DISPLACEMENT-BASED DESIGN OF HYSTERETIC DAMPERS FOR SEISMIC RETROFIT OF FRAMES

A simple and affordable displacement-based design procedure for the seismic rehabilitation of frame structures equipped with hysteretic dampers has been recently proposed in the literature. The method is based on the Capacity Spectrum Method and is developed in the acceleration-displacement response spectrum (ADRS) space. The procedure calculates the properties of the dampers required to achieve the desired structural performance in compliance with the assumed limit states. The required performance is expressed in terms of a target displacement demand, and the structural response is achieved by reducing the demand response spectrum as a function of the additional damping introduced by the supplementary energy dissipation. Iterations are required since the addition of hysteretic dampers increases the stiffness of the system; however, convergence is reached in very few steps. The present work aims at assessing the use of the procedure for the seismic retrofit of two existing structures taken as case-studies. One building is a 4-story residential RC structure designed according to outdated codes that ignored seismic actions, while the second building is a 4-story steel-framed structure designed according to old-code MRFs considering gravity loads and low earthquake-induced loads. The upgrade of both structures is performed in order to achieve an assigned ductility factor for the main frame µF in the range 1-1.5. Attention is also paid to limit the increase in axial force induced in the columns by the dampers in order to avoid buckling issues. To check the accuracy of the design procedure, non-linear static analyses are performed, showing a satisfactory agreement between the global performance and the design target. Non-linear dynamic analyses are eventually carried out considering a suite of bidirectional artificial ground motions in accordance with the provisions of the Italian and European building codes.