Shear brilliance

U of T Completes Groundbreaking Experiment on Shear Resistance

Supervised by Professors Evan Bentz and Michael Collins, master’s student Phil Quach (CivE 1T2, MASc 1T5) headed new groundbreaking experiment in the Structures Lab to discover the effect of extreme member depth on shear resistance.

A shear failure is a critical issue that all engineers want to prevent. It is one of the ways a structure can collapse catastrophically with little to no warning, such as  Quebec’s De La Concorde overpass in 2006.  As the demand for larger infrastructure and buildings increases, so does the need for greater understanding of how larger slabs of reinforced concrete—used in roofs or transfer slabs, for example—behave in shear.

“When we’re dealing with design of buildings and industrial facilities, we have one chance to get it right,” said Prof. Bentz. “We have to make sure we know the behaviour of the materials.”

The nature of reinforced concrete—its propensity to crack and susceptibility to shear failure—limits the applicability of scale model testing due to the size effect; different sizes of concrete slabs will behave differently under stress. Professors Frank Vecchio and Michael Collins developed the Modified Compression Field Theory (MCFT) to explain shear behaviour in 1986. According to the theory, the size effect should be substantial for some types of members such as thick slabs.

The experiment expanded on a previous series of tests on shear strength. Currently, the largest concrete slab in the world tested for shear was three metres thick in Japan. The U of T team doubled the size of its previous experiment—a two-metre deep slab in 2000—by creating a four-metre high, 20-metre long slab that is reinforced longitudinally in one end of the member and both longitudinally and transversely in the other end.

The results were as significant as the specimen itself: they validated the current provisions in the Canadian design code, and allow for thicker slabs as modern construction slabs increase in size. The results also find failures in current American building codes; the U of T study will be instrumental in changing what is currently prescribed as safe design.

The team has also engaged about 350 academic research groups and engineers worldwide in a prediction competition to estimate the strength of the slab, given the specifics of the specimen. They received close to 100 responses from the U.S., Germany, Turkey, Mexico, Italy, Australia, and Canada. The guesses varied in accuracy.

“The fact that the predictions have been so variable so far shows us that a lack of knowledge and inconsistencies in international building codes are a big problem,” said Bentz.