Effect of floor slabs on the seismic performance of RC frames

In monolithic reinforced concrete structures, portions of the floor slabs act as flanges to the girders, thereby increasing the strength and stiffness of the girders. The question of how much the slab contributes to the lateral strength is very important for the design of structures; therefore this paper describes the effect of slabs at the joints in moment-frame structures subjected to large seismic deformations. A simple method to model a beam-column joint subassembly including the effects of both beam growth/elongation and the floor slab is introduced. The model is developed by establishing the slab crack pattern at the joint and the state of strain in the slab bars. The results of the models excluding and including slab effects are verified with the experiential test results. The joint model is incorporated in the nonlinear dynamic analyses for a five-storey and four-bay moment frame structure. Two different ground motions (El-Centro 1940 and Northridge 1994) are considered for the analyses. The results show that the cyclic inelastic bending causes the beams to increase in length and the floor slabs significantly restrain this phenomenon and cause the columns to displace by different amounts, changing the distribution of shear among the columns, and increasing the base shear of the columns. These additional forces may lead to a failure mechanism different from the anticipated one. The effect of floor slab including beam elongation effect is thus illustrated for a two dimensional moment frame building and this model works well for the lateral load analysis of frames.