Bond Deformability of Reinforced Concrete Members

The usual earthquake resistant design philosophy of ductile frame buildings allows the beams to form plastic hinges adjacent to beam-column connections. In order to carry out this design philosophy, the ultimate bond or shear strength of the beam should be greater than the flexural yielding force and should not degrade before reaching its required ductility. The behavior of RC members dominated by bond or shear action reveals a dramatic reduction of energy dissipation in the hysteretic response due to the severe pinching effects. After flexural yielding, plastic hinges develop near both ends of these beams and reversed cyclic loading produces a progressive deterioration of bond that may lead to failure at cyclic bond stress levels lower than the ultimate stress under monotonic loading. In addition, flexural bond stress increases with the increasing of plastic hinge length induced during positive and negative loadings. The designer has to consider the effect of bending moment and shear when designing for bond since the bond seldom acts on its own but rather in combination with flexure and shear. In this study, a method was proposed to predict the deformability of reinforced concrete members with short-span-to-depth-ratios failing in bond after flexural yielding. Repeated or cyclic loading produces a progressive deterioration of bond that may lead to failure at cyclic bond stress levels lower than the ultimate stress under monotonic loading. Accumulation of bond damage is supposed to be caused by the propagation of micro-cracks and progressive crushing of concrete in front of the lugs. The proposed method takes into account bond deterioration due to the degradation of the concrete in the post yield range. In order to verify the bond deformability of the proposed method, the predicted results were compared with the experimental results of RC members, reported in the technical literature. Comparisons between the observed and calculated bond deformability of the tested RC members, showed reasonable agreement.