldng samples, analyss and determne the damage levels. The present approach assumes the followng: It s concevable that the study area would experence earthquake wth damage greater than ntensty VII+, n zone IV & V, the level assumed by the IS-1893:2002 (Part-I) for desgn purpose. [16] The buldng stock under consderaton s expected to behave as per known performance of each type of structure under earthquakes. The consequences of an earthquake causng buldng damage due to other dsasters such as fre, floods are gnored. For effectve categorzaton, varous buldng typologes assumed are : Type-A: Buldngs n feldstone, rural structures, unburnt brck house, clay houses; Type-B: Ordnary brck buldng, buldngs of the large block and prefabrcated type, half-tmbered structures, buldng n natural hewn stone; Type-C: Renforced buldng, well bult wooden structures. [16,18] It s well ngh possble to carry out survey of all exstng buldngs from sesmc vulnerablty pont of vew. In order to restrct buldng survey on lmted buldngs of study area, far number of representatve buldng samples of dfferent types are selected from varous pockets / colones for detaled nvestgatons. The varous pockets are dfferent muncpal wards as delneated by muncpal authortes. Approprate, structural scores have been assgned for varous sesmc vulnerablty parameters, based on the performance of dfferent types of buldng n past earthquake n Indan context. The structural scores have been derved after valdaton of ts results on the buldng sample consdered for dervng sosesmal map by dfferent agences after Jabalpur earthquake of [23,24] Methodology for Assessment of Sesmc Vulnerablty Exstng buldngs can become sesmcally defcent snce sesmc desgn code requrements are constantly upgraded and there s contnuous advancement n engneerng knowledge. Indan buldngs bult over past two decades are sesmcally defcent because of lack of awareness regardng sesmc resstng measures. Also sesmc desgn s not normally practced n most of the buldngs beng bult. In Inda. Therefore, sesmc vulnerablty estmaton s pre-requste for dsaster mtgaton & management. Vulnerablty estmaton s a complex process, whch has to take nto account not only the desgn of buldng but also the deteroraton of the materal and damage caused to the buldng, f any. The dffcultes faced n sesmc vulnerablty estmaton of a buldng are manfold. There s no relable nformaton/database avalable for exstng buldng stock, constructon practces, n-stu strength of

3 184 Mcrozonaton materal and components of the buldng, and therefore, sesmc vulnerablty estmaton manly reles on set of general evaluaton statements. For earthquake load defnton, ground moton parameters avalable n present code (IS: ) can be taken, f ste dependent accentuatons are not avalable for the area. As regards the effect of local sol condtons, whch are known to greatly modfy the earthquake ground moton, and data gven n code on ste response wll have to be consdered. The quanttatve approach, outlnes here covers demand-capacty computaton prmarly based on ATC-40, 1996 [10,13,14], whle qualtatve procedure estmates structural scores based on natonal & nternatonal state-of-the-art procedures vz. Rapd Screenng Procedure (ATC-21, 1988, ATC-21-1, 1988) [7,8]. The general procedures for sesmc vulnerablty estmaton of exstng buldngs proposed are ste vst & data collecton; selecton & revew of evaluaton statements; follow-up feldwork; and analyss of buldngs by quanttatve and qualtatve approach. Quanttatve Approach: Demand-Capacty Approach The approach s a comparson between some measures of demand that the earthquake places on a structure to a measure of capacty of buldng to resst. The Demand/capacty rato (DCR), thus evaluated s measure of earthquake resstance of a buldng. The DCR less than unty ndcate the buldng s safe for respectve stresses under consderaton. However, any DCR exceedng one, ndcates that buldng s vulnerable to earthquake loads as defned n IS: DCR computaton for masonry and RC buldngs are dscussed hereunder. DCR Computaton for Masonry Buldngs Demand All buldng components under evaluaton should be able to resst the effects of the sesmc forces as prescrbed n IS [16] The desgn sesmc