Fundamentals of Seismic Base Isolation

Transcription

1 Intrnational Training Programs for Sismic Dsign of Building Structurs Hostd by National Cntr for Rsarch on Earthquak Enginring Sponsord by Dpartmnt of Intrnational Programs, National Scinc Council Fundamntals of Sismic Bas Isolation Wang, Yn-Po 1 1 Introduction Ovr th past dcads, arthquak rsistant dsign of building structurs has bn largly basd on a ductility dsign concpt worldwid. Th prformancs of th intndd ductil structurs during major arthquaks (.g. Northridg, 1994; Kob, 1995; Chi-Chi, 1999 tc.), howvr, hav provd to b unsatisfactory and indd far blow xpctation. High uncrtainty of th ductility dsign stratgy is primarily attributd to: Co (1) Th dsird strong column wak bam mchanism may not form in rality, du to xistnc of walls. (2) Shar failur of columns du to inappropriat gomtrical proportions or short-column ffct. (3) Construction difficulty in grouting, spcially at bam-column joints, du to complxity of stl rinforcmnt rquird by th ductility dsign. To nhanc structural safty and intgrity against svr arthquaks, mor ffctiv and rliabl tchniqus for asismic dsign of structurs basd on structural control concpts ar dsird. Among th structural control schms dvlopd, sismic bas isolation is on of th most promising altrnativs. It can b adoptd for nw structurs as wll as th rtrofit of xisting buildings and bridgs. Stratgis to achiving sismic isolation includ: (1) Priod-shifting of structurs (2) Cutting-off load transmission path Th spring-lik isolation barings with considrabl latral flxibility hlp in rducing th arthquak forcs by changing th structur s fundamntal priod to avoid rsonanc with th prdominant frquncy contnts of th arthquaks, as indicatd by Fig. 1. Whras th sliding-typ isolation barings filtr out arthquak forcs via th discontinuous sliding intrfacs, btwn which th forcs transmittd to th suprstructur ar limitd by th maximum friction forcs, rgardlss of arthquak intnsity. 1 Profssor, Dpt. of Civil Enginring, National Chiao-Tung Univrsity, Hsinchu, Taiwan Dirctor, Larg-Scal Structural Lab, NCTU

2 Fig.1 Typical acclration rspons spctra Convntional Isolatd Fig. 2 Effcts of Bas Isolation During arthquaks, th convntional structur without sismic isolation is subjctd to substantial storydrifts, which may lad to damag or vn collaps of th building. Whras th isolatd structur vibrats almost lik a rigid body with larg dformations or displacmnts ndurd by th isolation barings, as illustratd in Fig 2.Th latral forcs of th isolatd building ar not only rducd in magnitud but also fairly rdistributd ovr th floors, which furthr mitigats th ovrturning momnt of th structur. Laminatd rubbr and stl plats Rubbr covr Lad cor Fig. 3 Construction of LRB Fig. 4 Construction of HDRB

3 Th spring-lik barings that hav sn widsprad applications includ th lad-rubbr baring (LRB) and th high-damping rubbr baring (HDRB). Th sliding-typ barings, on th othr hand, ar impractical du to lack of rstoring capability. To ovrcom this drawback, th friction pndulum systm (FPS) originatd from th sliding-typ barings is dvlopd by introducing a sphrical sliding intrfac to provid rstoring stiffnss, whil th friction btwn th sliding intrfacs hlps in dissipating nrgy. As a rsult, th FPS is functionally quivalnt to LRB and HDRB in lngthning structur s fundamntal priod, with additional advantagous faturs such as priod-invarianc, torsion-rsistanc, tmpratur-insnsitivity and durability. Although th rubbr barings hav bn xtnsivly adoptd for sismic isolation, th FPS has rcntly found incrasing applications (Buckl t al., 1990; Zayas t al., 1987; Kawamura t al., 1988 ). Th friction pndulum barings provid strngth and stability that xcd thos of rubbr barings. Its proprtis ar not affctd by aging or tmpratur. Th baring s low profil, high strngth, and high vrtical stiffnss rduc installation costs. Ths barings offr vrsatil proprtis which can satisfy th divrs rquirmnts of buildings, bridgs and industrial facilitis. This articl will addrss th basic mchanical proprtis of FPS, status of its dvlopmnt as wll as a prliminary dsign procdur basd on static analysis. Fig. 5 Construction of FPS Cntr position Displacd position Fig.6 Motions of FPS 2 Mchanical Proprtis of FPS Th friction pndulum barings ar stainlss stl sismic isolators consisting of a concav surfac, an articulatd slidr, and a covr plat. Th slidr is coatd with slflubricating composit linr (.g. Tflon). During an arthquak, th articulatd slidr within th baring slid along th concav surfac, causing th supportd structur to mov with gntl pndulum motions. Th motions of th FPS ar illustratd in Fig. 6.

