The effects of soil-foundation interface nonlinearity on seismic soil-structure interaction analysis

Transcription

1 The effect of oil-foundation interface nonlinearity on eimic oil-tructure interaction analyi M. Moghaddai Aurecon, Wlington, New Zealand. A. Carr, M. Cubrinovki, S. Pampanin & J.G. Chae Univerity of Canterbury, New Zealand. C.T. Chatzigogo & A. Pecker Geodynamique et Structure, France. NZSEE Conference ABSTRACT: Thi reearch preent the impact of bae fixity on eimic analyi including oil-tructure interaction (SSI) conidering linear and nonlinear oil-foundation interface condition. A et of inatic time hitory analye uing a yiding ingledegree-of-freedom tructural ytem with different fixity condition at the bae are ued. The bae fixity configuration conidered are: ) fixed-bae; ) linear flexible-bae; ) nonlinear flexible-bae without uplift; and 4) nonlinear flexible-bae with uplift. A uite of 4 ground motion with large-magnitude and moderate-ditance i choen to enure robutne of the reult acro realitic ground motion. The examination of SSI effect on the tructural repone under a deign bae earthquake (DBE) lev, i.e. 5-year return period event, i carried out for all conidered cenario. In addition, the effect of an increae in the eimic intenity up to a maximum credible earthquake (MCE) lev, i.e. 5-year return period event, are alo tudied for the cae of nonlinear flexible-bae with uplift. The reult illutrate the degree of reidual foundation deformation a wl a the impact of SSI on tructural acceration, total diplacement and ductility. In thi context, the importance of SSI effect for a deign procedure a wl a the difference between linear and nonlinear SSI conideration i highlighted. INTRODUCTION Conideration of oil-tructure interaction (SSI) during an earthquake in a dynamic analyi or a deign procedure ha been a controverial topic for the lat three decade []. In principal, when SSI i included in a dynamic analyi, the repone of three linked and interacting ytem, namy: ) the oil tratum urrounding the foundation; ) the foundation; and ) the tructure ha to be imultaneouly conidered. In thi context, intead of applying the eimic force directly to the tructure aumed to be fixed at the bae, a modified input excitation by the coupled dynamic behaviour of the oil, foundation and tructure ha to be ued []. A a reult, the induced deformation and force in the tructure are altered from their correponding value when the tructure i aumed to be fixed at the bae. Neglecting SSI effect on tructural repone i currently being uggeted in many eimic code [- 6]. However, there are ome tudie howing that SSI can reult in either beneficial or detrimental effect depending on the oil-tructure ytem parameter and input ground motion characteritic [7, 8]. Mot tudie on oil-tructure interaction in addition to the current deign procedure are baed on the aumption that the oil adjacent to the foundation behave a a linear or at mot an equivalent linear vicoatic material [9-]. In addition, the foundation i aumed to be fully bonded to the oil underneath. However, geotechnical invetigation after the Northridge 994, Kobe 995, Kocai 999 [] and Chritchurch earthquake [4] have hown that ignificant nonlinear action in the oil and oil-foundation interface can be expected due to high lev of eimic excitation and pectral acceration. Therefore, it i very important to invetigate the influence of oil-foundation interface nonlinearity on the effect of oil-tructure interaction [5]. Principally, neglecting uch phenomena Paper Number XXX

