SEISMIC BEHAVIOR OF POORLY DETAILED BEAM-COLUMN JOINTS RETROFITTED WITH FRP SYSTEMS: EXPERIMENTAL INVESTIGATION AND ANALYTICAL MODELING

Abstract

The high vulnerability of existing reinforced concrete (RC) structural systems is often related to brittle failures of critical members. Field surveys and relevant scientific studies showed that unconfined and poorly detailed beam–column joints were not able to resist moderate-to-large seismic events. Several strengthening techniques have been proposed to improve the seismic capacity of existing RC beam–column joints. The effectiveness of composite materials, such as fiber reinforced polymer (FRP) systems, has been demonstrated by experimental tests on joint subassemblies and entire structural systems. This, along with the simplicity of installation, has strongly promoted the use of the composite material for seismic retrofit of RC structures. However, the large number of parameters affecting the mechanical behavior of FRP strengthened beam–column joints makes the development of reliable capacity models complex. In recent years several proposal have been advanced, but a simple and generalized formulation is still lacking. This is because of the complex mechanical behavior of joint panel and the high variability of the debonding phenomena that strongly affect the FRP response. In order to investigate the behavior of unconfined joints that do not conform to current seismic codes and the effectiveness of externally bonded fiber reinforced polymers (FRPs) as a strengthening technique a wide experimental program has been designed. This study deals with the experimental test of seven full-scale RC corner joints under constant axial load and transverse cyclic loading in the as-built and FRP-strengthened configuration. The specimen design strategy and test setup are described and the experimental outcomes are illustrated and compared. Particular emphasis is given to comparing the experimental recorded and the predicted capacity of as-built joints, on the basis of models available in the literature. A discussion on the effectiveness of different FRP-strengthening layouts is also reported. Basing on the experimental evidence of the tested specimens and available literature studies, a new strength capacity model to account for the strength increase provided by FRP systems in the seismic retrofit of poorly detailed corner joints has been developed. A large database of experimental tests has been analyzed to assess the accuracy of the proposed model. The simple theoretical approach and the use of experimentally determined parameters make this model suitable for practical applications. In conclusion, a new modeling approach has been proposed to reproduce the joint hysteretic response in a FEM-based environment dedicated to shear critical members.