Study On Dynamic Response Of Simply Supported Isolated Bridge Subjected To Earthquake Load Considering Soil-Structural Interaction

Soil-Structural Interaction (SSI) is often ignored in practice on aseismic analysis. This leads to discrepancy between computed result and reality response when rigid foundation is assumed. This assumption is more common for isolated structures because of concept that SSI can be advantageous to structures. In fact, because of flexibility of foundation, SSI may be advantageous or detrimental to isolated structures or controlled structures. The effect of SSI on isolated structures is complex.Present isolated techniques and structural control techniques are getting more and more attractive in civil engineering. In practice, many stuctures are designed or retrofitted by the techniques, especially in bridge engineering. The research on SSI effect is getting deeper. Unfortunately, the research on combination of SSI and structural control is scarce.There is very limited literiture on the topic, in which controvesal conclusions sometimes got drawn. Therefore, further investigation on combination of SSI and structural control is needed. The present research on the combination focuses on rigid shallow foundation, seldom on flexible foundation, with simple mass point model under assumption that earth is elastic half space. In fact, most of bridges are built on pile foundation, one type of flexible foundations.The focus in this paper is on the isolated or hybrid isolated bridge (including aqueduct, a kind of bridge transporting water) with simply support considering SSI subjected to earthquake load.The reseach content is as follow.Chapter1introduces the development of seismic isolated stuctures considering SSI, and development of fragility analysis of bridge.Chapter2focuses on a simply supported aqueduct with isolated bearings. A refined finite element model is established in order to get data which is useful to designer. In the model, beam element is employed to represent pile, spring element is employed to represent SSI, relevant solid elements are used for pier, acqueduct body and water. By this model, dynamic analysis is accomplished. The effect of isolated bearings on structural response under earthquake is investigated.Chapter3uses the same model as in Chapter2. In the analytic model, keeping the superstructure unchanged, two kinds of methods of modeling foundation are applied. One is improved Penzien model, using Mindlin method to determinate spring and dashpot coeffecients. The orther one is m method recommended by the code. The comparison of these two methods is maked.Chapter4simulates the response of a scaled aqueduct model, which undergoed a quasi dynamic test in a large soil box. The analytic model employs the improved Penzien model to model pile-soil interaction.Chapter5studys a hybrid isolated aqueduct or intelligent control aqueduct subjected to earthquake load in different soil profiles. The hybrid isolated device is comprised of ordinary isolated bearing and MR damper, loacated on the top of pier. The control strategy is semi-active control. The influence of different soil profiles on the hybrid isolated aqueduct system is obtained.Chaper6takes a classic three-span simply supported isolated bridge as an example. Dynamic analysis is taken from the model, which considers the pounding between the decks or deck and abutment.The aqueduct aforementioned is also a simply supported bridge, just with different load. The analytic model complexity of bridge is varied according to analysis purposes. The models in Chapter2amd3are complex, but the model in Chapter5is simple. How to develop a reasonable model depends on types of bridge, layout of bridge, what you expect, analysis category et al.In this chapter, in order to perform full nonlinear dynamic time history analysis, a refined model is modeled, which includes some significant factors, such as materal nolinearity, bearing nonlinearity, soil-structure interaction and pounding between decks and deck and abutment. The column is modeled in fiber elements, pounding is modeled in pounding element and deck is modeled in beam elements, which is expected to be elastic. The whole model is not too complex, ortherwise it is unacceptable in fragility analysis in Chapter7.This model here is used to investigate the pounging effect on classic three-span isolated simply supported bridge.Chapter7takes fragility analysis to the aforementioned model in Chapter6. Furthermore, comparison of fragility before-isolated and after-isolated bridge is taken.Fragility is a conditional probability of structure reaching or exceeding a limit state damage under given earthquake intensity (such as PGA or Sa). In this Chapter,10earthquake waves are chosen from five earthquake wave bins,2waves per bin. Then incremental dynamic analysis is performed with0.1g incerement from0.1g to1g. Thus100time history analysis for non-isolated and isolated bridge are gathered respectively. The interested responses, such as column ductility, fixed bearing displacement, expansion bearing displacement et al, can be extracted. The extracted responses are earthquake demand. The relationship between demand and PGA can obtained through regression. Finally, based on limit state damage, fragility curve can be drawn. The system’s fragility can be got based on components fragility curves according reliability theory.The fragilitie of isolated and non-isolated bridge are compared in order to understand the isolation effect.The main conclusions are drawn as follows.For the dynamic response of aqueduct mounted seismic isolated bearingsThe peak dynamic stress and acceleration are mitigated after employing seismic isolated bearings. The effectness of mitigation depends on frequency of the input earthquake wave.For different pile-soil interaction modelsImproved Penzien model method and m method lead to similar analysis result. But m method has more flexible to obtain spring coefficient, which is hard to master. Therefore, improved Penzien method is better.For simulation and testThe responses, including displacement time history, peak displacement of pier, and pile displacement, are close between simulation and test under minor earthquake.For different soil profiles’ effect on the hybrid isolated aqueductThe control effect of hybrid isolation device is less when considering SSI, especially in soft foundation.For classic simply supported bridgeThe displacement, shear force and moment of column are increased or decreased depending on the earthquake wave in horizontal direction when considering pounding. The effectiveness in longitudal direction is good, but is bad in transverse direction. The responses in transverse direction are amplified when isolated device employed considering pounding.For fragility analysisThrough fragility analysis, fixed tranditional bearing is the weaket part, which needs to replace together with expansion bearing. After replacing tranditional bearings with isolated bearings, the fragility of components and system are improved.The highlight of the paper is to investigate pounding, between decks or deck and abutment, effect on the isolated simply supported bridge considering SSI through full nonlinear dynamic analysis.