Elsevier

Structures

Volume 28, December 2020, Pages 1244-1251
Structures

Effects of incident angles of earthquake sequences on seismic demands of structures

https://doi.org/10.1016/j.istruc.2020.09.064Get rights and content

Abstract

Numerous aftershocks can be triggered by a strong mainshock and may cause more severe and widespread structural damage with respect to a single seismic event. This study investigates the effect of relative differences between the incident angles of consecutive earthquakes on seismic demands of structures as bi-linear Single-Degree-Of-Freedom (SDOF) systems. To this aim, constant-strength spectra are developed based on different Normalized Engineering Demand Parameters (NEDPs) to identify this impact as a new seismic uncertainty. Several seismic sequences with one aftershock and also two aftershocks are generated considering different directions of both mainshocks and subsequent aftershocks as input excitations in nonlinear dynamic analyses of SDOF systems. The results demonstrate that taking into account the relative differences of the incident angles of sequential earthquakes can affect significantly seismic responses.

Introduction

A mainshock may trigger a cluster of aftershocks during a short time interval. Aftershocks can cause further structural and non-structural damage. Given that the current seismic codes are only based on single earthquakes [1], [2], thus, considering the effects aftershocks on seismic investigations is of paramount importance. Several efforts have been directed on assessment of structures subjected to multiple earthquakes, including SDOF systems [3], [4], [5], [6], [7], reinforced concrete and steel frames [8], [9], [10], [11], [12], [13], [14], [15], and existing structures that may exhibit stiffness degradation and strength reduction [16], [17].

Another critical concern in Earthquake Engineering is the impact of earthquake direction on structural responses. However, numerous researchers have examined the effect of seismic incident angle on the performance of structures under single earthquakes [18], [19], [20], [21], [22], [23], [24], limited works recently have evaluated seismic behavior of different structural systems, namely SDOF systems [25], MDOF systems [26], [27], [28], [29], [30], [31] against sequential earthquakes accounting for the effect of seismic incident angle. In these studies, the incident angles of consecutive earthquakes would be identical. In other words, the relative difference between successive ground motion orientations has not been considered. However, it is physically and seismologically possible to have rotated ground motion sequences. For instance, Yukutake and Lio [32] investigated the focal mechanisms of aftershocks after the Western Tottori Earthquake occurred in 2000. They showed that the aftershocks around the mainshock fault were distributed within zones of 1.0–1.5 km in thicknesses, and also their focal mechanisms were significantly diverse.

The objective of this paper is to reveal the effect of the relative difference between incident angles of mainshock and subsequent aftershocks, as a new seismic uncertainty, on nonlinear seismic responses of structures as SDOF systems. For this purpose, after generating mainshocks and aftershocks with various incident angles, constant-strength spectra based on different Normalized Engineering Demand Parameters (NEDPs) are developed. Furthermore, seismic sequences including one aftershock and also two aftershocks are considered in this regard.

Section snippets

Methodology

The epicenter location is not certainly determined before the occurrence of an earthquake, thus the direction of ground motion with respect to the orientation of a structure is not identified as a priori. Hence, one of the most important uncertainties in the process of determining the structural damage is the orientation of the earthquake record. Additionally, there are no specific and detailed provisions about determining the most critical direction of earthquake excitation in current seismic

Structures

The current paper aims to introduce the additional parameters induced by the rotation of successive earthquakes, as discussed in Section 2, for SDOF systems with the elastic-perfectly-plastic model, as a preliminary study to investigate the effect of this new seismic uncertainty on nonlinear responses. However, different hysteresis models can be used in this regard, as indicated in [33], [34], [35], [36], [37], [38] for better investigations, which will be considered in further research works.

Statistical results

After carrying out nonlinear time history analyses of the structures under mainshock-aftershock sequences rotated to various angles, the normalized responses, i.e. NEDPθ for three different EDPs, stated in sub-section 3.1, are computed using Eq. (1). Only some results are presented in this Section for the sake of brevity of the paper. The maximum value of normalized responses, NEDPθ, for all combinations of incident angles of mainshocks and subsequent aftershocks, namely 25 and 125 combinations

Summary and conclusions

This study investigates the effect of the relative differences between successive incident angles on various seismic responses of nonlinear structures. To this aim, structures as SDOF systems are taken into account with a variety of T and R. Moreover, a methodology to generate rotated seismic sequences is presented. Then constant-strength spectra are developed under rotated mainshock-aftershock sequences including one aftershock and two aftershocks, rotated to various angles, from 0° to 180°,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (41)

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