2 prohibit the effect and conequence of: ) energy diipation due to oil yiding; ) large foundation deformation a wl a reidual ettlement and rocking; and ) foundation toppling on the tructural repone. In thi regard, Gandomzadeh et al. [6] carried out a parametric tudy for an atic tructural ytem upported on a nonlinear oil tratum. The tructure wa modled a a ingle-bay, ingle-torey D frame having different mae, and the oil wa modled uing the Iwan contitutive nonlinear mod. The oil-tructure ytem conidered were then enforced to Ricker wavet with variou amplitude. They concluded that due to oil nonlinearity and, conequently, an additional energy diipation to the ytem, tructural repone decreae if SSI i conidered. Thi reduction i more pronounced for the ytem having a fundamental frequency cloe to the natural frequency of the oil. In addition, it wa tated that oil nonlinearity change fundamental frequency of the ytem and thi change i ignificantly affected by the ma of the ytem. Saez et al. [7] alo tudied the effect of atic and inatic oil-tructure interaction on eimic demand of ingle-degree-of-freedom (SDOF) tructure. In thi tudy, two inatic SDOF tructure, repreenting low-rie and mid-rie reinforced concrete moment reiting frame building, and two oil condition, repreenting a dry and a aturated homogenou dene Toyoura and profile of m depth overlaying bedrock, were conidered. The oil wa modled uing atic and ato-platic contitutive mod. The oil-tructure ytem generated were then excited by a uite of ground motion compriing different earthquake ection and trong-motion parameter. It wa concluded that when the oil i in a dry condition, the atic and inatic SSI reult in a imilar effect on tructural repone. However, when the oil i in a aturated condition, a ignificant variation exit between atic and inatic SSI effect. Thi variation i obviouly due to pore preure generation that cannot be captured by linear oil mod. In addition, it wa indicated that the influence of SSI on the tructural repone when low-rie tructural ytem on dry oil are conidered, might be either beneficial or detrimental. However, SSI i beneficial for other cenario conidered. Finally, the effect of SSI on tructural repone for linear and nonlinear oil-foundation interface condition were tudied by Pecker and Chatzigogo [8] following an incremental dynamic analyi (IDA) approach. The analye were facilitated uing a new dynamic macro-ement pecifically devoped to repreent oil-foundation interface nonlinearity [9, ]. Thi tudy covered a typical highway bridge pier excited by a uite of ground motion repreenting rativy large-magnitude earthquake with moderate ditance and no effect of directivity. In the oil-tructure mod ued, both oil and tructure were conidered to be nonlinear. It wa concluded that nonlinear SSI i alway beneficial and ignificantly reduce the tructural ductility demand. However, large diplacement and rotation at the foundation are alo reulted that might be unaccepted. Therefore, care mut be taken into account before moving toward a deign philoophy where the ductility demand can be tranferred from the tructure to the foundation (e.g. []). Following that ha been preented in the literature, an attempt wa made in thi reearch to expand thoe work and pecifically: ) invetigate the effect of linear and nonlinear SSI on tructural repone; ) compare the SSI effect for linear and nonlinear oil-foundation interface condition; and ) examine the effect of oil-foundation nonlinearity at the maximum credible earthquake (MCE) lev. In thi context, an idealied inatic SDOF tructural ytem attached to a oil-foundation interface ement repreenting either: ) a linear condition; ) a nonlinear condition without uplift; or ) a nonlinear condition with uplift wa ued. The oil-tructure mod generated were then enforced to a uite of 4 ground motion caled to a deired hazard lev. Finally, the trend and behaviour were comprehenivy quantified and preented. Thi tudy i the firt tep toward a more comprehenive probabilitic analyi and deign guidine uing nonlinear oil-tructure mod covering exiting uncertaintie in mod parameter and ground motion. SOIL-STRUCTURE MODEL DESCRIPTION The oil-tructure ytem invetigated in thi reearch (Figure ) denote a typical highway bridge pier upported by a rigid circular hallow-foundation and enforced to eimic excitation [8]. It wa

3 deigned baed on a direct diplacement-baed deign (DDBD) approach pecifically introduced to take into account the oil-tructure interaction effect []. The deign wa baed on the Eurocode 8 deign pectrum-type, conidering a firm oil condition and a peak ground acceration (PGA) of.5g. The deign performance criteria conidered are: Sytem drift limit, d. h Maximum foundation rotation,. lim Maximum tructural ductility demand, Figure. Soil-tructure ytem tudied: (a) phyical; (b) mod. Thi ytem wa then modled in the finite-ement program Ruaumoko D [] uing: ) a yiding SDOF tructure repreenting the bridge pier; and ) a ma-pring-dahpot aembly or a macroement repreenting the linear or nonlinear oil-foundation interface condition, repectivy. Clearly, thi modling approach follow the ubtructure technique introduced for SSI analyi.. Structural ytem The yiding SDOF tructural ytem ued i characteried by it height h e, ma m, lateral tiffne c. The damping ratio of 5% wa alo aumed. In k and equivalent tructural vicou damping addition, to cover tructural nonlinearity, a tiffne-degrading force-deformation hyterei rule a Takeda (bilinear envope with train hardening and tiffne degradation) wa conidered with 5% pot-atic tiffne and unloading and roading parameter of. and., repectivy ( and are defined in Figure ). Thi force-deformation behaviour wa then aigned to the pring repreenting the lateral tiffne of the ytem. The numerical parameter defining the tructural ytem are given in Table.. Linear oil-foundation interface To repreent the dynamic behaviour of oil-foundation interface auming a linear repone, the commonly ued ma-pring-dahpot aembly [4] wa attached to the bae of the SDOF tructural ytem conidered. In thi approach, the oil i aumed to behave linearly and the foundation i conidered to be fully bonded to the oil. The pring in thi aembly repreent the tatic tiffne of the oil-foundation ytem and the dahpot repreent the radiation damping. The formulation and the correponding numerical parameter ued to define thi ement are given in Table.

