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Softening Effects in Design of R C Planar Members

It has been a well known phenomenon that the principal compressive stress in concrete is a function not only of the compressive strain but also of the co-existing principal tensile strain. Thus concrete subjected to high tensile strains in the direction normal to the compression is softer and weaker than concrete in a standard cube or cylinder test. Such ‘softening’ effect has been confirmed by experiments by various researchers for in-plane structures. In this paper, the phenomenon is extended to the study of a plate bending structure having bending moments of opposite signs (one hogging and the other sagging) in two mutually perpendicular directions at the same point. As concrete will be having compression and tension in two perpendicular directions above and below the neutral axis, it is believed that the same softening effect will exist in the plate bending structure. However, the effects have been commonly ignored by most local engineers. In the paper, the authors propose simplified equations to account for the effects.