Arch Action and Bond in Concrete Shear Failures

Numerous investigators have suggested that a cracked reinforced beam without web steel behaves like a tied-arch. From a study of strain measurements in steel and on concrete, it is shown that a normally reinforced beam failing in shear behaves essentially like a beam up to failure, in spite of extensive cracking and deformation. External loads can be resisted by arch action only when the web acts in compression. Data reported show that such action can be developed in a beam only when the steel is fully unbonded between strong end anchorages. Arch action enhances the shear resistance of an unbonded beam beyond that of a bonded beam failing in shear but also increases deflection and reduced the flexural capacity. Lack of bond, however, is not a guarantee against shear failure or even diagonal cracking. The surface conditions of the longitudinal steel are shown to have limited influence on ultimate shear resistance.