中文摘要：

实际地震由水平地震动和竖向地震动组成。由于水平地震动可能沿任何方向发生，通常把引起结构最大动力反应的水平地震动角度称为地震对该结构的最不利入射角度。确定地震动的最不利入射角度是研究双向地震作用下结构动力响应的关键问题。该文提出了一种基于弹性时程分析的地震动最不利入射角度的确定方法，并给出了该方法的图形解法。该文通过具体算例证实，当结构为线弹性时，该方法给出的是动力响应的精确解。当结构进入非线性阶段，产生塑性变形时，该文对该方法引起的误差进行了讨论，发现误差随强度折减系数（Rd）的增加而增大，在Rd小于6时，误差通常不超过10％，证明该文所提出的方法在结构发生塑性变形时依然具有很好的适用性。

英文摘要：

Translational ground motion is decomposed usually into three components： two horizontal components and one vertical component. As the maximum structure response may occur in any direction along the structure, usually the direction of the horizontal component, in which the maximum structure response occurs, is known as the critical orientation. The determination of the critical orientation is one of the key issues in bi-directional seismic excitation studies. In this paper, based on time history analysis results, a method is proposed to determine the critical angle of the structure response under a bi-directional seismic excitation as well as a graphic solution to the method. It is proved that for structures of linear elastic materials, this method gives exact solutions. In the meantime, the accuracy of the method for nonlinear-material structure is analyzed in this paper. The analysis shows that the method introduces an error increasing as the section strength reduction factor （Rd） grows, which stays below 10% while Rd is less than 6. It is shown that the structural response caused by the earthquake excitation in the critical angle of seismic incidence calculated by the proposed method is reasonably close to the maximum response in the random orientation of earthquakes.