Experimental response of a substandard RC frame building retrofitted with GFRP sheets and slab-weakening subjected to earthquake motions

This paper presents an experimental study on the seismic response of a 2/5 scale, 3-storey, 2-bays non-ductile reinforced concrete frame building retrofitted with glass fibre reinforced polymer sheets and weakening of the floor slab around the strengthened beam hinge region, following a partial retrofit strategy involving an upgrade of the exterior beam column (b-c) joints only. The research comprised two uni-directional shake-table experiments, including a different earthquake record each, with similar PGA, but different duration and frequency content. During the short-duration test, the specimen experienced limited inter-storey drift ratios (less than 0.6%) and presented no visible damage. During the long-duration test, instead, the specimen undertook large inter-storey drifts ratios, particularly in the first storey (3.7%), and showed large cracking in the exterior beams and crushing of the concrete at both ends of the central columns. An analysis of the spectrograms of the input and top-storey displacement motions shows that the large-amplitude response of the specimen during the long-duration test was apparently due to resonance. It was concluded that the proposed retrofit strategy was able to attain, under an adverse scenario with large inter-storey drift demands, the fundamental objectives of: (a) inverting the hierarchy of strengths and sequence of events in the critically vulnerable exterior b-c joints, relocating the damage from the panel zone into the beams at the weakened part of the slab; and (b) improving the global inelastic mechanism of the structure by shifting it from brittle/unstable to ductile/stable.