Seismic Retrofit of Low-Rise Reinforced-Concrete Buildings: A Modified Displacement-Based Design Procedure

Abstract

A noniterative procedure to determine the retrofitted configuration of an existing building, to be assessed according to the seismic code, is proposed. Among possible rehabilitation strategies, the approach involving both stiffness and strength increments has been followed, maintaining yielding and ultimate displacements unchanged. Limiting our focus to low-rise reinforced-concrete buildings, under the hypotheses of shear-type behavior and constant axial forces in the columns, the yielding and ultimate displacements of a single-degree-of-freedom (SDOF) system, equivalent to the existing multi-degree-of-freedom (MDOF) system, are determined. Using the SDOF displacement capacity, the displacement design spectrum furnishes the corresponding period and, then, the required stiffness and strength are determined. Hence, returning to the MDOF system, the demand at each floor in terms of shear resistance can be assessed. The difference between required and available shear resistances is provided by additional elements, in this case steel bracings are adopted. The discussion of a case study clarifies the various steps described, also confirming the suitability of the assumptions introduced, with special attention paid to the vertical regularity of the whole structure or of the added elements alone. Moreover, nonlinear dynamic responses to a set of natural accelerograms are used to evaluate the effectiveness of the proposed approximate design procedure.