A strong mainshock is able to trigger several aftershocks during a short time interval, causing additional damage to structures. The effect of the polarity of aftershock ground motions (positive and negative) is of great significance, particularly on the residual displacement (RD) demand. The positive and negative polarity occurs when the direction of aftershock is the same and the opposite of that of mainshock, respectively, while these are only two of the various situations of the incident angle of aftershock with respect to that of mainshock. This study evaluates the sufficiency of the positive and negative polarity in multiple earthquakes for the determination of critical residual displacements of structures by considering various relative differences between the incident angles of sequential earthquakes. The structures are schematized as bi-linear single-degree-of-freedom systems to develop constant-strength spectra in terms of the Normalized Residual Displacement (NRD_θ). The results reveal that considering only the conventional aftershock polarity (positive and negative) may not necessarily lead to critical residual displacement demands, and also accounting for the variability of various incident angles for consecutive ground motions is necessary for the reliable evaluation of structures.

强烈的主震能够在很短的时间间隔内触发多次余震，对结构造成额外的破坏。余震地震动极性（正负）的影响意义重大，特别是对残余位移（RD) 需求。正、负极性分别发生在余震与主震方向相同和相反时，而这只是余震相对于主震入射角的各种情况中的两种。本研究通过考虑连续地震入射角之间的各种相对差异，评估多次地震中正负极性对确定结构临界残余位移的充分性。结构被示意化为双线性单自由度系统，以根据归一化残余位移 ( NRD _θ）。结果表明，仅考虑常规余震极性（正和负）可能不一定会导致临界残余位移需求，并且考虑连续地面运动的各种入射角的可变性对于结构的可靠评估是必要的。