Influence of Axial Stress on Shear Response of Reinforced Concrete Elements

There is strong disagreement between different code provisions as to the influence of axial stress on shear strength. To examine this influence, six nominally identical reinforced concrete elements representing web regions of girders or walls were loaded under different ratios of longitudinal axial stress to shear stress. The results demonstrated that the application of the basic ACI 318-08 shear approach can significantly overestimate both the beneficial effect of compression on shear and the detrimental effect of tension on shear strength. The ACI 318-08 simple expression for the benefits of compression gave excellent predictions, whereas the simple expression for tension was very conservative. The CSA A23.3-04 shear provisions based on the modified compression field theory (MCFT) provided the best code-based estimates of the shear strength. The full MCFT provided not only the best estimates of conditions at failure including failure shear stresses and failure crack angles for the full range of axial stresses but also provided predictions of the complete load-deformation response of the elements. For the two highest compression-to-shear ratios, these load-deformation predictions were reasonable, whereas for the other axial load levels, they were excellent.