base shear (V B ) calculated as per codal provsons s the basc sesmc demand placed on the structure by sesmc ground moton n a partcular zone. Desgn Sesmc Base Shear The desgn sesmc base shear s the total desgn lateral force at the base of a structure. The total desgn lateral force or sesmc base shear (V B ) along any prncpal drecton shall be determned by the followng expresson V B = A h x W Where, A h s desgn horzontal acceleraton spectrum value, calculated from the followng equaton Z I Sa Ah = 2R g where Z = Zone factor, 0.24 for Delh (Zone-IV) I = Importance factor R= Response reducton factor S a /g= Average response spectra coeffcent, based on tme perod, and W = Sesmc weghts of all the floors of buldng. The detaled explanaton s gven n IS (Part-I). Fundamental Natural Perod The approxmate fundamental natural perod of vbraton (T a ), n second for moment resstng frame buldngs wth brck nfll panels may be estmated by the emprcal expresson: 0.09 h Ta = d

4 Methodology for Sesmc Vulnerablty 185 where, h = heght of buldng, n meters and d = base dmenson of buldng at plnth level, n m along the consdered drecton of the lateral force. The desgned base shear computed (V B ) wll be dstrbuted along the heght of the buldng as per the followng expresson: 2 Wh Q = VB n 2 W h where, Q = desgned lateral force at floor W = sesmc weght of floor, h = heght of floor I measured form base, and n = number of storeys n the buldng s the number of levels at whch the masses are located Average Shear Stress The average shear stress (V avg, ) of a buldng at any level s derved by followng expresson: Q v avg, = Aw where, Q = storey shear at the level under consderaton determned = = 1 A w = summaton of the horzontal cross sectonal area of all shear walls n the drecton of loadng. The wall area shall be reduced by the area of any openngs. Drect & Bendng Stresses The total horzontal shear (Q ), at any level be dstrbuted n varous walls n proporton to ther shear stffness. Ths shear causes bendng moment equal to (Q h / 2) at the top and bottom secton of the wall where h s the heght of wall. The bendng moment wll cause bendng stress ( σ b, ) n addton to the drect stress ( σ ) due to dead load of the buldng above the floor level under consderaton [17]. d, σ ± t / c, = σ d, σ b, Overturnng Moment The total horzontal shear (Q ) also causes overturnng moment (M o ) n the walls, whch s equal to (Q h / 2) at the bottom of wall, as per clause of IS: , whereas free standng walls shall be checked aganst overturnng under the acton of desgn sesmc force allowng for a factor of safety of 1.5. The factor of safety aganst overturnng s gven as under: M s FOS = M o where, M s = stablzng moment due to dead load at the bottom of wall Capacty The capacty of a masonry wall s ts allowable stress dependng upon mortar type n accordance wth relevant codal provsons (IS: ).

5 186 Mcrozonaton Shear Stress The permssble shear stress calculated on the area of bed jont shall be n accordance wth clause of IS: , and s reproduced here under: [17] f =.1+ f / 6 s 0 d where, f d = compressve stress due to dead loads Permssble Compressve & Tensle Stress The permssble compressve (f b ) stresses shall be based on the values gven n Table-8 of IS: , after multplyng wth approprate reducton & modfcaton factors (clause 5.4, IS: ). The permssble tensle stress (f t ) shall be as per clause 5.4.2, IS: Demand Capacty Rato (DCR) Based on above, DCR computaton for varous parameters vz. shear stress, compressve & tensle stress, and overturnng are to be performed for assessng vulnerablty of masonry buldng as under: DCR Computaton for RC Buldngs V DCR ShearStress = f DCR Tensle DCR Compressve avg, σ Stress = f s t, σ Stress = f 1.5 DCR Overturnng = FOS The proposed DCR computaton for RC buldngs comprses of two stages: Input Data Stage: Step 1: Study of sol condtons at the ste. Step 2: Measurements of actual geometry of buldng and ts components. Step 3: Non-Destructve Testng (NDT) to estmate actual strength of concrete n the buldng components, f possble Step 4: Tests to estmate extent of corroson to carefully estmate ther avalable dameters and verfy the sze, number and spacng of renforcng bars, f possble. Analyss Stage: Step 5: Preparaton of 3-D model of buldng frame, usng measured geometry, and materal propertes. [22] Step 6: Estmaton of desgn lateral force on buldng usng IS (Part-1) for 5% dampng. Step 7: Applcaton of desgn lateral force on 3-D buldng model to determne stress-resultants (.e. axal forces, shear forces, bendng moments) n the beams & columns and determnaton of nter storey drfts. t c, b

