Design of high-strength concrete beams subjected to small axial loads

In the design of reinforced concrete beams, the effects of small axial loads (axial load ≤ 0% of axial capacity of concrete section) are often ignored or at most only nominally allowed for. Moreover, the existing guidelines given in the various design codes are applicable only to beams cast of normal-strength concrete and designers are left to themselves to decide what to do when high-strength concrete is used. In this study, the effects of small axial loads on the flexural strength and ductility of normal- and high-strength concrete beams have been investigated by complete moment–curvature analysis of beam sections cast of different grades of concrete and subjected to different levels of axial load. The results revealed that the application of a small axial load has the beneficial effect of increasing the flexural strength but also the adverse effect of decreasing the flexural ductility. Based on the numerical results, simple formulae for estimating the effects of small axial loads and guidelines for the design of normal- and high-strength concrete beams subjected to small axial loads have been developed.