DISTRIBUTION IN PLAN AND ELEVATION OF DISSIPATIVE BRACING FOR SEISMIC RETROFIT OF R.C. FRAMES

Abstract

Dissipative bracing systems have been one of the approaches for improving the seismic performance of Reinforced Concrete (RC) frames in earthquake-prone areas since the 1980s. However, design methods for dissipative bracing systems are still under research. Most existing studies focus on evaluating the global stiffness and strength of the dissipative braces. At the same time, a smaller number of those define the criteria and methods for their distribution both in elevation and plan. These issues are particularly important for irregular structures, as the precise design of each brace and dissipative device must comply with local features. In this area, inadequate consideration is given to controlling the increase of axial loads in columns, which affects their deformation capacity. Since the 1980s, seismic strengthening interventions using dissipative bracing have been developed for RC frames originally designed to withstand only gravity loads. Most of the design methods are derived based on Pushover analysis (POA), able to identify structural weaknesses at different stages under lateral displacement. However, in the literature there is an absence of retrofitting of structures designed with modern earthquake codes, in which strengthening is required due to an increase in the seismicity of the area or change in use, resulting in a change in the importance class and consequently the return period of seismic action. In this research, criteria for the distribution of stiffnesses and resistances of dissipative bracing both in elevation and in the plan are provided, aiming at reducing structural irregularities and minimizing the reduction of column deformation capacity due to variation of the axial load. A reinforced concrete structure is designed in a medium seismicity area according to current seismic codes using modal response spectrum analysis and a capacity design approach. POA is conducted with finite element modeling to identify structural performances and address design deficiencies. Then, due to an increment of the design seismic action, strengthening interventions by dissipative bracing are designed, using standard procedure, or based on the aforementioned criteria. Comparative evaluations through static and nonlinear dynamic analyses reveal significant improvements, including increment of deformation capacity, reduced inter-story drift demands, better stress distribution, ensuring enhanced seismic performance.