Determining Shear Capacity of Prestressed Concrete Beams

Project summary:

The shear provisions of the American Association of State Highway and Transportation Officials bridge design code have changed significantly in recent years. The 2004 Load and Resistance Factor Design (LRFD) and 2002 Standard shear provisions for the design of prestressed concrete bridge girders typically require more shear reinforcement than the 1979 Interim shear provisions. The purpose of this research was to determine whether or not bridge girders designed according to the 1979 interim shear provisions were under-designed for shear and develop a method to identify potentially under-designed girders. Two shear capacity tests were performed on opposite ends of a bridge girder removed from MnDOT Bridge No. 73023. The stirrup spacing in the girder suggested it was designed according to the 1979 Interim shear provisions. The results from the shear tests indicated the girder was capable of holding the required shear demand because the applied shear at failure for both tests was larger than the factored shear strength required by the 2004 LRFD HL-93 and 2002 Standard HS20-44 loading. The results of a parametric study showed that girders designed using the 1979 Interim were most likely to be under-designed for shear near the support, and that the girders most likely to be under-designed in this region had smaller length-to-girder spacing ratios.