6 Methodology for Sesmc Vulnerablty 187 Step 8: The most crtcal stress resultants thus obtaned for the varous RC members are the demands for the members Step 9: Determnaton of RC member flexural & shear capactes wth actual cross secton geometry & materal propertes as per IS and Demand/Capacty ratos of RC members at crtcal locatons. [5,15,16,22] The DCR less than unty of any member ndcate that the member s safe, however, any DCR exceedng one, ndcates that the member s vulnerable. DCR Computaton for Non-Structural Elements : Parapet Walls Non-structural fallng hazards such as chmneys, parapets, cornces, veneers, and overhangs can pose lfe safety hazards f not adequately anchored to the buldng. The maxmum heght of unbraced URM parapet above the roof should not exceed 1.5 tmes the thckness of parapet wall. Accordngly, DCR for parapet walls can be computed by comparng heght wth 1.5 tmes thckness of parapet wall. Qualtatve Approach : Rapd Screenng Procedure (RSP) The Rapd Screenng Procedure (RSP) [7,8] s amed for dentfyng potentally hazardous buldngs n the study area, wthout gong nto detaled analyss. RSP utlzes a methodology based on vsual nspecton of a buldng and notng the structural confguraton. The methodology begns wth dentfyng the prmary structural lateral load resstng system and materals of the buldng. The method generates a Structural Score S, whch conssts of a seres of scores and modfers based on buldng attrbutes that can be seen durng buldng survey. The Structural Score S s related to probablty of the buldng sustanng lfe-threatenng damage should a severe earthquake n the regon occur. A low S score suggests that the buldng s vulnerable and needs detaled analyss, whereas a hgh S score ndcates that the buldng s probably safe for defned earthquake loads. Thus, the expresson for structural score s: S (Structural score) = BSH (Basc Structural Hazard) + PMFs (Performance Modfcaton Factor) Basc Structural Hazard (BSH) & Performance Modfcaton Factors (PMFs) Each buldng type s assgned wth basc structural hazard (BSH) score. Ths BSH reflects the estmated lkelhood of a typcal buldng of that category sustanng major damage gven ts sesmc envronment. ATC and ATC , presents BSH for varous buldng types applcable to state of Calforna. These scores have been sutably modfed n Indan context, based upon 1997 Jabalpur earthquake damage survey data. [12,23,24,25] These values have been determned so that the sesmcally good buldng has a hgh value, and a potentally weak/hazardous buldng has a low value. The BSH scores estmated for Type-A, Type-B, and Type-C are 2.0, 2.5 and 3.0 respectvely. In order to arrve at fnal structural score S for the buldng under revew, a seres of Performance Modfcaton Factors (PMFs) are subtracted from BSH. These PMFs account for sgnfcant factors such as hgh rse, qualty of constructon, vertcal & plan rregulartes n the structural system, soft storey, poundng, claddng, sol/ground condton and ambence, that can negatvely affect a buldng s sesmc performance or adequacy. There are number of factors that can modfy the sesmc performance of a structure causng the performance of an ndvdual buldng to dffer from the average. These factors bascally are related to sgnfcant devatons from the normal structural practces or condtons, or have to do wth the effects of sol amplfcaton on the expected ground moton. The number and varety of such performance modfcaton factors, for all types of buldngs s very large. However, based on experence ganed durng the damage survey n past earthquakes, a lmted number of the most sgnfcant factors were dentfed. These PMFs (Table-I) were assgned values based on judgment such that when subtracted to BSH, the resultng modfed score would approxmate the possblty of major damage.

