Effects of compression reinforcement on the shear strength of reinforced concrete bridge beams

The predicted shear capacity of existing reinforced concrete bridge beams is an important subject that needs to be addressed in more detail. Concern has been Expressed about whether the current bridge assessment code for the United Kingdom is too conservative when evaluating the shear resistance of existing concrete beams that contain considerable amounts of top (compression) steel, ignored during assessment. This paper looks at the beneficial effects of such compression steel on the shear strength of reinforced concrete beams. The results of laboratory testing are compared with the current code predictions for the shear strength of beams assumed to contain solely tension steel. Further comparisons are made with an upper-bound plasticity solution which is able to consider all steel reinforcement in a concrete beam. There are several reasons why bridges exhibit hidden reserves of strength, with compressive membrane action probably the most important. However reasons such as the presence of compressive steel will contribute to hidden strength, so that research of this kind is required to enable realistic assessments and strength predictions to be carried our. It is shown that the presence of top (compression) steel has a significant effect on the shear capacity of reinforced concrete bridge beams which contain shear stirrup reinforcement.