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Evaluation of Seismic Performance of Geotechnical Structures View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by UC Research Repository Evaluation of seismic performance of geotechnical structures M. Cubrinovski University of Canterbury, Christchurch, New Zealand B. Bradley University of Canterbury, Christchurch, New Zealand ABSTRACT: Three different approaches for assessment of seismic performance of earth structures and soil- structure systems are discussed in this paper. These approaches use different models, analysis procedures and are of vastly different complexity. All three methods are consistent with the performance-based design phi- losophy according to which the seismic performance is assessed using deformational criteria and associated damage. Even though the methods nominally have the same objective, it is shown that they focus on different aspects in the assessment and provide alternative performance measures. Key features of the approaches and their specific contribution in the assessment of geotechnical structures are illustrated using a case study. 1 INTRODUCTION The assessment of seismic performance of geo- Methods for assessment of the seismic performance technical structures is further complicated by uncer- of earth structures and soil-structure systems have tainties and unknowns in the seismic analysis. Par- evolved significantly over the past couple of dec- ticularly significant are the uncertainties associated ades. This involves improvement of both practical with the characterization of deformational behaviour design-oriented approaches and advanced numerical of soils and ground motion itself. Namely, the com- procedures for a rigorous dynamic analysis. In paral- monly encountered lack of geotechnical data for lel with the improved understanding of the physical adequate characterization of the soil profile, in-situ phenomena and overall computational capability, soil conditions and stress-strain behaviour of soils new design concepts have been also developed. In results in uncertainties in the modelling and predic- particular, the Performance Based Earthquake Engi- tion of ground deformation. Even more pronounced neering (PBEE) concept has emerged. In broad are the uncertainties regarding the ground motion terms, this general framework implies engineering (earthquake excitation to be used in the analysis) evaluation and design of structures whose seismic arising from the inability to predict the actual ground performance meets the objectives of the modern so- motion that will occur at the site in the future. ciety. In engineering terms, PBEE specifically re- The above uncertainties affect key elements in quires evaluation of deformations and associated the analysis, the input load (ground motion or earth- damage to structures in seismic events. Thus, the quake load) and constitutive model (stress-strain key objective in the evaluation of the seismic per- curve or load-deformation relationship). Clearly, the formance is to assess the level of damage and this in output of the analysis will be adversely affected by turn requires detailed evaluation of the seismic re- these uncertainties and would therefore require care- sponse of earth structures and soil-structure systems. ful interpretation. One may argue that, strictly Clearly this is an onerous task since the stress-strain speaking, a prediction of the seismic response is not behaviour of soils under earthquake loading is very possible under these circumstances; instead, the aim complex involving effects of excess pore-water pres- should be an assessment of the seismic performance. sures and significant nonlinearity. The ground re- This argument is not in the realm of semantics, but it sponse usually involves other complex features such rather implies difference in philosophy. It alludes to as: the importance of the process and engineering inter- - Modification of the ground motion (earth- pretation rather than the outcome alone, which is in quake excitation for engineering structures) agreement with the traditional role that engineering - Large ground deformation and excessive per- judgement has played in geotechnical engineering. manent ground displacements In this paper, three approaches for assessment of - A significant loss of strength, instability and seismic performance are applied to a case study of a ground failure, and bridge on pile foundations. Conventional methods of - Soil-structure interaction effects. seismic analysis are used in the assessment and comparatively examined. Key features in the imple- Simple Advanced Other methods SEISMIC EFFECTIVE PSEUDO-STATIC ANALYSIS STRESS ANALYSIS - Equivalent static analysis - Dynamic time-history analysis Figure 1. Methods for seismic analysis of earth structures and soil-structure systems mentation of the methods, their advantages and dis- Despite its complexity, the seismic effective stress advantages are discussed. It is demonstrated that the analysis is now frequently used in geotechnical prac- examined approaches focus on different aspects and tice for assessment of the seismic performance of make different contribution in the assessment. important structures. As indicated in Figure 1, a large number of alternative analysis methods are available in the range between these two benchmark 2 METHODS FOR ASSESSMENT OF SEISMIC approaches. PERFORMANCE 2.1 Analysis methods 2.2 Deterministic versus probabilistic approaches There are various approaches for seismic analysis of Generally speaking, the seismic response can be earth structures and soil-structure systems ranging evaluated either deterministically or probabilisti- from relatively simple approximate methods to very cally. Figure 2 illustrates the three approaches scru- rigorous but complex analysis procedures. These tinized in this study in this regard: (i) Deterministic approaches differ significantly in the theoretical ba- approach (DA) in which a single scenario is consid- sis, models they use, required geotechnical data and ered; in this case, only one analysis is conducted and overall complexity. The simplest methods are based respectively a single response of the system is com- on the pseudo-static approach in which an equiva- puted; (ii) Deterministic approach (DAP) in which a lent static analysis is used to estimate the dynamic series of analyses are conducted in a parametric response induced by the earthquake. The pseudo- manner in order to account for the uncertainties and static analysis is based on routine computations and unknowns in the analysis; as indicated in Figure 2, use of relatively simple models, and hence is easy to this approach results in a range of different re- implement in practice. For this reason, it is the com- sponses for the analyzed system; (iii) Probabilistic monly adopted approach in seismic design codes. approach (PA) in which “all possible” earthquake On the other hand, the most rigorous analysis proce- scenarios are considered for the site in question; this dure currently available for evaluation of the seismic approach also results in a range of different re- response of soil deposits and earth structures is the sponses for the system and, in addition, provides an seismic effective stress analysis. This analysis per- estimate for the likelihood of each response. mits detailed evaluation of the seismic response The key difference between these three ap- while considering the complex effects of excess pore proaches is in the treatment of the uncertainties. The water pressures and highly nonlinear behaviour of deterministic approach with a single scenario (DA) soils in a rigorous dynamic (time history) analysis. effectively ignores the uncertainties in the analysis DETERMINISTIC (DA) Single scenario Single response APPROACH DETERMINISTIC Range of (DAp ) Parametric analyses APPROACH responses Likelihood PROBABILISTIC "All possible" scenarios Range of (PA) & of their APPROACH are considered responses occurence Figure 2. General approaches for assessment of seismic performance of geotechnical structures (a) Bridge deck (b) Pier Footing 0 m N =10 2.0 m 1 N =15 1 8.5 m Existing RC piles D = 0.3m 0 N =10 1 West 15.0 m pile East N =25 pile 1 New steel encased RC (non liquefiable) piles D = 1.5m 0 20.0 m Figure 3. Central pier of the bridge: (a) cross section; (b) simplified soil profile used in seismic effective stress analyses (Bo- wen and Cubrinovski, 2008) while the probabilistic approach (PA) offers the Figure 4 depicts the SPT blow count and soil pro- most rigorous treatment of uncertainties and quanti- file at the northeast corner of the bridge. This soil fies their effects on the computed seismic response. profile was adopted in the pseudo-static analyses. The soil deposit consists of relatively loose liquefi- able sandy soils with a thickness of about 15 m over- 2.3 Adopted approaches lying a denser sand layer. The sand layers have low This paper examines three approaches for assess- fines content predominantly in the range between ment of the seismic performance in the context out- 3% and 15% by weight. Detailed SPT and CPT in- lined above as follows: vestigations revealed a large spatial variability of the (1) Pseudo-static analysis within a deterministic penetration resistance at the site. Hence, a rigorous approach incorporating parametric evaluation investigation of the seismic response of the bridge (DAp) and its foundation would require consideration of (2) Seismic effective stress analysis using a sin- 3-D effects and spatial variability of soils. These gle scenario (DA) complexities are beyond the scope of this
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