Assessment of seismic behavior of RC frames retrofitted using concrete jacket system

The vulnerability and poor seismic performance of the columns and beams of old non-seismically designed reinforced concrete (RC) frame structures has been proven time and again, both at laboratory level as well as by natural disasters in real life situations. On the other hand, a vast majority of existing structures designed with non-seismic considerations is not practically viable so that they designed and constructed with improper reinforcement detailing, with no consideration of the principles of the capacity design, inappropriate of the seismic actions, etc. Such structures need to be correctly assessed to predict their seismic performance and retrofitted, if required.

Therefore, the assessment of seismic behavior of RC frames retrofitted using concrete jacket system would be effective. In order to realistically predict the seismic performance of such structures, providing practical and accurate models to simulate the inelastic behavior of structural members are significantly importance. Evaluation of the response of non-seismically designed RC structures by using such tools would estimate the seismic performance of the structures with high degree of confidence. According to such assessments, an efficient and cost-effective solution can be designed and developed.

In order to improve the seismic behaviour of old non-seismically designed RC frame structures, numerous retrofitting techniques of structural members have been adopted and studied in practical applications. Among these retrofitting techniques, the application of concrete jacketing has extensively increased. The current study has focused on evaluating the effects of concrete jacketing on flexural behaviour of RC jacketed members and seismic performance of non-seismically designed RC structures through analytical modeling approaches.

For this purpose, an analytical model including rotational springs in members was developed. The nonlinear characteristics of these springs can be computed using moment – curvature analysis of a RC member before and after retrofitting so that each level of curvature would be converted into rotation based on the first moment-area theorem. Experimental and analytical studies have confirmed that the confinement induced by transverse reinforcements would lead to considerable enhancement in terms of strength and ductility of RC columns and beams. Thus, this effect was taken into account in determining characteristics of stress – strain concrete. On the other hands, it is well known that the response of RC members in terms of ultimate deformation could be influenced when buckling in longitudinal steel bar occurs. Therefore, in this study, a simplified methodology has been developed to take into account this effect as ultimate condition in the calculation of the moment – curvature analysis of a RC jacketed member.

In order to prove the reliability of the proposed model, it has been validated against experiments conducted by other researchers in the literature. The results showed that the proposed model can be successfully predicted the response of columns retrofitted by RC jacketing. Moreover, the application of the developed model to assess and retrofit RC structures has been demonstrated by a case study and using pushover analysis. The response of structures in terms of initial stiffness, strength, ductility and damage patterns corresponding to performance point and ultimate displacement indicated that the seismic behaviour and performance level of the retrofitted structure were significantly improved in comparison with the original structure.