Shear strength of concrete masonry walls

Abstract

There is general consensus that the New Zealand masonry design standard, NZS 4230:1990, is overly conservative in its treatment of masonry shear strength, therefore restricting cost-effective masonry design. This report presents an experimental study that investigated the shear strength of reinforced concrete masonry walls. Also included in this report is a study that compares the accuracy of shear equations in predicting masonry shear strength.

Valuable information about masonry shear strength was captured during the testing of full scale masonry walls. It was concluded that horizontal reinforcement and axial compression load provided additional shear resistance to masonry walls. Therefore the nominal shear strength of reinforced masonry walls could be evaluated as a sum of contributions from masonry, shear reinforcement and applied axial stress. In addition, it was concluded that masonry shear strength decreases inversely in relation to an increase in H/L ratio.

Seven shear equations were selected to examine their accuracy in predicting masonry shear strength. Of the seven predictive equations, there were four equations currently prescribed by codes and the remaining three were proposed formulations which used different functional forms for the effect of various parameters on masonry shear strength. It was found that the NEHPR shear expression provides the closest shear prediction for both the fully and partially grouted masonry walls. However, the NEHPR shear expression does not address masonry shear strength within potential plastic hinge zone. Also, the use of 0.5phfyh in its vs term is contrary to well established split beam theory. Consequently, a new shear equation was developed in this study.

It was shown that the newly developed shear equation provided significant improved masonry shear prediction than the NZS 4230:1990 shear expression. It was also shown that shear prediction using the newly developed shear equation produced results with similar accuracy to that of NEHPR. Consequently, it is recommended that this shear equation to be implemented into the New Zealand masonry design standard.