In-plane shear strengthening of reinforced concrete diaphragms using fiber reinforced polymer composites

Reinforced concrete (RC) diaphragms play a fundamental role in seismic resistance of RC buildings. In addition to transferring the gravity loads to the beams and columns, diaphragms need to be sufficiently rigid with high shear capacity to resist and distribute the earthquake loads to the lateral force resisting system. Despite the large building inventory with under designed diaphragms to resist the shear loads from earthquakes, the number of studies investigating possible approaches to increase the in-plane shear capacity of diaphragms is limited. External equally distributed narrow strips of fiber-reinforced polymers (FRP) providing additional shear strength for RC diaphragms is proposed in this study. Five full scale RC diaphragms with overall dimensions of 4.5 m long by 1.5 m wide by 0.1 m thick were built and tested under cyclic shear loading. One out of the five specimens was tested as a reference with no strengthening, while the remaining four were strengthened with external glass or carbon fiber-reinforced polymer (GFRP or CFRP) composites. The test results showed that the FRP strengthening system effectively enhances the in-plane shear capacity of the RC diaphragms. Compared to the reference specimen, the FRP layout used for testing increased the shear capacity by up to 40%. This paper presents the test specimens, strengthening systems, and in-plane shear response of tested diaphragms in terms of failure modes, strength, and deformation. The experimental results were also compared to theoretical values determined according to the evaluation criteria in the International Code Council (ICC)-Evaluation Services (ES), Acceptance Criteria (AC) 125.