7 188 Mcrozonaton Table-I Performance Modfcaton Factors Modfers Descrpton Modfcaton Factor Hgh Rse Upto 2 storey 0 Between 3-7 storey -0.2 More than 7 storey -0.5 Qualty of Hgh 0 Constructon Medum Vertcal Irregularty Soft Storey Plan Irregularty Poundng Claddng Sol Condton Ground Condton & Slope Ambence Low Steps n elevaton, nclned walls, dscontnutes n load path, buldng on hlls Wthout vertcal rregularty 0 Open on all sdes of buldngs, tall ground floor, buldngs on stlts Wthout soft storey 0 L, U, E, T, or other rregular buldng shape Wthout plan rregularty 0 Floor levels of adjacent buldngs not algned and less than mm of separaton per storey Wthout poundng 0 Many large heavy stone or concrete panels, glass panels and masonry veneer do not qualfy Wthout vertcal rregularty 0 Buldngs founded on rocks (SR) 0 Buldngs founded on cohesonless sol (SC) -0.3 Buldngs founded on black cotton sol (BC) -0.6 Buldngs n flat/plan land doman 0 Buldngs on hll slopes/tank bunds/reservor rms wth slope > 10 o - gentle -do- - moderate do- - steep Generally speakng f a buldng s structural score S s less than 2, then the sesmc performance of buldng may not meet the codal requrements. Hence, such buldngs are classfed under vulnerable buldngs n the present study recommended for detaled nvestgatons. Dscusson The pragmatc methodology for determnstc vulnerablty analyss of bult envronment has been attempted here wth an objectve to generate sesmc vulnerablty map of buldngs of Indan Ctes. Intensve feld survey can be conducted by fllng a comprehensve questonnare. The sample buldngs are qualtatvely explored for ascertanng PMFs pertanng to specfc structure, sol & ground ambence. As regards to quanttatve estmaton of vulnerablty of buldngs, emprcal methods as stated n IS: & IS: have been followed. In order to evaluate demand placed on structure vs-à-vs structural capacty, Demand Capacty Rato (DCR) on varous parameters lke shear stress, compressve/bendng stress, overturnng of walls, and damage to non-structural members attrbutng to possble falures vz. excessve crackng, fallng of walls, fallng hazard, and ts combnaton thereof, are calculated for Type B structures. Smlarly, DCRs n terms of flexure, shear, fallng hazard of non-structural member (parapet wall), attrbutng to possble falures vz. excessve crackng, dagonal crackng, fallng hazard, and ts combnaton thereof, are computed for Type-C structures. The authors experence based on the sesmc vulnerablty analyss of buldngs carred out n Delh and Jabalpur ctes ndcate that dwellngs of Type-A category, are constructed based on soco-

8 Methodology for Sesmc Vulnerablty 189 economc consderaton and lack sesmc resstant measures and deemed to be sesmcally vulnerable. As these dwellngs mostly constructed of lght weght materal for roofng, t would result n relatvely low casualty, even after ther falure. The vulnerablty analyss for masonry & RC buldngs can play an mportant role n selecton of strateges for repar/rehabltaton of exstng buldng of a cty. In order to reduce the consequences of a major earthquake n Indan ctes, t s necessary that approprate structural as well as non-structural measures be undertaken n buldng constructon. [13,14,19,20,21] The structural mtgaton measures are those that drectly nfluence the performance of buldng stock through strengthenng of code provsons and the prevalent constructon practces. The vulnerablty of any buldng type can be reduced by ncorporatng the approprate structural mtgaton measures. The non-structural mtgaton measures nclude mprovement n the state of preparedness before a dsaster. Further, the mpact of ntroducton of strengthenng measures wll not only affect the new constructons that are desgned and bult n future but also assst n reducng rsk assocated wth exstng buldng stock. Concluson The paper presents methodology for estmaton of sesmc vulnerablty of Indan Cty takng nto account as bult nformaton, prevalent constructon practces, materal of constructon, qualty/workmanshp of constructon, types of buldngs, ambence, geologcal / geotechncal parameters and s based on ground realtes. The developed procedure can be readly appled to any urban regon of our country n order to assess the necessty of more detaled nvestgaton for earthquake damage