Shear Deformation in Seismic Frame Structures

Inelastic cyclic loading in reinforced concrete members, which may occur in ductile frame structures under severe seismic conditions, causes shear deformation to develop in hinge zones. Experimental results illustrating this are presented. The form of the shear vs. shear deformation curve changes with repeated inelastic cycling. With each such cycle, the shear displacement is increased and most of this deformation occurs when the shear force reverses in direction. This accounts for a large part of the stiffness degradation of flexural members. For beams with closely spaced stirrups which are proportioned to resist all the shear, and where shear stresses are not high, strength degradation does not appear to be a problem. Two different forms of plastic hinge may occur in structures forming sway mechanisms under earthquake conditions. In the first, the hinges are subjected to both positive and negative rotations, while, in the second, inelastic rotations in one direction accumulate in the hinge. It is only in the first case that stiffness degradation occurs due to shear deformation.