Seismic Risk Assessment Using Different Soil Conditions

Transcription

1 Canadian Journal on Environmntal, Construction and Civil Enginring Vol. 3, No. 1, January 2012 Sismic Assssmnt Using iffrnt Soil Conditions Md Aftabur Rahman 1*, Md Shajib Ullah 1 Abstract With rapid population growth ovr th past 30 yars and frqunt sismic activity during this priod, th unchckd dvlopmnts of th built nvironmnt in Bangladsh hav bn rsultd in toughnss to arthquak. Idntifying structurs that hav high vulnrability is of critical importanc for both rliabl loss stimation as a rsult of an xpctd stratgic arthquak and stting priority critria for strngthning of thos buildings. In this study building assssmnt allowing for diffrnt sit conditions basd on statistical mthod as dmonstratd in Istanbul, Turky has bn considrd to valuat sismic vulnrability of rinforcd concrt structurs. In ordr to xtrapolat this risk assssmnt procdur to othr rgions that ar likly to b subjctd to major arthquaks, a Cut off valu is dtrmind which dpnds on th soil condition. Th cutoff valus rcommndd ar considrd to b valid for damaging arthquaks and th rgions that hav similar distanc to sourc and sit conditions to that of 1999 uzc arthquak. To apply th procdur to th sits, which hav diffrnt distanc to sourc and soil proprtis than uzc, modifications to th cutoff valus hav bn computd. Th sits ar classifid basd on shar wav vlocity of th soil. iffrnt soil sits wr considrd using diffrnt shar wav vlocity of th soil. Finally vulnrability of RC structurs was dtrmind for diffrnt sit conditions basd on shar wav vlocity of soil. Kywords RC structurs, vulnrability, discriminant analysis, shar wav vlocity, cutoff valu I. INTROUCTION Earthquak occurs whn nrgy stord within th arth, usually in th form of strain in rocks suddnly rlass. 1* Lcturr, partmnt of Civil Enginring, Chittagong Univrsity of Enginring & Tchnology, Chittagong-4349, Bangladsh. aftabur028@yahoo.com 1 Lcturr, partmnt of Civil Enginring, Chittagong Univrsity of Enginring &Tchnology, Chittagong-4349, Bangladsh. ngrshajib101@yahoo.com Th nrgy is transmittd to th surfac of th arth by wavs. Most arthquaks ar minor trmors, whil largr arthquaks usually bgin with slight trmors, rapidly tak th form of on or mor violnt shocks and nd in vibrations of gradually diminishing forc calld aftrshocks. Earthquak is a form of nrgy of wav motion, which originats in a limitd rgion and thn sprads out in all dirctions from th sourc of disturbanc. Sismic wavs gnratd by an arthquak sourc ar commonly classifid into thr main typs. Th first two, th P and S wavs ar propagatd within th arth, whil th third consisting of Lov and Rayligh wavs, is propagatd along its surfac. P wavs ar th first to rach any point on th arth s surfac. At all distancs from th focus, th mchanical proprtis of th rocks, such as incomprssibility, rigidity and dnsity, play an rol in th spd with which th wavs travl and th shap and duration of th wav trains. Th layring of rocks and th physical proprtis of surfac soil is also affcting ths charactristics of th wavs. In most cass, lastic bhavior occurs in arthquaks, but th shaking of surfac soils from th incidnt sismic wavs somtims rsults in nonlastic bhavior, including slumping and th liqufaction of sandy soil. Whn a sismic wav ncountrs an intrfac or boundary that sparats rocks of diffrnt lastic proprtis, it undrgos rflction and rfraction. Thr is a spcial complication if a convrsion btwn th wav typs occurs at such a boundary: ithr an incidnt P or S wav can yild in gnral rflctd P and S wavs and rfractd P