4 Table. Propertie of the oil-tructure mod. Parameter Formulation/Decription Value Structural Parameter:. height, h e effective height to centre of the ma m. ma, m including the ma of the deck and pier. kt. initial lateral tiffne, ( k ) i - 5 MN/m 4. coefficient of vicou damping, c c (5%) m ( k ).55 MN./m i 5. yid trength, f y -.7 MN 6. yid diplacement, y f ( k ) y y i.5 m Soil-Foundation Interface Parameter: Soil and Foundation. oil ma denity, -.6 t/m. Poion ratio, -.. oil hear wave vocity, V - 55 m/ 4. oil hear modulu, G - 4 MPa 5. oil coheion, c -.5 MPa 6. foundation radiu, r -.75 m 7. foundation ma, M fh. kt 8. foundation ma moment of inertia, M fr.78 kt.m Ma-Spring-Dahpot Aembly. Vertical tiffne, K v. Horizontal tiffne, K h. Rocking tiffne, K r Macro-Element a) Platicity parameter 4Gr K v 9 MN/m 8Gr K h 85 MN/m 8Gr K r 89 MN.m ( ). Maximum cantered vertical force, N N 6. 6c A max max 4 MN. Maximum normalized horizontal force, Q V, max c A N Q V,max.65 max. Maximum normalized moment, Q M, max 4. Numerical parameter expreing the extent of initial platic tiffne, p 5. Numerical parameter expreing the extent of tiffne degradation in roading, p b) Uplift parameter.67c AD DN Q M,max. max Q 6 N Q exp( Q ) M, N..5. vr vr, q q M M, t q, M, v ( ) qm q M q M,.75 4

5 Table. Continued. 4. t, q q.5 r M M, r q q M, ( ) t M, t 5. ( ) ( ), q q r v M M,. q q M M 6. Q / Q q M M, M, Q Q M M, q M Radiation Damping. Vertical damping, C v. Horizontal damping, C h C.4 ( ) V A, A r v 8 MN./m C V A h 8 MN./m.4 4. Rocking damping, C C V I, I r 4 r r r r 98 MN./m ( ) For detailed decription of the macro-ement formulation and it parameter refer to [9]. Becaue of pace limitation, the full explanation of the mod i avoided herein.. Nonlinear oil-foundation interface The dynamic behaviour of the oil-foundation interface with nonlinear condition wa included in the mod by a link ement, denoted a macro-ement, introduced by Chatzigogo et al. [9, ]. Thi macro-ement i pecifically formulated to reproduce all nonlinearity expected at the foundation lev including: ) oil material nonlinearity (yiding); and ) interface nonlinearity (uplift). It principally ue a nonlinear contitutive law linking force parameter to diplacement parameter. Thee parameter are ected uch that to be directly linked to thoe rated to the tructure upported by the foundation. The intereted reader i referred to the original work by Chatzigogo et al. for complete detail. In addition, the macro-ement i coupled with the ame dahpot a for the linear oilfoundation mod to cover radiation damping. The formulation and the correponding numerical parameter ued to define macro-ement are given in Table. GROUND MOTIONS AND SCALING SCHEME To cover the uncertaintie reulting from record-to-record variability, the generated oil-tructure mod wa ubjected to a large number of ground motion with different characteritic. An enemble of 4 earthquake ground motion recorded on tiff/oft oil (oil type C, V 8 6 m/, and D, V 8 m/ to a depth of m, baed on USGS claification) wa ued in the analye. All ected record are from earthquake with magnitude of and have ource-to-ite ditance (cloet ditance to fault rupture) in the range of 5-4 km. Detailed information about the ected uite of ground motion can be found in [8]. The ected ground motion were then caled uing the method introduced in New Zealand Standard [5] to match the target pectrum over the period range of interet. The target pectrum choen repreent a 5% damped atic acceration repone pectrum for: ) oil cla C; ) hazard factor (Z=PGA) of.4g; ) return period factor of. correponding to a deign baed earthquake (DBE) lev. The period range of interet conidered i.5-. covering period between.4tfb and.t SSI, where T FB i the fundamental period of the fixed-bae ytem and T SSI i the fundamental period of the correponding oil-tructure ytem. Scaled acceration repone pectra are preented in Figure. In addition, to invetigate the effect of oil-foundation nonlinearity on the repone of oil-tructure ytem when the maximum credible earthquake (MCE) lev i conidered, the ground motion ected were alo caled for the return period of 5 year. It hould be noted that thee caled record were only ued for the reult preented in Section

