Avoiding Shear Failures and Excessive Deflection of Concrete Flat Plates

Abstract

The paper presents an up-to-date procedure to design shear-resistant concrete flat plates of optimum thickness. Excessive long-term deflection is avoided by the reliable prediction of probable deflection while accounting for the effects of cracking, creep, and shrinkage of concrete and the relaxation of prestressed reinforcement. Safety against shear failure is considered for concrete flat plates without drop panels, beams, or column capitals. Presented analyses satisfy the compatibility of deformations and equilibrium of forces (requirement of ACI 318 code); few empirical provisions in the latest ACI 318 code, not used here, are identified. Empirical procedures aiming at reducing calculations are not needed in practice; thus, they are avoided. The objective is to recommend the most effective analysis in the design of concrete flat plates to avoid shear failure and excessive deflection. Hundreds of experiments done by many researchers are the basis of the depicted design procedure to avoid shear failure. Measurements of bridge deflections calibrate the analytical procedure for predicting long-term deflections; the analysis for long-term deflection assumes that externally applied load is introduced at age t0 and sustained to age t; time-dependent changes in strain and deflection are predicted by equations that are based on compatibility and equilibrium principles. Cracking is assumed to have occurred before t0 due to shoring of consecutive upper floors during construction.