Factors affecting web-shear capacity of deep hollow-core units

Shear-strength estimates for precast, prestressed hollow-core units in the United States follow the shear recommendations of the American Concrete Institute's (ACI's) Committee 318. These recommendations are based on experimental research done mainly on members other than hollow-core units, and they have been extended to hollow-core units based on a limited amount of hollow-core shear tests. Recent shear tests that several U.S. manufacturers have performed on thick hollow-core units (units with depths larger than 12 in. [300 mm]) have indicated failures in the web at forces smaller than those predicted by Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R05). Similar observations have been made by researchers in Europe, where web-shear provisions are similar in nature to those in ACI 318-05. As a stopgap measure, ACI 318-08 waives the exception on minimum shear reinforcement for hollow-core units deeper than 12.5 in. (318 mm). An analytical research program at the University of Minnesota investigated the reasons that thicker hollow-core units fail at lower shear forces than those predicted using the ACI 318-05 provisions. This paper summarizes the findings of the analytical program, including comparisons of the ACI 318-05, Eurocode No. 2: Design of Concrete Structures-Part 1: General Rules and Rules for Buildings, the modified compression field theory, the American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications, and Yang's shear prediction methods.