6 u tot F, (MN) Input (m/ ) a (m/ ) S a (g) S a (g).5 Scaled-Acceration Repone Spectra Target Spectrum Mean Spectrum Cloe-Up for Sected Period Range Period () Figure. Scaled acceration repone pectra for the ground motion ected. 4 SSI EFFECTS ON STRUCTURAL RESPONSE 4. Typical reult of a dynamic analyi A typical dynamic repone of the fixed-bae mod and the correponding nonlinear flexible-bae mod with uplift i hown in Figure and 4, repectivy. The quantitie depicted are: ) tructural acceration a, that i the total acceration of the tructural ma repreenting the bae hear; ) total diplacement u tot, that i a meaure of the diplacement at the roof lev including lateral diplacement reulted from foundation motion and tructural ditortion, which can caue the pounding between adjacent tructure; and ) tructural force-deformation hyteretic behaviour F v. u that how the, maximum tructural force and ditortion in addition to the degree of tructural nonlinearity experienced. The other quantitie only conidered for the flexible-bae mode are: 4) horizontal foundation diplacement u ; 5) foundation rocking u ; and 6) vertical foundation diplacement u. fh Thee parameter are all defined at the foundation centre. It hould be noted that when u, the foundation centre move downward (ettle), and when u, a eparation between the foundation centre and the ground urface occur. However, thi eparation doe not mean toppling. fr fv fv fv EQ 6 No SSI - - Time () Time () 4 5. No SSI No SSI Time () Figure. Example of a dynamic tructural repone of the fixed-bae mod ubjected to EQ

7 u tot F, (MN) u fv a (m/ ) u fh u fr x - Nonlinear SSI x - 4 Nonlinear SSI - - Time () Time () x - Nonlinear SSI Nonlinear SSI Time () Time () 4 5. Nonlinear SSI Nonlinear SSI Figure 4. Example of a dynamic tructural repone of the nonlinear flexible-bae mod ubjected to EQ 6. A illutrated in thee figure, the incluion of nonlinear oil-tructure interaction in dynamic analyi reult in a reidual foundation deformation that, in turn, caue a more ignificant reidual total diplacement compared to that for the fixed-bae mod. In addition, foundation ettlement exit a a reult of thi integration that cannot be captured in a traditional fixed-bae mod. Finally, the nonlinear behaviour of the foundation mot probably make the ytem to how a maller tructural acceration and le degree of nonlinearity. 4. SSI effect preentation To illutrate SSI effect on tructural repone for the different oil-foundation interface condition examined, the maximum value of: ) tructural acceration a ; ) total diplacement u ; and ) normalized tructural ditortion by the yid diplacement and flexible-bae (SSI) mod. In addition, the reidual foundation ettlement ( u ) are alo illutrated for flexible-bae mod. fr -. Time () 4 5 re 4. Linear SSI effect u y are compared for the fixed-bae (FB) fv re tot ( u ) and rocking The reult of the numerical imulation uing mod with linear oil-foundation interface are preented in Figure 5. Clearly, a the foundation behave linearly and the vertical and rocking foundation repone are independent, the foundation ettlement under the total weight of the ytem i contant for all ground motion conidered, and no reidual foundation rocking i oberved. Furthermore, in term of tructural acceration, in contrat to the current deign proviion, SSI can either decreae or increae the repone. In thi context, the probability of amplification i 5%, a percentage value that cannot be imply neglected. However, it hould be noted that the degree of 7

