Seismic damage mechanism of weak beam-column joint frames

The use of old building design codes and improper execution of recent seismic design practices resulted in substandard and vulnerable reinforced concrete building stock, the majority of which is built with weak beam-column joint connections (i.e. joint panels having no transverse reinforcement and built in low strength concrete). In order to understand the seismic response and damage behavior of recent special moment resisting frame (SMRF) structures with the defect of weak beam-column joints, shake table tests were performed on two 1:3 reduced scale two storey and one bay RC frame models. Reference code design and weak beam-column joint frame models were subjected to unidirectional dynamic excitation of increasing intensities using the natural record of 1994 Northridge Earthquake. The input scaled excitation were applied from 5 % to 130 % of the maximum input peak ground acceleration record, to force the test models from elastic to inelastic stage and then to fully plastic incipient collapse stage. The weak beam-column frame experienced flexural cracking of the columns, longitudinal bar-slip in beam members, cover concrete spalling and severe damageability of the joint panels under multiple dynamic excitations. The deficient frame was able to resist only 40 % of the maximum acceleration record as compared to the code design frame, which was able to resist up to130 %. Based on the experimental observations, a drift-based damage scale was developed for different performance limits states that can be employed for the global performance assessment of deficient weak beam-column joint RC frames.