The report describes an experimental and analytical investigation of the vertical shear strength of composite bridge beams formed from a standard precast prestressed beam of M5 section with the deck slab added in situ. Fourteen beams, four of them with deflected strands, were tested to failure under static loading and a further four were tested under repeated loading. The factors governing the static shear strength are considered theoretically and experimentally with particular attention to the effect of the degree of prestress and amount of web reinforcement. The effect of flexural cracking before shear failure is also examined and the influence of deflected tendons on the shear strength is considered in a separate section. The tests under repeated loading are used to discuss inclined cracking of the concrete due to fatigue and consequent fatigue of the web reinforcement. The behaviour of tack-welded stirrups, used in three of the four beams, is also examined. The investigation shows the static shear design recommendations in BS5400: 1984 to be mainly conservative but two small modifications relating to sections cracked in flexure are proposed. However, at the ends of the prestressing strands, when sections are subjected to high shearing forces, the recommendations are unconservative in that allowance needs to be made for slippage because this results in a reduced ultimate shear strength. The considerable increase in the stirrup stress under repeated loading requires further investigation.

Want to know more about this project?