Collapse strength ratios for weak first-story buildings under soft soil intraslab earthquakes

Abstract

Weak first-story buildings are susceptible to experience severe structural damage, and even collapse, during strong earthquakes as was evidenced during the September 19, 2017 (Mw = 7.1) intraslab earthquake that hit Mexico City. Such type of buildings are prone to exhibit a negative slope in their capacity curve due to weak first-story mechanism that might lead to dynamic instability during earthquake excitation. Therefore, this paper presents the main results of an investigation focused on evaluating the collapse strength ratios, Rc (i.e., the normalized lateral strength ratio in the onset of dynamic instability) of single-degree-of-freedom (SDOF) systems whose force-displacement nonlinear envelope (backbone) behavior is representative of that observed in existing multistory weak first-story buildings. Particularly, their envelope exhibit several levels of negative slope after a post-yield displacement ductility capacity. Median and dispersion of Rc ratios were obtained for degrading SDOF systems subjected to a set of 136 earthquake ground motions recorded in soft soil sites of Mexico City from instraslab earthquakes. Results of this investigation showed that Rc ratios depend on the normalized period of vibration with respect to the predominant period of the ground motion, T/Tg, the displacement ductility capacity, μc, and the negative slope after peak strength, αc. Additionally, the spectral trend of Rc ratios at soft soil sites is different to that reported for firm soil sites. A continuous functional form is introduced to obtain estimates of median Rc ratios for the preliminary seismic assessment of weak first-story buildings in this seismic environment.