Stiffness reduction factor for post-tensioned flat plate slabs

The effective beam width model (EBWM) has been used for estimating force and drift demands on flat plate frames under lateral loads. The accuracy of this model depends strongly on how to estimate the effective stiffness of flat plate slabs. With an increase in slab moments due to lateral loads, slab cracks occur and propagate on the slab, which leads to a reduction in slab flexural stiffness. For post-tensioned (PT) flat plate slabs, the initiation and propagation of slab cracks can be delayed due to slab in-plane compressive stress induced by PT tendon forces. This study investigates the contribution of slab in-plane compressive stress to the slab stiffness reduction factor, which is defined as the ratio of slab stiffness obtained from a cracked slab section to that from the corresponding uncracked slab section. For this purpose, this study collected the test results of PT flat plate slab-column connection specimens and estimated the slab stiffness reduction factors (beta) for those specimens with respect to a level of applied slab moment. Non-linear regression analysis is used to propose an empirical equation computing beta for PT flat plate slabs using beta estimated from the test results. The accuracy of the empirical equation for beta is verified by comparing the lateral stiffness of a two-storey PT flat plate frame, obtained using the EBWM with the empirical equations for beta, with that estimated from shaking table test results on the frame.