Relevance of beam-column joint damage and collapse in RC frame assessment

The role of joint damage and collapse in the seismic response assessment of existing reinforced concrete frame buildings is herein investigated. Based on recent results from experimental investigations on frame system and subassemblies designed for gravity-load-only, considerations on structural performance based on hybrid local and global failure mechanisms related to joint damage are provided, with particular attention to displacement demand, interstorey drift and damage distribution. Effects of bond deterioration and slip of reinforcing bars passing through an interior joint are discussed in terms of local hierarchy of strength and sequence of events. A simple analytical model for joint response is proposed and adopted for preliminary investigations on frame systems with substandard structural details. The occurrence of a "shear hinge" in the joint might protect to some extent soft-storey mechanisms, reducing the interstorey drift demand, with no significant effects on the global displacement demand. On the other hand, typical inadequacies of structural details (i.e. end-hook anchorage in beam bars combined with use of smooth bars) might cause severe strength degradation leading to particularly brittle failure mechanism.