and S wavs. Boundaris btwn structural layrs also giv ris to diffractd and scattrd wavs. Ths additional wavs ar in part rsponsibl for th complications obsrvd in ground motion during arthquaks. Th arthquak risk at any location dpnds on th sismic hazard as wll as th vulnrability of its structur. Th sismic hazard valuation considrs th liklihood of arthquak of a particular magnitud or intnsity affcting a sit. Sismic vulnrability, on th othr hand, dpnds on th construction practic in th ara and quality of building and soil condition. Evry damaging arthquak raffirms th importanc of sismic hazard and risk 47

2 analysis for stimating th consquncs of an arthquak. Hr hazard mans a thratning vnt, or th probability of occurrnc of a potntially damaging phnomnon within a givn tim priod and ara. mans xpctd losss du to a particular hazard for a givn ara and rfrnc priod. Basd on mathmatical calculations, risk is th product of hazard and vulnrability. In viw of th arthquak hazard mitigation, various organizations in th arthquak thratnd countris hav com up with documnts which srv as guidlins for th assssmnt of th xpctd prformanc and safty of xisting building as wll as for carrying out th ncssary rhabilitation. In USA, stpwis sismic vulnrability valuation procss is proposd. Th valuation stps ar namly rapid visual scrning, valuation in dtail and nginring valuation. Ohkubo and Otani dscrib Japans standard for th valuation of th sismic vulnrability of th xisting low-ris rinforcd concrt buildings. Th scrning procdur in Japans standards is mainly basd on xamination of story shar of columns and structural walls. Hasan and Sozn proposd a simplifid mthod for th classification of low-ris monolithic rinforcd structurs in a givn rgion according to thir sismic vulnrability. Th main thm of this mthod is to idntify buildings that hav high probability of svr damag during a strong motion. Th ranking procss is basically dpnds on th total floor ara of th building and cross-sctional aras of columns, shar walls and masonry walls. Gulkan and Sozn also proposd a mthodology in ordr to stimat th sismic vulnrability of rinforcd concrt fram buildings with masonry infill. Th mthod rquirs only total floor ara, cross sctional dimnsions of columns and masonry infill walls. Most of th mthodology rquirs only structural configuration to valuat th sismic vulnrability. Sismic risk assssmnt procdur givn by Yucmn t al considr th ffct of diffrnt sit conditions for vulnrability. This mthod is applid considring variabl sit conditions for a typical rinforcd concrt structurs and analyzd in this papr. II. SEISMIC RISK ASSESSMENT PROCEURE In many instancs statistical analysis basd on th obsrvd damag and significant building attributs would provid rliabl and accurat rsults for rgional assssmnts. Yucmnt t al mployd th discriminant analysis tchniqu to dvlop a prliminary valuation mthodology for low to mdium ris RC buildings in Turky. This procdur rlis on th damag cutoff valus dvlopd using a statistical approach basd on th damag data compild from uzc in 1999 arthquaks. Som slctd building attributs ar ntrd into a rlation obtaind from th discriminant analysis to comput a damag scor. Th analyss ar conductd for both Lif Safty Prformanc Lvl and Immdiat Occupancy Prformanc Lvl. In Lif Safty Prformanc Lvl, th main objctiv is to idntify th buildings, which hav a tndncy to suffr svr damag or to collaps undr th dsign arthquak. Howvr, Immdiat Occupancy Prformanc Lvl, th main concrn is to idntify th buildings that ar suitabl for habitation immdiatly aftr th dsign