包括震后固结沉降在内的多层结构物基础抗震性分析

Seismic Performance of Foundations of Multi-story Buildings Considering Post-earthquake Consolidation Settlement

摘要:

以一多层框架结构停车场的地基基础为研究对象,分析其在将来可能发生大地震中的动力特性。除地基可能出现的液化,还包括地震中的瞬时沉降及地震后地基长期固结沉降,尤其是不均匀沉降。采用水土耦合2维有限元分析法,对研究领域的地基基础及上部结构进行整体建模。计算中采用的地震波为一三连动人工地震波,最大加速度为182 gal,主震持续150 s。为比较不同的基础形式对地基液化和沉降的影响,对采用长桩和密集型短柱两种基础形式做分析比较。有限元计算中,采用能反映其地层土交变移动特性的弹塑性本构模型来描述土的动力学特性,桩基础和上部框架结构采用梁单元模型,密集型短柱基础采用弹性单元模型。结果表明,除地震中地基的液化,震后随着超孔隙水压的消散,地基基础长期不均匀沉降也是不可忽略的重要问题。

Abstract:

In evaluating the damage caused by earthquakes,attention has been paid in the past to ground liquefaction and displacement during or immediately after the earthquakes.For this reason,only the analysis of liquefaction in sandy ground during earthquakes is performed in most dynamic analyses.However,the damage to complex ground that contains sand,silt,or clay layers and long-term settlement over several weeks or years after the earthquake cannot be ignored mainly because of the long time required for the dissipation of excess pore water pressure (EPWP) and the recovery of the ground rigidity.In this study,a multi-story car park with a steel frame is designed and constructed according to Japanese Architectural Building Standards.This study will investigate the seismic performance of the building during and after a great earthquake that is predicted to hit the central part of Japan in the near future.Special attention is paid to the differential settlement caused by liquefaction and long-term settlement after the earthquake.The analysis is performed using a 2D soil-water coupled dynamic/static finite element analysis program DBLEAVES,considering ground-foundation-superstructure as one whole system.The program can analyze not only the static and dynamic behavior of natural complex ground but also solve soil-structure interaction problems.The applicability and accuracy of the program have been verified by many investigations.A rotational kinematic hardening elasto-plastic model called the cyclic mobility (CM) model is adopted in this analysis to describe the nonlinear behavior of cohesionless soils under both dynamic and static loadings,particularly the cyclic mobility of sand during liquefaction.With the CM model and an effective-stress-based FEM code,the mechanical behavior of soil,change of EPWP,and consolidation can be defined.The input earthquake wave is an approximation of three synchronized seismic waves whose main shock lasts about 150 s with a maximum acceleration of 182 gal.Before the dynamic analysis,a static analysis considering the ground-structure as one whole system is performed to determine the initial effective stress of the ground.In the dynamic analysis,an equal displacement boundary condition,sometimes called a periodic boundary condition,is used for two side boundaries to manage the energy-loss problem.In this study,a comparison of long-pile and short-pile foundations is presented.As mentioned above,the ground behavior is described by the CM model;in addition,the long piles and super structure are modeled as beam elements,and the short piles are modeled as elastic solid elements.The analysis shows that liquefaction occurred mainly in loose and medium dense sand layers.The long-pile foundation has a better capacity for resisting differential settlement,whereas the short-pile foundation (improved ground) has a better capacity for resisting ground liquefaction.In all cases,most of the differential settlement occurs during earthquake motion,while the post-liquefaction settlement is relatively uniform despite its large amplitude.Therefore,serious consideration should be given not only to the liquefaction behavior of the ground during earthquake motion but also to the long-term settlement after the earthquake.