Description
The nonlinear static procedures are very popular for the design and assessment of buildings subjected to earthquake ground motion in current structural engineering practice. Their application and relative accuracy are very relevant issues, especially in the case of seismic retrofit of irregular and/or high-rise buildings. Many retrofit design procedures are valid for low-rise buildings and/or neglect torsional effects and higher modes contribution. The vibration properties are considered unchanged after retrofit, and the higher modes are often neglected. Other ones are based on the proportional stiffness criterion (i.e., lateral story stiffness due to the additive structures is considered proportional to that of the original main structure). Still others rely on the hypothesis that the main structure remains elastic. Often, they neglect the interaction between the main structure and the additive structures used for retrofit. These are very significant drawbacks in the case of plan-asymmetric buildings, where torsional effects are important. In this case, the seismic response is dominated by harmful torsional effects and, thus, the retrofit strategy should significantly modify the dynamic response. To overcome such drawbacks but still keep the simplicity of using equivalent pushover analysis, this paper develops a “two-step” pushover procedure for seismic retrofit of plan-asymmetric buildings using buckling restrained braces. To this aim, a real case-study school building has been considered in the analyses. A design method has been implemented to size buckling restrained braces to be placed on selected spans and stories of the building. The effectiveness of the retrofit strategy has been finally demonstrated by nonlinear time-history analyses under different sets of earthquake-strong ground motions.
