Earthquake retrofitting of R/C frames with soft first story using hysteretic dampers: Energy-based design method and evaluation

Abstract

Soft stories constitute a major reason for building collapses during severe earthquakes. This paper investigates a solution for the seismic upgrade of existing reinforced concrete frames with open first story and masonry infills at the upper stories. It combines the addition of hysteretic dampers with the strengthening (if necessary) of the columns of the first story. The energy-based design procedure put forth provides the strength, stiffness and energy dissipation capacity required for the dampers so that the overall structure can endure the design earthquake without exceeding a prescribed maximum drift at the first story. The strength required for the dampers depends on the strength of the columns of the first story therefore multiple solutions are possible. The proposed method takes into account the contribution of the elastic deformation of the upper stories to the elastic vibrational energy, using simple formulae to determine the strength for these stories that will guarantee they remain elastic. Finally, non-linear dynamic analyses are carried out to evaluate the seismic performance of the retrofitted structures and to validate the proposed procedure.