8 u tot FB (u fv ) re a FB (m/ ) u tot FB (u fv ) re a FB (m/ ) reduction i higher than the degree of amplification. The maximum reduction in the tructural acceration due to SSI i about 4%, while the maximum amplification i about %. In addition, at the 84 th percentile lev, a linear SSI mod appear to reduce the tructural acceration by about %. The repone amplification effect of SSI are more pronounced when total diplacement i conidered. Specifically, for almot 95% of the cae, SSI reult in an amplified total diplacement, with a maximum amplification of about 9%. Moreover, at the 84 th percentile lev, SSI increae the total diplacement by almot %. The rik of uch non-negligible lev of amplification emphaize that SSI hould be alway conidered in tudie where pounding effect are of concern. Finally, if tructural ditortion i conidered, SSI can alo reult in either a reduction or an amplification in the repone. In thi cae, the probability of amplification i about 45% with the maximum reduction and amplification being in the order of 4%. Note that the tructural ditortion i reduced at the 84 th percentile lev by about % when a linear SSI mod i ued Linear SSI ) re a Linear SSI (m/ ) u tot Linear SSI / y FB / y Linear SSI Figure 5. SSI effect on tructural repone for mod with linear oil-foundation interface Nonlinear SSI without Uplift ) re a Nonlinear SSI without uplift (m/ ) u tot Nonlinear SSI without uplift / y FB / y Nonlinear SSI without uplift Figure 6. SSI effect on tructural repone for mod with nonlinear oil-foundation interface without uplift. 8

9 u tot FB (u fv ) re a FB (m/ ) Nonlinear SSI.5..5 ) re.5.5 a Nonlinear SSI (m/ ) u tot Nonlinear SSI / y FB / y Nonlinear SSI Figure 7. SSI effect on tructural repone for mod with nonlinear oil-foundation interface. The SSI effect on tructural repone preented above for oil-tructure ytem with linear oilfoundation interface are in complete agreement with the reult that ha been previouly preented by the author [8]. Explicitly, the effect of linear oil-tructure interaction may not be alway a beneficial a conidered in practice. However, it hould be noted that the aumption of having linear oil-foundation interface might not be appropriate for ome oil-tructure-earthquake cenario. 4.4 Nonlinear SSI effect conidering only material nonlinearity The role of oil material nonlinearity (yiding) on SSI effect i dicued next. In thi regard, the macro-ement wa ued in the dynamic analye adopted to repreent oil-foundation interface. The uplift wa at thi tage deactivated in the ement to avoid the effect of geometrical nonlinearity being included. The reult of the correponding analye are preented in Figure 6. A expected, the foundation ement repond nonlinearly and, a a conequence, the oil-tructure ytem conidered experience reidual ettlement and rocking at the foundation lev. It i intereting to note that although the vertical force on the foundation i contant, the reidual ettlement will have different value. The reaon i that in the macro-ement formulation, the vertical force and diplacement are interconnected with the applied moment and correponding rocking. Therefore, depending on the input excitation characteritic and foundation rocking behaviour, different reidual ettlement might reult. In addition, note that for all cae examined, reidual diplacement i a negative value repreenting a ettlement condition compared to the eparation from the ground urface. Furthermore, in contrat to what oberved in the cae of linear SSI, tructural acceration and tructural ditortion alway decreae due to SSI conideration. The maximum reduction in the repone i 6% for tructural acceration and imilarly for tructural ditortion. Thi high degree of reduction in the repone i obviouly due to a large amount of energy diipation occurring at the oil-foundation interface lev. In addition, conideration of oil material nonlinearity can reduce tructural acceration and tructural ditortion at the 84 th percentile lev by almot % and 5%, repectivy. It i alo intereting to note that although foundation yiding decreae tructural ductility lev, it cannot totally prevent the tructure from yiding. However, when total diplacement i conidered, different trend and concluion are oberved. SSI increae total diplacement for almot 7% of the cae. In addition, the amplification in the repone 9

