A partial-interaction ductility model for FRP plated RC flexural members

Abstract

Structural engineers have long recognised that ductility is a major consideration in the safe design of reinforced concrete structure. FRP retrofitting is now a well established technique for strengthening reinforced concrete structures but because FRP is a brittle material this can lead to the misconception that FRP retrofitted structures behave in a brittle fashion which has restricted the use of FRP retrofitting. Furthermore, quantification of the ductility of FRP plated RC structures is much more difficult than the quantification of the ductility of unplated RC structures as the latter depends only on the limit of concrete crushing. Hence the necessity in FRP retrofitted RC members to fully quantify the rotational capacity for all limits. In this paper a novel ductility model for FRP internally or externally strengthened or FRP confined RC members is described which fully simulates the behaviour of flexural members all the way through to concrete softening and then member failure. The model simulates the rotation due to material non-linearity, flexural cracks and the occurrence of both disturbed and undisturbed regions. However and more importantly, the model fully simulates the behaviour of the hinge in the region where concrete softening occurs. It is shown that in the hinge the rotation is limited by concrete softening and that this softening limit can be derived directly from shear-friction theory. It is also shown that the rotation of the hinge is also limited by intermediate crack (IC) debonding of the FRP reinforcement and by yield-penetration failure of the steel reinforcing bars. All of these rotational limits have been quantified and the mechanisms used in the quantification is presented in this paper. This research will allow the expansion of FRP reinforcement into regions where ductility is required and hence considerably expand the use of FRP.