The soft-story failure is one of the most typical types of damage observed in buildings during earthquakes. The failure of an intermediate soft story of a structure results in large impact loading acting on the lower floors due to the fall of the upper stories. The aim of the present paper is to study the behavior of the structural members exposed to such extreme load conditions occurring under ground motion excitation. Special attention is paid to investigate the influence of the degradation of horizontal stiffness of columns on the response of the building during earthquake. The results of the study show that the stiffness degradation of building columns, due to impact caused by the fall of the upper stories, may have a considerable influence on the structural response. Moreover, the results clearly indicate that the time of impact plays a substantial role in the overall behavior of the building suffering from the soft-story failure. It has been shown that the structural response may be increased significantly if impact takes place when the building is in the range of its peak deformations during the earthquake, especially when the structure is just approaching the extreme position.
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