Assessment of shear stress limits for high-strength concrete bridge beams

Abstract

The design of concrete beams for shear loading is governed in New Zealand by the provisions of NZS 3101. The shear design provisions of NZS 3101 impose two limits on the permissible design shear capacity, including a maximum shear capacity of 8MPa. This 8MPa limit influences the efficiency of concrete beam design, and in particular the design of concrete bridge beams that have concrete compressive strengths greater than 40MPa. The validity of this limit was assessed through an examination of a number of other international design standards, statistical analyses using databases composed of all previous experimental testing of reinforced concrete (RC) and prestressed concrete (PC) beams, and results from an experimental investigation aimed at addressing deficiencies in the compiled databases. The research found that the limits in NZS 3101 are excessively conservative compared with the limits imposed in most other design standards. This observation was reinforced by analysis of the databases and results of the experimental investigation, which supported the need for a limit on the nominal design shear capacity but found that an absolute limit of 8MPa was overly restrictive. Alternative limits were proposed, and the absolute limit of 10MPa was found to provide improved design accuracy without compromising safety.