A Simple Evaluation Method of Seismic Resistance of Residential House under Two Consecutive Severe Ground Motions with Intensity 7

January 31, 2018

By Kotaro Kojima and Izuru Takewaki, Department of Architecture and Architectural Engineering, Graduate School of Engineering, Kyoto University

July 26, 2016, Frontiers in Built Environment.

In the 2016 Kumamoto earthquake in Japan, two severe ground shakings with the seismic intensity 7 (the highest level in Japan Meteorological Agency scale; approximately X–XII in Mercalli scale) occurred consecutively on April 14 and 16. In the seismic regulations of most countries, it is usually prescribed that such severe earthquake ground motion occurs once in the working period of buildings. In this paper, a simple evaluation method is presented on the seismic resistance of residential houses under two consecutive severe ground motions with intensity 7. Therefore, the proposed method can be used for the design of buildings under two consecutive severe ground motions. The present paper adopts an impulse as a representative of near-fault ground motion and two separated impulses are used as the repetition of intensive ground shakings with the seismic intensity 7. Two scenarios to building collapse (collapse limit in terms of zero restoring force with P-delta effect and collapse limit in terms of maximum deformation) under two repeated severe ground shakings are provided and energy consideration is devised for the response evaluation. The validity and accuracy of the proposed theories are discussed through numerical analysis using recorded ground motions.

Introduction

The general well-accepted theory of main-shock, after-shock occurrence was severely distorted in the 2016 Kumamoto earthquake in Japan and two intensive ground shakings with the seismic intensity 7 [the highest level in Japan Meteorological Agency (JMA) scale; approximately X–XII in Mercalli scale] occurred consecutively on April 14 and 16. In most seismic regulations in earthquake-prone countries, it is usually prescribed that such intensive earthquake ground motion occurs once in the working period of buildings and the after-shock is relatively small compared to the main-shock. In this circumstance, some changes of design philosophy may be necessary. In this paper, the degree of necessary upgrade is investigated on the seismic resistance of residential houses under two consecutive severe ground motions with intensity 7.

Several attempts have been conducted on the damage analysis of structures under repeated ground motions (Mahin, 1980; Amadio et al., 2003; Fragiacomo et al., 2004; Li and Ellingwood, 2007; Hatzigeorgiou and Beskos, 2009; Hatzigeorgiou, 2010; Moustafa and Takewaki, 2011, 2012; Motosaka, 2012; Ruiz-Garcia, 2012; Hatzivassiliou and Hatzigeorgiou, 2015). The formulations of residual deformation and member deterioration after one ground motion may be key issues. It seems that most previous papers deal with the response characteristics of structures under repeated ground motions and do not mention directly the necessary strength upgrade due to input repeat. In other words, while the previous researches are aimed at the analysis of damage for the main-shock–after-shock sequence, the purpose of the present paper is to propose a design method for preventing from collapse under two consecutive intensive ground shakings.

The present paper adopts an impulse of the velocity V as a representative of near-fault ground motion and two separated impulses are used as the repetition of intensive ground shakings with the seismic intensity 7 (see Figure 1). The modeling of earthquake ground motion into an impulse corresponds to the evaluation of the input energy under a monotonic loading that is a well-known and well-accepted concept in understanding the earthquake input energy demand (see Figure 2). It is not intended to extract a pulse from a record because the pulse represents an impulsive input symbolically in this case. A residential house is modeled by three models. The first one is an undamped single-degree-of-freedom (SDOF) model of normal bilinear hysteresis with negative second slope (steel structures), the second one is an SDOF model of slip-type restoring-force characteristic, including a bilinear hysteresis (wooden structures), and the third one is an SDOF model of degrading hysteresis (reinforced-concrete structures). Two scenarios to building collapse (collapse limit in terms of zero restoring force with P-delta effect and collapse limit in terms of maximum deformation) under two repeated severe ground shakings are provided and energy consideration is devised for the response evaluation. The validity and accuracy of the proposed theories are discussed through numerical analysis using recorded ground motions.