Development and Validation of a Metallic Haunch Seismic Retrofit Solution for Existing Under-Designed Reinforced Concrete Frame Buildings

Te-Hsiu Chen

Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand

Constantin Christopoulos

Department of Civil Engineering, University of Toronto, Canada

Stefano Pampanin

Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand

The feasibility and efficiency of a seismic retrofit solution for existing reinforced concrete frame systems, designed before the introduction of modern seismic-oriented design codes in the mid 1970s, is conceptually presented and experimentally investigated. A diagonal metallic haunch system is introduced at the beam–column connections to protect the joint panel zone from extensive damage and brittle shear mechanisms, while inverting the hierarchy of strength within the beam–column subassemblies and forming a plastic hinge in the beam. A complete step-by -step design procedure is suggested for the proposed retrofit strategy to achieve the desired reversal of strength hierarchy. Analytical formulations of the internal force flow at the beam-column-joint level are derived for the retrofitted joints. The study is particularly focused on exterior beam–column joints, since it is recognized that they are the most vulnerable, due to their lack of a reliable joint shear transfer mechanism. Results from an experimental program carried out to validate the concept and the design procedure are also presented. The program consisted of quasi-static cyclic tests on four exterior, 2/3 scaled, beam–column joint subassemblies, typical of pre-1970 construction practice using plain round bars with end-hooks, with limited joint transverse reinforcement and detailed without capacity design considerations. The first (control specimen) emulated the as-built connection while the three others incorporated the proposed retrofitted configurations. The experimental results demonstrated the effectiveness of the proposed solution for upgrading non-seismically designed Reinforced Concrete frames and also confirmed the applicability of the proposed design procedure and of the analytical derivations.

Recent experimental investigations on the seismic performance of existing reinforced concrete frame buildings, designed for gravity loads only, as typically found in seismic prone countries before the introduction of adequate seismic code provisions in the mid-1970s, have confirmed the expected inherent weaknesses of these systems, that have been observed in past earthquake events. Because of the poor detailing of the reinforcement, the absence of capacity design philosophy and the use of plain round reinforcing bars, undesirable brittle failure mechanisms are observed at either the local level (i.e. shear failures in joints, beam or column members) or globally in the structure (i.e. soft-storey mechanisms). The beam–column joint panel region is of particular interest in such systems, as it is likely to be the critical and possibly the weakest link according to capacity design or hierarchy of strength considerations. Joint damage and failure can in fact lead to severe deterioration of the overall lateral load carrying capacity of the structure and even result in total collapse. Appropriate retrofit strategies, capable of providing adequate protection to the joint region while modifying the strength hierarchy between the different components of the beam–column connections, according to a capacity design philosophy, are thus required for improving the seismic response of such structures.

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