Stiffness of Reinforced Concrete Plates

A general formulation for the in-plane and flexural stiffnesses of isotropically and nonisotropically reinforced concrete plates is presented. The stiffness of plates is related quantitatively to the relative orientation of the reinforcement with respect to the applied forces, the combination of the applied forces and the amounts of reinforcement in the two orthogonal directions. By rotating the reinforcement from 0° to 45° with respect to the axes of the applied forces the in-plane or flexural stiffness may be decreased to one-half or less. The investigation did not support conventional theories to calculate flexural stiffness such as those in the 1971 ACI Building Code. In general, the ACI Code Equations considerably overestimate flexural stiffness. Results of this investigation can be used to calculate bending moments, deflections and effectiveness of reinforcing bars in cracked concrete sections.