arthquak. Th main structural paramtrs ar numbr of storis, minimum normalizd latral stiffnss indx, minimum normalizd latral strngth indx, normalizd rdundancy scor and soft story indx. Th basic quations ar givn blow: = (1) = (2) In th abov quation, I LS : damag indx corrsponds to Lif Safty Prformanc Lvl, I IO : damag indx corrsponds to Immdiat Occupancy Prformanc Lvl, n: numbr of story, mnlsti: minimum normalizd latral stiffnss indx (indicat latral rigidity of ground story), mnlsi: minimum normalizd latral strngth indx (indicat bas shar capacity of th most critical story), nrs: normalizd rdundancy scor, ssi: soft story indx, or: ovrhang ratio. This damag scor is thn compard with a cutoff valu, which idntifis th buildings as, or Modrat. Th cutoff valus rcommndd ar considrd to b valid for damaging arthquaks and th rgions that hav similar distanc to sourc and sit conditions to that of 1999 uzc arthquak. To apply this procdur to th sits, which hav diffrnt distanc to sourc and soil proprtis than uzc, modifications to th cutoff valus hav bn computd. Th ffcts of distanc to sourc and soil typ ar th two main paramtrs that should b takn into account. Th sits ar classifid by distanc to sourc (ds) and soil typ. Fiv distancs to sourc wr slctd in viw of th variation in th rspons spctra with th distanc. Soil typs wr dtrmind basd on th shar wav vlocity of th soil. Twnty diffrnt sit classs wr obtaind from th combination of distanc to sourc and soil typs, which ar illustratd in th following tabl. 48

3 Soil Typ Shar wav vlocity,v s (m/sc) Tabl 1: Sit classification istanc to Sourc (km) A A1 A2 A3 B B1 B2 B3 C C1 C2 C A4 B4 C4 4 A5 B5 C5 5 th quations. amag indx corrsponds to th Lif Safty Prformanc Lvl and Immdiat Occupancy Prformanc Lvl ar calculatd. Aftr that, cutoff valus rlating to diffrnt soil typ is calculatd for two arthquak magnitud M=6.0 and M=6.5. Finally vulnrability of th structur is dtrmind. Th cut off modification cofficint valu for dif- to sourc frnt typs of soil with diffrnt distanc and for diffrnt magnitud ar shown in th tabl 2. Tabl 2: Cutoff Modification Cofficints (CMC) Magnitud of Earthquak, M=6.0 Shar istanc to Sourc (km) wav vlocity,v s (m/sc) A B C Magnitud of Earthquak, M=6.5 Soil Typ Soil Typ Shar wav vlocity,v s (m/sc) istanc to Sourc (km) A B C cutoff valus for ach prformanc lvls is thn calculatd using th following quations: = + 1 = + 1 Whr, : cutoff valus corrsponds to Lif Safty Prformanc Lvl, : cutoff valus corrsponds to Immdiat Occupancy Prformanc Lvl. Th damag indx for both prformanc lvls ar computd and compard with th cut off valus to idntify th structural vulnrability. III. ANALYSIS OF TYPICAL STRUCTURE (3) (4) In this analysis, a typical rinforcd concrt fram structur is considrd having 5 spans in X- dirction and 4 spans in Y-dirction and shown in Figur 1. Column siz is takn as 3000 x 300 mm for xtrior column and 300x450 mmm for intrior column. All th structural paramtrs (mnlsti, mnlsi, nrs, or) hav bn calculatd and by using Figur 1: Plan viw of a typical RC structur IV. RESULT Sismic vulnrability assssmnt of a typical RC structur for diffrnt typs of soil and diffrnt distanc to sourc has bn analyzd and shown in th following figur 2 to figur 11 A B C Figur 2: Vulnrability for diffrnt soil typs (istanc to sourc: 0-4 km, M=6.0) Figur 3: Vulnrability for diffrnt soil typs (istanc to sourc: 5-8 km, M=6.0) 49

