Shear failure of reinforced concrete beams M. D. Kotsovos hnperial College of Science and Technology, Department o/"Civil Engineering, Imperial College Road, London SW72BU, UK (Received February 1985: revised May 1986) It has been suggested in a previous work that the causes of shear failure exhibited by reinforced concrete (RC) beams are associated with the stress conditions in the region of the path along which the compressive force is transmitted from support to support. The work described in this paper presents experimental evidence sup- porting the above concept. The work is based on a comparative study of the behaviour of concrete beams reinforced in compliance with this concept and that of beams reinforced in compliance with current design procedures. Keywords: shear failure, reinforced concrete Codes of practice, such as CPll0, ~ require structural members to be designed so as to exhibit ductile beha- viour, since such behaviour gives ample warning of impending collapse. Shear failures, therefore, are unde- sirable because of their brittle nature which allows little or no such warning. Current shear design procedures are based on the assumption that shear failures occur when the shear capacity of a critical section is exceeded. Thus, the objective of these procedures is to realistically assess the amount of transverse reinforcement required to carry that portion of the shear force in excess of the value which can be sustained by concrete alone. For beams subjected to two-point loading, the critical sec- tion is any section situated within the shear span (av) and, therefore, reinforcing this span in compliance with the current strength requirements is generally con- sidered to safeguard against shear failure. It has, however, been argued elsewhere, 2 that the concept of shear capacity of a critical section is insuffi- cient to describe the underlying causes of the observed behaviour of RC beams, Shear failure appears to be associated with the stress conditions in the region of the path along which the compressive force is transmit- ted from support to support and not with the shear capa- city of particular sections. As a result, it has been suggested that compliance with current design proce- dures, although preventing shear failure, may not be sufficient to provide adequate ductility.-" The present work, therefore, has been aimed at pro- viding experimental evidence to support the above argu- ments. This evidence has been obtained from tests on RC beams with various arrangements of transverse rein- forcement. The beams have been subjected to two-point loading with values of av varying between approximately 1 and 4 times the beam depth. The fundamental causes of shear failure are discussed concisely and this discus- sion forms the basis of a comparative study of the strength, deformation and failure characteristics exhi- bited by the beams tested in the programme. An under- standing of these causes is essential for the development of new shear design models compatible with the concept of member rather than section design stipulated by the CEB commission IV (Stuttgart meeting, 9 May, 1984). Causes of shear failure It is generally considered that the behaviour of RC beams without transverse reinforcement is dependent on the value of the shear span to depth (av/d) ratio. For values of a,/d between approximately 1 and 6, such beams are expected to fail in shear before their flexural capacity is attained. For any other value, the beams should attain their flexural capacity. The behaviour of the latter beams has been the subject of previous experi- mental work. 3 As stated elsewhere, 2 the causes of shear failure are likely to be associated with the stress conditions in the region of the path along which the compressive force is transmitted to the supports after the occurrence of diagonal cracking; an analytical description of these con- ditions could lead to the formulation of a lower bound criterion for failure. On the basis of this argument, the shear modes of failure exhibited by the beams under 0141-0296/87/01032 -07 / $03.00 32 Eng. Struct. 1987, Vol. 9, January © 1987Butterworth & Co (Publishers) Ltd