Demand diagrams based on inelastic spectra: a proposal for the implementation of the capacity spectrum method in the Romanian seismic design code

During recent years, an increasing number of studies advocate the application of the capacity spectrum method based demand diagrams computed directly from inelastic response spectra, as an alternative to the basic formulation of the method, which uses elastic spectra computed for single degree-of-freedom oscillators with modified characteristics to account for inelastic behavior. When demand diagrams are based on the code-specified elastic spectrum, inelastic spectra are typically determined by dividing the elastic spectrum by strength reduction factors. The paper presents the application of the method, based on strength reduction factors computed with a unified analytical expression, deduced by the author in previous studies. The expression and is valid both for broadband and narrow band ground motions. Taking as a starting point the newly-enforced elastic design spectra in the 2013 edition of the Romanian seismic design code, the method is exemplified for the capital city, Bucharest. It is shown that, for the analyzed case, the use of the proposed strength reduction factors leads to results that are very close to those obtained by the "classical" nonlinear static procedure used in the code. This suggests a good potential for the implementation of the capacity spectrum method in the Romanian code, based on these factors.