Inelastic Seismic Response Of Reinforced Concrete Buildings With Floor Diaphragm Openings

Floor and roof systems are designed to carry gravity loads and transfer these loads to supporting beams, columns or walls. Furthermore, they play a key role in distributing earthquake-induced loads to the lateral load resisting systems by diaphragm action. In reinforced concrete buildings, the in-plane flexibility of the floor diaphragms is often ignored for simplicity in practical design (i.e., the floor systems are frequently treated as perfectly rigid diaphragms). Past research, which is acknowledged in recent building standards, has shown that this assumption can result in considerable error when predicting seismic response of reinforced concrete buildings when diaphragm plan aspect ratio is greater than 3:1 (Kunnath, 1991 & ASCE-7, 2005). However, the influence of floor diaphragm openings (typically for the purpose of stairways, shafts, and other architectural applications) has never been considered. In order to investigate the influence of diaphragm openings on the seismic response of reinforced concrete buildings; two 3-story reinforced concrete buildings are designed as a Building Frame System according to the International Building Code (IBC, 2006). Each building is assumed to be in the Saint Louis, Missouri area and it’s analyzed with and without floor openings -4 cases. The inelastic behavior of the buildings is investigated under both static lateral loads (push-over) and dynamic ground motions (time-history), where a suite of three well-known earthquakes is scaled to model moderate ground motions in the Saint Louis, Missouri region. The diaphragm parametric study conducted involves two opening size/locations and two lateral load resisting frames stiffness/locations, where three types of floor diaphragm models (rigid, elastic, and inelastic) are assumed. The result summary is presented in this paper and discussed. It was concluded that in order to capture the seismic response of reinforced concrete buildings with floor diaphragm openings accurately; it is necessary to use an inelastic diaphragm model.

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