Shear Reinforcement Requirements for High-Strength Concrete Bridge Girders

Abstract

A research program was conducted on the shear strength of high-strength concrete members. The objective was to evaluate the shear behavior and strength of concrete bridge members with compressive strengths in the range of 10 000 to 15 000 psi. The goal was to determine if the current minimum amount of shear reinforcement together with maximum spacing limits in the 2004 AASHTO LRFD Specifications, and the upper limit on the nominal shear strength were applicable to concrete compressive strengths up to 15 000 psi. A total of twenty I-shaped specimens were tested monotonically to failure. Sixteen specimens were reinforced concrete beams, half of them without shear reinforcement. Four AASHTO Type I prestressed concrete beams were also tested. The main variables were the compressive strength of concrete and the amount of longitudinal and transverse reinforcement. Measured concrete compressive strengths ranged from 7 000 to 17 000 psi. Longitudinal reinforcement ratios on the basis of web width, ρw, varied from 1.32 to 7.92%. All specimens met the flexural requirements in Section 5.7.3.3.1 of the 2004 AASHTO LRFD Specifications. The amounts of shear reinforcement, ρvfyv, provided were in the range of 0 to 1 300 psi. Main findings support the notion that the current prescribed minimum amounts of shear reinforcement in both the 2004 AASHTO LRFD Specifications and the ACI 318-05 Code provide sufficient reserve strength after first inclined cracking, and adequate crack width control at estimated service load levels for reinforced and prestressed concrete beams with concrete compressive strengths up to 15 000 psi. Based on the test results of reinforced concrete specimens, an upper limit for the nominal shear strength of 12 f'c in concretes with compressive strength up to 15 000 psi was shown to be adequate to prevent web crushing failures prior to the yielding of stirrups. This limit is similar to the current upper limit on the nominal shear strength in the ACI 318-05 Code.