Analysis of floor diaphragms made of precast concrete slabs with high length-to-breadth aspect ratio

Abstract

During an earthquake, floor slabs in reinforced concrete buildings act as diaphragms and transfer the in-plane lateral inertial forces to the frames and walls. In slabs made of precast concrete units, these forces have to be transferred through the horizontal joints between the precast units. The jointed nature and high length-to-breadth aspect ratio make precast diaphragms flexible. Floor cutouts further increase the flexibility. This study investigated the modelling of a regular mid-rise building with precast slab units, without and with central floor cutouts. First, 2D finite element (FE) models of standalone Cast-In-Place (CIP) and precast diaphragms were analysed. The effects of cutout and lack of integration of chord beams with the slab units were studied. It was observed that precast diaphragms without proper connection between chord beams and slab units, and with architectural topping alone, were substantially flexible than CIP diaphragms. The flexibility further increased due to the presence of a cutout. Next, 3D FE models of mid-rise buildings made of precast slabs, with and without central cutouts were analysed. Storey drift ratios were substantially higher when the diaphragm was modelled as jointed, as compared to an analysis with rigid diaphragm assumption. The storey drift ratios were even more in the building with cutouts. This indicates that flexible diaphragm analysis is necessary in buildings made of precast slab units without adequate structural topping, especially with high length-to-breadth aspect ratio in plan.