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). In recent building standards: ASCE-7, 2005), it is acknowledged that this assumption can result in considerable errors when predicting the seismic response of reinforced concrete buildings with diaphragm plan aspect ratio of 3:1 or greater. However, the influence of floor diaphragm openings: typically for the purpose of stairways, shafts, or other architectural features) has not been considered. In order to investigate the influence of diaphragm openings on the seismic response of reinforced concrete buildings; several 3-story reinforced concrete buildings are designed as a Building Frame System according to the International Building Code: 2006). Each building is assumed to be in the Saint Louis, Missouri area, and it\u27s analyzed using IDARC2, a non-commercial program capable of conducting nonlinear analysis of RC buildings with rigid, elastic, or inelastic floor diaphragms, under both static lateral loads: pushover) 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 region. The comprehensive analytical study conducted involves placing different opening sizes: none, 11%, 15% and 22% of total floor area) in various floor plan locations with respect to the location of the shear walls: located at end frames or at the interior frames), where three types of floor diaphragm models: rigid, elastic, and inelastic) are assumed. Building floor plan aspect ratios of 3:1 and 4:1 are investigated. IDARC2 is enhanced by modifying the fiber model: strain compatibility) computation routine involved in obtaining the idealized moment-curvature curves of floor slabs with openings: symmetric and nonsymmetric). Also, a new option is added so that the user can over-ride IDARC2 idealized moment-curvature curves for slabs with openings and by defining their own. The results are then presented and discussed. It is 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 for floor diaphragm aspect ratio of 3:1 or greater. Thus, using a rigid diaphragm assumption, as specified by ASCE7-05 for buildings concrete floor diaphragms with aspect ratio of 3:1, and elastic diaphragm assumption, as allowed by ASCE7-05 for floor diaphragm with aspect ratio of 4:1, can result in significant underestimations of the lateral loads resisted by the interior building frames and building maximum frame displacements, particularly when the diaphragm openings are located in the middle two-thirds of the building plan. The base shear redistribution due to inelastic slab deformations increases the load subjected to the interior frames significantly. Hence, the influence of inelastic inplane diaphragm deformations due to floor openings cannot be overlooked in such buildings. Simple design recommendation is given for determining proper diaphragm chord reinforcement to prevent in-plane floor slab yielding when openings are present