Computation of deflections of edge supported reinforced concrete slabs under service loads

Abstract

Excessive deflection of reinforced concrete slabs can cause severe serviceability problems. In recent years, realistic estimation of slab deflection under service loads has become more important due to the increasing use of high-strength materials and due to the use of ultimate limit state design which generally result in thinner members. This thesis is mainly concerned with computation of deflections of two-way edge supported slabs. Deflection calculation of slabs is complicated and time consuming due to the fact that it is affected by cracking, creep and shrinkage etc. The main objective of the current work is to develop a simplified method for prediction of immediate and long-term deflections of two-way edge supported slabs. The work started with the elastic analysis of two-way edge supported slabs. Two finite element softwares, FE-77 and ANSYS have been used to analyze slabs with different end conditions. Moment and deflection coefficients charts have been prepared for easy calculation of elastic and immediate deflections. Hossain (1999) developed a nonlinear FE module, which was incorporated in the finite element software FE-77, to model effect of cracking. This has been used in the current work after proper validation against experimental results. A detailed parametric study has been carried out and it has been observed that most of the edge supported slabs are cracked even under service loads if restraint shrinkage IS considered. Results of the parametric study show that deflection increase due to cracking is only influenced by level of cracking, aspect ratio and boundary conditions. Finally design charts have been proposed for easy calculation of immediate deflections of edge supported slabs. Charts are also validated by comparing with experiments results. As for long-term deflection, ACI Code (2002) multiplier approach has been used. ACI Code thicknesses are found to be adequate for most of the slabs with usual panel size and loading as the incremental and total deflections are within tolerable limits. Comparison between results of FE analysis and design chart method have been shown good agreement. With the help of these charts, a designer can easily calculate immediate as well as long-term deflections of edge supported slabs.