scenaro predcton. The methodology can be mplemented to estmate sesmc vulnerablty of dfferent types of exstng buldng stock n Indan Ctes and further extended to nvestgate the mpact of mtgaton measures on the consequences of an earthquake. Based on the vulnerablty analyss, rsk n the form of casualtes and economc losses can further be estmated after collectng wardwse demographc and census nformaton for a cty. Further, the vulnerablty studes demands specal attenton wth reference to hertage/monumental buldngs, lfelnes lke ral/road, water supply, electrc supply, sewage, communcaton, dams, hosptals & schools, vulnerable ndustres. Also there s a need to dentfy safe zones/domans/structures and secure routes to work as a relef centers and relef dsperson on ncdence of future dsaster. References 1. Arya, A. S., and Kumar, A., (1982), An Earthquake Resstant as well as Economcal Brck Buldng System, Seventh Symposum on Earthquake Engneerng, Roorkee, Inda. 2. Agrawal, S.K., Chourasa, Ajay, and Parashar, J. (2004), Estmaton of Sesmc Vulnerablty of Buldngs n Delh, DST Report, CBRI, Roorkee. 3. Agrawal, S.K., Chourasa, Ajay, and Parashar, J. (2003), Sesmc Vulnerablty of Jabalpur Urban Area, DST Report, CBRI, Roorkee. 4. Agrawal, S.K., and Chourasa, Ajay, (2006), Estmaton of Sesmc Vulnerablty of Buldngs of Delh Muncpal Area, 1 st Inda Dsaster Management Congress, Nov.28-30, 2006, New Delh. (n prnt) 5. Agrawal, S.K., Chourasa, Ajay, and Parashar, J. (2002), Sesmc Evaluaton & Retrofttng of Exstng Buldngs, 12 th Symposum on Earthquake Engneerng, Indan Insttute of Technology, Roorkee. 6. Agrawal, S.K., Chourasa, Ajay, Parashar, J and Dutta, J. (2002), Expermental Investgaton on Earthquake Resstance and Retrofttng Measures of Masonry Houses, Advances n Concrete & Constructon Technology, Publcaton 3, Interlne Publshng, Bangalore. 7. ATC-21, (1988), Rapd Vsual Screenng of Buldngs for Potental Sesmc Hazards: A Handbook, Appled Technology Councl, Redwood cty, CA, USA.

9 190 Mcrozonaton 8. ATC-21-1(1988), Rapd Vsual Screenng of Buldngs for Potental Sesmc Hazards: Supportng Documentaton, Appled Technology Councl, Redwood cty, CA, USA. 9. ATC-28, (1991), Development of Recommended Gudelnes for Sesmc Strengthenng of Buldngs Phase: Issue Identfcaton and Resoluton, Appled Technology Councl, Redwood cty, CA, USA. 10. ATC-40, (1996), Sesmc Evaluaton and Retroft of Concrete Buldngs, Vol. 1, Appled Technology Councl, Redwood cty, CA, USA. 11. CBRI Report No. S(S)-008 (2002), Earthquake Behavour of Masonry and Prefabrcated Buldngs, CBRI, Roorkee. 12. CBRI Report (1994), Plot Project on Repars & Strengthenng of Earthquake Damaged Houses n Maharashtra, CBRI, Roorkee. 13. FEMA , (1997), NEHRP Gudelnes for the Sesmc Rehabltaton of the Buldngs, JRS/J.A. Blume & Assocates, Engneers, San Francsco, CA FEMA , (1992), NEHRP Handbook for the Sesmc Evaluaton of Exstng Buldngs, Buldng Sesmc Safety Councl, Washngton, DC. 15. IS: , (2000), Code of Practce for Plane and Renforced Concrete, Bureau of Indan Standards, New Delh. 16. IS: , (2002), Crtera for Earthquake Resstant Desgn of Structure, Bureau of Indan Standards, New Delh. 17. IS: , (1987), Code of Practce for Structural Use of Unrenforced Masonry, Bureau of Indan Standards, New Delh. 18. IS: , (1993), Code of Practce for Earthquake Resstant and Constructon of Buldngs, Bureau of Indan Standards, New Delh. 19. IS: , (1993), Improvng Earthquake Resstance of Earthen Buldngs-Gudelnes, Bureau of Indan Standards, New Delh. 20. IS: , (1993), Improvng Earthquake Resstance of Low Strength Masonry Buldngs- Gudelnes, Bureau of Indan Standards, New Delh. 21. IS: , (1993), Repar and Sesmc Strengthenng of Buldngs-Gudelnes, Bureau of Indan Standards, New Delh. 22. STAAD.Pro 2003, Structural Analyss & Desgn Software, Research Engneers Internatonal. 23. Mshra, P.S. et al (2000), GSI Specal Publcaton No. 51, Jabalpur Earthquake 22 May A Geoscentfc Study, Geologcal Survey of Inda, Calcutta. 24. Mshra, P.S. et al (2001), Base Level Sesmc Hazard Mcrozonaton of Jabalpur Urban Area, MP, Natonal Symposum on Role of Integrated Development & Related Issues, GSI Specal Publcaton No. 65(), Calcutta, pp Vulnerablty Atlas of Inda (1999), Buldng Materals & Technology Promoton Councl, New Delh.