Significance of elongation on seismic resistance of concrete frame structures or elongation in ductile seismic-resistant reinforced concrete frames

Technical Abstract

To survive a major earthquake, current practice requires seismic resistant frames to be designed to be ductile. To achieve the required level of ductility in multi-storey frames, the majority of the potential plastic hinge zones are located in the beams. The inelastic rotation, which may develop in these zones, arises predominantly from the tensile yielding of the reinforcement. The associated compressive strains are small and as a consequence elongation occurs.

Test results show that elongations of the order of 2 to 4 percent of the member depth develop in plastic hinge zones of beams subjected to cyclic loading before strength degradation occurs. The factors influencing elongation are reviewed. The results of a time history analysis, in which elongation effects are modelled, shows that this action, which is neglected in current design practice, has important implications for the detailing of columns and the design of supports for precast components and external cladding.