Shear behavior of large reinforced concrete deep beams and code comparisons

In this paper, an experimental program on the size effect in reinforced concrete deep beams is described. A total of 12 large- and medium-sized specimens with overall height h ranging from 500 to 1750 mm and effective span 1_e from 1300 to 4520 mm were tested to failure under two-point symmetric top loading. The beams had compressive cylinder strengths f′_c of about 40 MPa and main steel ratio of 2.60 percent. Test results reveal that the ultimate shear stress is size-dependent and that Bazant's law can best describe this size effect. On the other hand, the diagonal cracking stress is hardly size-dependent. Besides the shear span-to-height ratio a/h, the size effect h also has a significant influence on the failure mode; larger deep beams are more brittle in comparison with smaller ones. Some plausible explanations are given to the source of size effect in deep beams. It may arise from different rates of release of fracture energies associated with crack propagation in beams of different sizes. This is demonstrated from the crack patterns of geometrically similar beams at the same nominal shear stress, where it is obvious that crack development was more extensive in larger specimens. The 12 test results are then compared with predictions from the current ACI Code, the UK CIRIA Guide-2, and the Canadian CSA Code. Comparison study shows that while the ACI Code predictions do not have uniform safety margin, and estimations from CIRIA are generally unsafe for large deep beams, the strut-and-tie-model predictions in the Canadian Code yield uniform safety margin.