10 at the 84 th percentile lev i about % with the poibility of an increae up to %. It highlight that the beneficial role of SSI in decreaing tructural acceration and ductility demand i compenated by large foundation diplacement and rocking that might become totally unacceptable. 4.5 Nonlinear SSI effect conidering material and geometrical nonlinearity Obviouly, material and geometrical oil-foundation interface nonlinearity are two ineparable phenomena. Thu, their combined role on SSI effect hould be invetigated when oil-foundation interface nonlinearity i included in the SSI analyi. In thi regard, the macro-ement with an activated uplift option wa ued in the imulation adopted. Note that when foundation uplift i conidered, an extra type of foundation failure, referred to a toppling, i introduced. Principally, foundation toppling occur when the eparation between the oil and foundation exceed a predefined limit. The reult of the numerical analye conidering both material and geometrical nonlinearity are preented in Figure 7. A foundation uplift i included in the dynamic analyi, the oil-tructure ytem conidered experience a larger degree of nonlinearity at the foundation lev. Specifically, foundation failure due to oil yiding occur in 6 cae, where the foundation motion i getting very large without being tabilized. Having larger degree of nonlinearity alo can be ditinguihed in term of reidual ettlement and rocking at the foundation lev. A hown in Figure 7, reidual foundation rocking can increae up to.5 rad compared to.5 rad when foundation uplift ha been neglected. Due to thi large foundation rocking, the centre of foundation even may experience a reidual eparation from the ground lev. In addition to the cae of failure due to due to oil yiding, toppling failure wa alo oberved. Therefore, 7 cae out of 4 cenario invetigated experienced foundation failure. Thee failure cae are not hown in the graph preenting ( u ) v. ( u ). Similar trend and concluion to thoe decribed for the cae of nonlinear SSI without uplift are valid in the cae of nonlinear SSI with both material and geometric nonlinearity. More pecifically, nonlinear SSI with uplift alo alway decreae tructural acceration and normalized tructural ditortion. The maximum reduction in the repone i alo 6% with the reduction at the 84 th percentile lev being 4% and 6% for tructural acceration and tructural ditortion, repectivy. It implicitly conclude that foundation uplift doe not have a ignificant effect on the original tructural repone. However, when total diplacement i conidered, the effect of foundation uplift i notable. Thi i due to the fact that the total diplacement include foundation motion a a rigid body that, in turn, i ignificantly affected by foundation uplift. Nonlinear SSI with uplift increae the total diplacement for almot 75% of the cae, and the maximum amplification in the repone can be up to %, even before howing foundation failure due to oil yiding. Note that the cae with the larget value of total diplacement (6 in total) correpond to the cae where the foundation failure i due to oil yiding. Nonlinear SSI with uplift at 84 th percentile lev increae the total diplacement by about 8%. An attempt i alo made to better illutrate and compare SSI effect when different oil-foundation interface condition are conidered. In thi regard, Figure 8 and 9 compare the previouly defined repone parameter for the cae of nonlinear SSI v. linear SSI and nonlinear SSI v. nonlinear SSI without uplift, repectivy. When nonlinear SSI effect on oil-tructure ytem repone are compared with the linear SSI effect (Figure 8), it i clear that reidual foundation rocking, which might have a ignificant conequence in term of deign and recovery after earthquake event, i not taken into account in linear SSI. In addition, it can be concluded that linear SSI overetimate tructural acceration and tructural ditortion. Thi overetimate i in the range of -8% for tructural acceration and in the range of -5% for normalized tructural ditortion. Therefore, uing a linear SSI analyi to define tructural reaction and deformation when oil-foundation interface nonlinearity i probable to occur, can lead to mileading reult and concluion. On the other hand, if total diplacement i conidered, oil-foundation interface nonlinearity can reult in either reduction or amplification in the repone compared to the cae when linear SSI i conidered. For almot 5% of the cae conidered, oilfoundation interface nonlinearity reult in larger total diplacement. The maximum amplification in fv re fr re

11 u tot Nonlinear SSI / y Nonlinear SSI ) re Nonlinear SSI a Nonlinear SSI (m/ ) u tot Nonlinear SSI / y Nonlinear SSI ) re Nonlinear SSI a Nonlinear SSI (m/ ) the repone, ignoring failed cenario due to oil yiding, i about %, while the maximum reduction i about 4%. It clearly demontrate that oil-foundation interface nonlinearity alo play an important role in term of total diplacement. If the reult from nonlinear SSI analye are compared with thoe from nonlinear SSI without uplift, a hown in Figure 9, it i noticeable that foundation uplift ha a ignificant effect on the foundation rocking and, conequently, total diplacement. For almot all the cae conidered foundation uplift reult in an equal or a larger reidual foundation rocking and total diplacement. However, the effect of foundation uplift on tructural acceration and tructural ditortion i not ignificant. Principally, conidering both material and geometrical nonlinearity only lightly reduce tructural acceration and tructural ditortion a compared to the cae when foundation uplift i ignored ) re Linear SSI a Linear SSI (m/ ) u tot Linear SSI / y Linear SSI Figure 8.Comparion of dynamic tructural repone between mod with nonlinear and linear oil-foundation interface `.5..5 ) re Nonlinear SSI without uplift.5.5 a Nonlinear SSI without uplift (m/ ) u tot Nonlinear SSI without uplift / y Nonlinear SSI without uplift Figure 9.Comparion of dynamic tructural repone between mod with nonlinear and nonlinear without uplift oil-foundation interface.