4 Modrat A B C Figur 4: Vulnrability for diffrnt soil typs (istanc to sourc: 9-15 km, M=6.0) Figur 7: Vulnrability for diffrnt soil typs (istanc to sourc: 0-4 km, M=6.5) Modrat Modrat Figur 5: Vulnrability for diffrnt soil typs (istanc to sourc: km, M=6.0) Figur 8: Vulnrability for diffrnt soil typs (istanc to sourc: 5-8 km, M=6.5) Modrat Modra at Modrat Figur 6: Vulnrability for diffrnt soil typs (istanc to sourc: km, M=6.0) Figur 9: Vulnrability for diffrnt soil typs (istanc to sourc: 9-15 km, M=6.5) 50

5 Modrat Modrat th abov sismic risk assssmnt procdur hav compltd within limitd facilitis and short priod. Furthr xtnsiv rsarch can b carrid out con- dtaild soil sidring dynamic structural analysis, classification and considring dpth of foundation. Figur 10: Vulnrability for diffrnt soil typs (istanc to sourc: km, M=6.5) Modrat Figur 11: Vulnrability for diffrnt soil typs (istanc to sourc: km, M=6.5) V. CONCLUSION In th abov analysis, soil classification is basd on shar wav vlocity of soil. Typical RC structur for 3 storid, 4 storid and 5 storid has bn con- as sidrd in th analysis. Structur groupd group whn it is locatd nar th sourc of xcitation. From th analysis it can b shown that, structur lis in group for th distanc to sourc up to 5-8 kilomtrs for magnitud 6.0. On- soil lis in ly a fiv storid structur built on typ Modrat risk group in cas of 9-15 kilomtr dis- can b tanc to sourc. Any low ris structurs tratd as group built on A & B typ soil whn distanc to sourc is gratr than 15 kilomstorid build- trs. For C typ soil, 4 storid and 5 ings lis in group. Structur can b tratd as Modrat group as thy ar con- only th structd on typ soil. Furthrmor, structurs build on typ soil and distanc to sourc gratr than 16 km has bn groupd as Modrat risk group. As th distanc to sourc incrass and also with th incras in shar wav vlocity of soil, th structur can b catgorizd from to Modrat group. Howvr, Modrat 5 REFERENCES [1] Ozcb, G., Yucmn, M. S., Aydogan, V., and A. Ya- Assssmnt of kut, Prliminary Sismic Vulnrability Existing Rinforcd Concrt Buildings in Turky- Part I: Statistical Modl Basd on Structural Charactristics, Sismic Assssmnt and Rhabilitation of Existing Building s, NATO Scinc Sris IV/29, 29-42,2003 [2] Yakut, A., Aydogan, V., Ozcb, G., and M. S. Yuc- Assssmnt of mn, Prliminary Sismic Vulnrability Existing Rinforcd Concrt Buildings in Turky -Part II: Inclusion of Sit Charactristics, Sismic Assssmnt and Rhabilitation of Existing Buildings, NATO Scinc Sris IV/29, pp ,20033 [3] M.R.Sadat, M.S.Huq and M.A.Ansary, Sismic vulnrability assssmnt of buildings of haka city. Journal of Civil Enginring (IEB), 38(2), ,2010 [4] Sucuoglu,H. and Yazgan,U, Simpl Survy Procdurs for Sismic Assssmnt in Urban Building Stocks. Sismic Assssmnt and Rhabilitation of Existing Buildings, NATO Scinc Sris,IV/29,97-118,2003 [5] Alam, M. Jahangir, Larning Earthquak Fundamntals and Guidlins for Sismic dsign and construction, Workshop Organizd by Earthquak Enginring Rsarch Cntr (EERC), CUET, Chittagong, Bangladsh,2007 [6] Alam, M. Jahangir, Lsson Larnt From Rcnt Earth- Managmnt, quak-prcautionary Masurs & isastr Workshop Organizd by Earthquak Enginring Rsarch Cntr (EERC), CUET, Chittagong, Bangladsh,2007 [7] Alam, M. Jahangir, Liqufaction Potntial Study of Chittagong City, Workshop Organizd by Earthquak Enginring Rsarch Cntr (EERC), CUET, Chitta- gong, Bangladsh,2007 BIOGRAPHIES Mr. Md Aftabur Rahman was born in Sonapur, Noakhali in Bangladsh on cmbr 31, H obtaind th Bachlor of Scinc in Civil Enginring from a rnownd univrsity in Bangladsh namd Chittagong Univrsity of Enginring & Tchnology (CUET) in th yar Aftr that, h joind as a faculty mmbr of Civil Enginring partmnt of CUET in th sam yar. 51