Tests on beams with sustained loading

Summary Six pairs of concrete beams, each containing one beam reinforced with mild steel (M.S.) and one with Tentor steel (H.T.), with spanldepth ratios (L/d) of 20, 30 and 40, and all designed for the same bending moment, were kept under the full design load for 4¾ years. During this period the deflexions increased to 3·14–3·94 times the original. All the beams became unserviceable, although their factor of safety against failure was over 3. The test results were compared with deftexions calculated by various methods. The ratio of the cross-sectional areas of the reinforcement was M.S./H.T. = 1·70, but the average ratio of the maximum deflexions was only H.T./M.S. = 1·14. The average depth of the neutral axis increased considerably. The tests proved that the limiting slenderness ratios in CP 114(1957), Clause 309a, Table 13 are excessive if a substantial proportion of the design superimposed load is to be supported for a long period. It is suggested that Table 13 in the Code should be revised and that the limiting slenderness ratio should vary with the stress in both the steel and the concrete.