AN INVESTIGATION OF MINIMUM REINFORCEMENT REQUIREMENTS IN BOTTLE-SHAPED STRUTS

Abstract

The strut-and-tie method (STM) is a rational methodology for the design of D-(Discontinuity) regions in structural concrete. D-regions are those portions of a structure in which there is a complex no-linear variation in strain and beam bending theory cannot be used. The flow of forces within a D-region can be modelled using a network of struts and ties, called as a strut-and-tie model, which serves to transfer the applied loads to the supports or to adjacent B(Beam or Bernoulli) regions. Bottle-shaped struts are formed within a D-region when the external load is applied to a relatively small area of the member and the stresses disperse as they flow through the member. As the compression disperses along the length of a bottle-shaped strut, tensile stresses perpendicular to the axis of the strut are developed to counteract the lateral component of the angled compression forces. Because of its susceptibility to longitudinal splitting due to the transverse tensile force, transverse reinforcement has to be provided to control longitudinal splitting in bottle-shaped struts. The Indian code IS 456:2000 is silent on transverse reinforcement requirements in bottle-shaped struts. There is no consensus amongst International codes particularly the ACI, CSA and the AASHTO Codes on the desirability, the influence of and the quantity of transverse reinforcement required in bottle-shaped struts. At the same time, researchers have also pointed out an error in the formula specified in Appendix A of ACI 318-05 for calculating the effective transverse reinforcement in the bottle-shaped struts. The present experimental investigation has been carried out to comparatively evaluate the transverse reinforcement requirements for bottle-shaped struts specified by various codes and investigate the influence of parameters related to transverse reinforcement on the strength and serviceability behavior of bottle-shaped struts. The results of this investigation reveal that the minimum web reinforcement content specified in codes is sufficient for satisfying the serviceability requirements with respect to cracking though orthotropic detailing of web reinforcement is best from serviceability point of view. The effective transverse reinforcement content in bottle-shaped struts has been found to have a significant influence on the efficiency and hence the strength of bottle-shaped struts when interpreted in terms of the corrections suggested by researchers in the relevant ACI formula for effective transverse reinforcement content. The ACI specified efficiency factors for bottle-shaped struts have been found to be too conservative.