Evaluation of Seismic Performance of Reinforced Concrete Frame Buildings with Retrofitted Columns

Abstract

In the US, ASCE/SEI 41 serves as the primary standard for the seismic evaluation of existing buildings, as well as for retrofit design. In this study, 3-, 6-, and 9-story nonductile reinforced concrete (RC) frame buildings, designed to the 1967 Uniform Building Code, are retrofitted according to that document’s nonlinear static procedures. For each building, retrofits are designed to meet ASCE 41 collapse prevention (CP), life safety (LS), and immediate occupancy (IO) performance levels by retrofitting vulnerable columns. Multiple column retrofit techniques are considered for each performance level. Once retrofits are designed, the retrofitted buildings are assessed using a nonlinear time history analysis and the framework of performance-based earthquake engineering to quantify seismic performance, collapse risk, and economic losses. These metrics are compared to the stated retrofit performance objectives to evaluate how well the retrofits align with the objectives for the global seismic performance of buildings. The results show that retrofits to IO generally perform as expected, with buildings retaining nearly all their strength and stiffness after design level shaking events and damage concentrating away from the columns that were the target of the retrofits. However, improvements in the nonstructural performance may not be significant without design efforts specifically targeting those components. For retrofits designed to CP and LS, the performance of the retrofitted structures is better than the unretrofit structures but highly variable.