Evaluation of Minimum Shear Reinforcement Requirements in Non-Prestressed Beams without Distributed Horizontal Reinforcement

摘要

This paper presents an evaluation of the minimum shear reinforcement requirements in the ACI 318 code for nonprestressed concrete beams exempt from distributed horizontal reinforcement requirements. A total of 34 tests performed by different researchers on reinforced concrete beams with heights in the range of 24 to 36 in. (600 to 900 mm) are used to examine the reserve shear strength defined as the shear strength in excess of the nominal shear strength provided by the concrete, V_c, calculated in accordance with ACI 318. Additionally, the design shear force limitations for these beams containing minimum shear reinforcement are examined. Tests evaluated in this study include beams without shear reinforcement as well as beams with shear reinforcement levels that are less than ACI 318-08 minimum requirements. From the evaluation conducted in this study, it is concluded that the addition of low amounts of shear reinforcement, even less than the minimum amount required by ACI 318-08, V_(s,min), provide a reserve strength beyond V_c calculated in accordance with the code. Results also show that low amounts of shear reinforcement tend to eliminate the trend of decreasing shear strength with increasing height. When low levels of shear reinforcement are taken into account in the strength calculation (that is, V_n=V_c + V_(s,min)), however, specific concerns are raised regarding the reliance on minimum shear reinforcement to mitigate low values of the concrete contribution to the shear strength as well as provide shear resistance above V_c without the use of the strength reduction φ factor. Modifications to the minimum shear reinforcement requirement exceptions for beams in ACI 318-08 are also examined.