4 Th natural priod (T ) of th friction pndulum baring is slctd simply by choosing th radius of curvatur of th concav surfac ( R ) as T = 2π R / g (1) whr g is gravitational acclration. It is indpndnt of th mass of th supportd structur. Th latral stiffnss ( K ) of th baring providing th rstoring capability of th systm is b K b = W / R (2) whr W is th wight of th structur. As a rsult, th torsional motions of th structur ar minimizd sinc th cntr of stiffnss of th barings coincids with th cntr of mass of th supportd structur. Th movmnt of th slidr gnrats a dynamic friction forc that provids th rquird damping for absorbing th nrgy of th arthquak. Th latral loads (i.. th bas shar), V, transmittd to th structur as th baring slids to a distanc, u, away from th nutral position includ th rstoring forcs and th friction forcs as u V = ( + µ ) W (3) R whr µ is th cofficint of friction. Typical hystrtic loops of th latral forc of FPS in cyclic motion ar shown in Fig. 7. Th nrgy ( E D ) dissipatd for on cycl of sliding with amplitud D is stimatd as E D = 4µWD (4) Th cofficint of friction is dpndnt on th contact prssur btwn th Tflon-coatd slidr and th stainlss stl surfac. Th cofficint dcrass as th prssur incrasd. Th valu of µ btwn 3~10% is considrd rasonabl for th FPS to b ffctiv. latral load displacmnt dissipatd nrgy Fig. 7 Hystrsis of Latral Load 3 Dsign Procdur Basd on Static Analysis Dsign of th sismic isolation systm includs dtrmination of th bas shar, baring displacmnt, tc., in accordanc with th sit-spcific conditions by cod provisions, at dsird baring proprtis (.g. friction cofficint, µ, and radius of curvatur of th concav surfac, R, for FPS). In this sction, a simpl dsign procdur basd on static analysis is introducd for th prliminary dsign of isolation systms. This procdur alon is sufficint for building structurs with fair gomtrical rgularitis. Howvr, a mor complx

5 nonlinar dynamic analysis is dsird for thos with gomtrical irrgularitis. Th hystrsis of th bas shar against sliding displacmnt of an FPS can b idalizd by a rigid-plastic linar-hardning modl, as dpictd in Fig. 8. Th charactristic constant (Q ) of th isolation systm is th maximum friction forc dfind as Q = µw (5) whil th ffctiv stiffnss ( k ff ) of th isolation systm as a function of th xpctd largst baring displacmnt ( D ) with givn µ and R is dtrmind by k ff 1 µ = + W R D (6) As a rsult, th quivalnt natural priod ( T ) of th isolatd building can b approximatd as T W = 2π (7) k g ff by taking th suprstructur as a rigid body, and this quivalnt natural priod is again dpndnt on D. Bas shar V max Q 1 k b k ff 1 D Baring displ. Fig. 8 Idalizd Hystrsis of FPS On th othr hand, th maximum bas shar ( V max spctral analysis using th lastic spctrum as ) of th isolation systm can b obtaind by Vmax = ZICC DW (8) whr Z is th sismic-hazard-basd PGA of th dsign arthquak, I is th important factor of th structur, C is th sit-spcific normalizd (PGA=1g) 5% lastic acclration rspons spctrum spcifid in th cod, and C D is th modification factor for structurs with a damping factor othr than 5%. It is dfind as

6 C D 1.5 = ς + 1 whr th quivalnt damping factor, ς, is in turn dtrmind by (9) 1 E D ς = 2 (10) 2π kff D whr E D is again th dissipatd nrgy dfind in Equ. (4). Th maximum bas shar is quivalnt to th mass of th structur ( W / g ) tims th spctral acclration, S a, thrfor, Using th spctral rlation S = a ZICCD g (11) S a 2 ( 2π / T ) D 2 = ω D = (12) and taking th important factor I as unity for sismic isolatd buildings, th spctral displacmnt ( D ) turns out to b 2 DT 2 Th abov quation suggsts that D is a function of T and ZCC g D = (13) 4π C D which in turn ar functions of D implicitly. Thrfor, an itrativ procdur is rquird until convrgnc of th spctral displacmnt, D, is achivd. Th bas shar is thn stimatd by V = k D (14) ff Distribution of th latral forc at th i- th floor is suggstd as F i V = N k= 0 w u i k i w u k (15) whr w k rprsnts th wight of th k- th floor of th N-story building; u k is th latral displacmnt of th k- th floor du to latral loads at ach floor proportional to its wight, that is wk f k =, k = 0,..., N N w j= 0 j (16) With th latral forc dtrmind, th suprstructur is thn dsignd in a mannr similar to convntional structurs in accordanc with th building cods.

