场地地震反应分析中侧向人工边界距离选取及影响因素分析

Selection of lateral artificial boundary distance and analysis of its influence factors in seismic responses analysis of sites

结合均匀场地模型和分层均匀场地模型,采用数值试验的方法研究了场地地震反应分析中侧向人工边界距离的选取问题。分析了各主要因素对人工边界距离选取的影响,得到了如下结论:单侧人工边界距离与土层深度之比L/h是影响人工边界距离选取的主要参数;随着侧向人工边界距离的增加,有限元模型的计算误差并不一直减小,在局部范围内,会出现计算误差随着边界距离的增加而增大的现象;场地硬度、阻尼比和激振频率等因素不会影响计算误差随边界距离的变化规律,但采用自由场边界时受输入地震动频谱特性的影响较为明显;对水平激振下的一维场地模型来说,采用结合自由度边界时所需边界距离最短,自由场边界次之,采用粘性边界达到收敛时所需边界距离最长;无论哪种边界条件,当边界距离大于某一范围后,都能得到稳定的计算结果;相对于均质场地模型来说,实际的分层均匀场地建模时可适当减小侧向人工边界距离。

The selection of lateral artificial boundary distance （LABD） in the seismic response analysis of sites was studied by numerical simulation for uniform site model and layered uniform site model. And main factors of influencing the selection of the LABD were analyzed. The results are as follows. The ratio of one side LABD to soil depth （IMh） is main parameter of influencing the selection of the LABD. The calculation errors of the finite element model do not always decrease with the increase of the L/h. In some range of the L/h, the calculation errors will increase with the increase of the L/h. The calculation errors are not affected by those parameters such as site hardness, damping ratio and excitation frequency, however, they are obviously affected by spectral characteristics of input ground motions for the free-field boundary condition. For the one - dimensional site model under horizontal excitation, the LABD required for the tied degrees of freedom boundary condition is the shortest, followed by free- field boundary condition, and the distance for the viscous boundary condition is the longest. No matter which boundary condition, we can get the steady results when the L/h is greater than a certain range. Compared with the uniform site model, L/h can be appropriately reduced in the layered uniform site.