Evaluation of Shear Design Methods for Large, Lightly-reinforced Concrete Beams

This paper describes one phase of an extensive experimental program that has recently been completed at the University of Toronto. In this phase, eighteen lightly-reinforced shear-critical reinforced concrete beams were loaded to failure. The abilities of the ACI-318 shear design method and a simplified design method based on the Modified Compression Field Theory to predict the failure loads are compared. It is found that the ACI design method is dangerously unconservative when applied to large beams and oneway slabs constructed without stirrups, while the simplified MCFT design method is both safe and accurate. Studies of the mechanism of shear transfer indicate that approximately one quarter of the shear in a reinforced concrete beam constructed without stirrups is transferred in the compression zone, with the rest carried primarily by aggregate interlock. The development of theoretically-sound shear design methods must therefore be based on the fact that aggregate interlock plays a critical role in the shear behaviour of reinforced concrete structures.