7 It is notd that, torsional ffct du to accidntal ccntricity on th baring displacmnt should b accountd for, spcially for thos to b placd at th cornrs of th building. Morovr, to nsur baring stability at critical conditions, a safty factor of 1.5 is furthr imposd whn sizing th barings. Stability vrification of th prototyp barings prior to implmntation is basd on this factord displacmnt dmand. Although th abov discussions mphasiz on th friction pndulum systms, th dsign procdur dscribd in this sction is in fact common for structurs isolatd with any typ of sismic isolators, rgardlss of FPS, LRB or HDRB. 4 FPS in Buildings, Bridgs and Industrial Applications Th friction pndulum barings hav bn spcifid for many sismic isolation projcts in buildings, bridgs and industrial storag tanks. Among which, th U.S. Court of Appal; San Francisco Airport Intrnational Trminal; Grc s LNG tanks; and th Bnicia-Martinz Bridg ar th world s largst sismic isolation projcts to dat. (Earthquak Protctiv Systms, Inc., 1998). Th sismic rtrofit of th U.S. Court of Appals building in San Francisco, upon its compltion in 1994, was th largst building in th world to hav bn rtrofittd with sismic isolators. Th advantags of th friction pndulum barings- its novl tchnical approach, supportd tst rsults and othr analysis, ar found to b mor ffctivly nhanc th buildings survivability in th vnt of an arthquak. Fig. 9 U.S. Court of Appals Th San Francisco Airport Intrnational Trminal is th largst nw building in th world constructd using sismic isolation. It has dramatic architctural faturs, including: xpansiv intrior spacs, 80 ft tall columns, 700 ft long roof trusss, and glass xtrior walls. Th building was dsignd to rsist a magnitud 8 arthquak occurring on th San

8 Andras fault. Sismic isolation provids th lowst construction cost for achiving th dsird sismic prformanc. Morovr, th us of friction pndulum barings, as compard to rubbr barings, allowd for a furthr rduction in column and bam sizs and savd an additional 680 ton of structural stl. Fig. 10 San Francisco Airport Int l Trminal Grc s cntralizd liqufid natural gas (LNG) storag tanks ar locatd just outsid of Athns. Ths tanks contain 38 million gallons of flammabl LNG, and ar situatd within on of Europ s highst sismic rgions. Th baring prformanc rquirmnts for this projct wr th mosstringnt in th history of sismic isolation. Th isolation barings wr rquird to maintain thir dsign proprtis whil fully accommodating th ffcts of: 35 yars of aging in a marin nvironmnt; simultanous latral and vrtical arthquak motions; tmpraturs ranging from 10 to 86 F. Friction barings wr slctd ovr lastomric barings aftr tsts of full siz barings showd that thy wr bst abl to satisfy ths dmanding prformanc rquirmnts, and would thrby achiv th safst tank prformanc. Th Bnicia-Martinz bridg is on of th largst bridgs to dat to undrtak a sismic isolation rtrofit, and uss th largst sismic isolation barings vr manufacturd. Th friction pndulum barings for this projct hav a 5 scond priod, a latral displacmnt capacity of 53 inchs, a 5 million lg. Dsign vrtical load, masur 13 ft in diamtr, and wigh 40,000 lbs.

9 Fig. 11 LNG Tank in Rvithoussa, Grc Fig. 12 Bnicia-Martinz Bridg in San Francisco 5 Rfrnc Buckl, I. G., and Mays, R. L. Sismic isolation history: application and prformanc - a world rviw. Earthquak Spctra, 6, , (1990). Earthquak Protctiv Systms, Inc., DM of August, Elsssr, E., Jokrst, M. and Naash, S. Historic Upgrads in San Francisco, Civil Enginring, ASCE, p.50-57, Octobr, (1997).

10 Kawamura, S., Kitazawa, K., Hisano, M., and Nagashima, I. Study of a sliding-typ bas isolation systm-systm composition and lmnt proprtis., Proc. 9 th WCEE, Tokyo- Kyoto, Vol. V, , (1988). Zayas, V., Low, S. S. and Main, S. A. Th FPS arthquak rsisting systm, xprimntal rport, Rport No. UCB/EERC-87/01, Earthquak Enginring Rsarch Cntr, Univrsity of California, Brkly, CA., Jun, (1987).