12 u tot FB (u fv ) re a FB (m/ ) 4.6 The role of earthquake deign lev A hown in the Chritchurch earthquake equence in -, tructure deigned for the DBE lev might be ubjected to extreme event during their life time. Therefore, from a deign point of view, it i important to aure that tructure can reit the higher eimic demand without collaping or, where poible, without being damaged beyond reparability lev. In thi regard, to invetigate the role of earthquake deign lev on the repone of oil-tructure ytem with nonlinear oilfoundation interface, the uite of ground motion ected were caled up to the maximum credible earthquake (MCE) lev, i.e. with a 5-year return period, and the dynamic imulation previouly decribed were repeated with the record caled to MCE. The reult of thee imulation are ummaried in Figure. A expected, puhing foundation with larger force will reult in a larger number of failure cae. In thi regard, 6 cae failed due to foundation toppling, 8 cae failed due to oil yiding, and only 6 cae avoided any foundation failure. Large foundation motion obviouly reult in a very large total and reidual diplacement that i further out of the acceptable range. However, large oil-foundation interface nonlinearity wa in favour of tructural repone in term of tructural acceration and tructural ditortion. Specifically, nonlinear SSI, in general, reduced tructural acceration by a factor of and tructural ditortion by a factor of.5. It implicitly mean that oil-foundation nonlinearity can act a an iolation mechanim preventing the damage to be tranferred to the tructure. The tatitic preented give the crude impreion that, uing nonlinear SSI a an iolation mechanim in extreme event can reult in ytem collape due to foundation toppling with 4% probability, tructural protection but with large foundation movement with 45% probability and full tructural protection with 5% probability Nonlinear SSI (MCE).5..5 ) re a Nonlinear SSI (MCE) (m/ ) 4 / y FB u tot Nonlinear SSI (MCE) 4 5 / y Nonlinear SSI (MCE) Figure.SSI effect on tructural repone for mod with nonlinear oil-foundation interface conidering MCE hazard lev. 5 CONCLUSIONS Thi tudy aimed to invetigate the role of oil-foundation interface nonlinearity on the eimic oiltructure interaction analyi. With thi purpoe, a comparative analyi wa performed between oiltructure mod with four different bae fixity condition, including: ) fixed-bae; ) linear flexiblebae; ) nonlinear flexible-bae without uplift; and 4) nonlinear flexible-bae with uplift. In thi context, the tructure wa modled a a yiding ingle-degree-of-freedom ytem with Takeda type force-deformation behaviour, and the oil-foundation interface wa modled either with a pringma-dahpot aembly (for linear cae) or macro-ement (for nonlinear cae). The generated mod

13 were then ubjected to a uite of recorded ground motion repreenting large-magnitude, moderateditance earthquake event. The reult of the imulation adopted can be ummaried a: In contrat to what typically bieved in practice, linear SSI can reult in beneficial or detrimental effect on the tructural repone depending on the oil-tructure-earthquake cenario conidered. The role of oil material nonlinearity on the SSI effect i ignificant. Specifically, thi role i favourable in reducing the tructural repone compared to that of the fixed-bae condition. However, thi beneficial role might be compenated with large foundation diplacement and rocking that might be totally unacceptable. Foundation uplift increae the degree of nonlinearity on the foundation behaviour and, conequently, caue larger foundation diplacement and rocking. In thi context, foundation might fail due to exceive oil material nonlinearity or toppling, while it would not captured if the uplift wa not conidered. However, the effect of foundation uplift on the tructural repone are negligible compared to the effect of oil material nonlinearity. Soil-foundation interface nonlinearity in the extreme event can be ued a a damage prevention mechanim if the toppling and large rigid body deformation can be appropriaty treated. Finally, it hould be noted that thi tudy only covered a implified SDOF ytem. Thu, the differential movement of the individual foundation wa not taken into account. Thee individual foundation movement, and poibly failure, might introduce large deformation/tree in the tructure above and, conequently, caue greater damage than that predicted in fixed-bae analyi. Therefore, a further tudy i required to invetigate thee effect in more detail. REFERENCES: [] E. Kau, "Early hitory of oil-tructure interaction," Soil Dynamic and Earthquake Engineering, vol., pp. 8-8,. [] J. P. Wolf, Dynamic Soil-Structure Interaction. Englewood Cliff, N.J.: Prentice Hall, 985. [] ATC-4, "Seimic evaluation and retrofit of concrete building," ed: Applied Technology Concil, 996. [4] ASCE-7, "Minimum deign load for building and other tructure," ed: Amarican Society of Civil Engineer, 998. [5] EC8, "Deign proviion for earthquake reitance of tructure, part 5: foundation, retaining tructure and geotechnical apect," ed. Brue: European Committee for Standardizatoin,. [6] FEMA-45, "NEHRP recommended proviion for eimic regulation for new building and other tructure," ed. Wahington, D.C.: Building Seimic Safety Council,. [7] G. Mylonaki and G. Gazeta, "Seimic oil-tructure interaction: beneficial or detrimental?," Journal of Earthquake Engineering, vol. 4, pp. 77-, Jul. [8] M. Moghaddai, et al., "Probabilitic evaluation of oil-foundation-tructure interaction effect on eimic tructure repone," Earthquake Engineering & Structural Dynamic, vol. 4, pp. 5-54,. [9] A. S. Veto and J. W. Meek, "Dynamic behaviour of building-foundation ytem," Earthquake Engineering and Structural Dynamic, vol., pp. -8, 974. [] M. Ciampoli and P. E. Pinto, "Effect of oil-tructure interaction on inatic eimic repone of bridge pier," Journal of Structural Engineering, vol., pp , 995. [] J. P. Stewart, et al., "Seimic oil-tructure interaction in building. I: analytical method," Journal of Geotechnical and Geoenvironmental Engineering, vol. 5, pp. 6-7, Jan 999. [] J. P. Stewart, et al., "Reviion to oil-tructure interaction procedure in NEHRP deign proviion,"

14 Earthquake Spectra, vol. 9, pp ,. [] E. E. R. Intitute, "999 Kocai, Turkey," Earthquake Reconnaiance Report, vol. Special Iue of Earthquake Spectra,. [4] M. Cubrinovki, et al., "Geotechnical apect of the February Chritchurch earthquake," Bulletin of the New Zealand Society of Earthquake Engineering, vol. 44, pp. 5-6,. [5] G. Gazeta and M. Apotolou, "Nonlinear oil-tructure interaction: foundation uplifting and oil yiding," preented at the Proceeding Third UJNR Workhop on Soil-Structure Interaction, Menlo Park, California, USA, 4. [6] A. Gandomzadeh, et al., "Influence of oil nonlinearitie on dynamic oil-tructure interaction," preented at the Fifth International Conference on Recent Advance in Geotechnical Earthquake Engineering and Soil Dynamic, San Diego, California, USA,. [7] E. Saez, et al., "Effect of atic and inatic DSSI on eimic demand of SDOFS tructure," preented at the Fifth International Conference on Recent Advance in Geotechnical Earthquake Engineering and Soil Dynamic, San Diego, California, USA,. [8] A. Pecker and C. T. Chatzigogo, "Non linear oil tructure interaction: impact on the eimic repone of tructure," preented at the 4th European Conference on Earthquake Engineering, Ohrid, Republic of Macedonia,. [9] C. T. Chatzigogo, "Macroement moding of hallow foundation," vol. 9, ed, 9, pp [] C. T. Chatzigogo, et al., "A macroement formulation for hallow foundation on coheive and frictional oil," International Journal for Numerical and Analytical Method in Geomechanic,. [] I. Anataopoulo, et al., "Soil failure can be ued for eimic protection of tructure," Bulletin of Earthquake Engineering, vol. 7, 9. [] R. Figini, "Nonlinear dynamic oil-tructure interaction: application to eimic analyi and deign of tructure on hallow foundation," PhD Thei, Politecnico di Milano,. [] A. Carr, "Ruaumoko D, Nonlinear FEM Computer Program," ed. New Zealand: Univerity of Canterbury,. [4] G. Gazeta, "Formula and chart for impedance of urface and embedded foundation," Journal of geotechnical engineering, vol. 7, pp. 6-8, 99. [5] NZS7.5, "Structural deign action, part 5: earthquake action," ed. New Zealand, 4. 4