Harmonizing Seismic Performance via Risk Targeted Spectra: State of the art, dependencies, and implementation proposals

Design spectra are commonly defined for a constant value of hazard associated to an "ultimate" limit state (LS) of reference, typically 10% in 50 years. Given a specific structure, this approach results in different limit-state exceedance risk levels even for sites characterized by the same design peak ground acceleration (PGA), mainly because of differences in hazard curve shape/slope. Instead, Risk Targeted design maps (first applied in ASCE7-10) suggest the application of suitable spectra adjustment factors, the so-called risk coefficients, in order to ensure a uniform collapse risk across different sites and buildings. Making use of simplified single-degree-of-freedom (SDoF) structures for several configurations of vibration period and ductility, we test the effectiveness of the adjustment factors computed under different assumptions in matching a specific target risk or, at least, in harmonizing the risk of multiple buildings at different sites with respect to different LSs. Although risk matching is shown to be only theoretically possible and unachievable in practice, we claim that harmonization remains a viable target, and we offer insights for possible future adoption of Risk Targeted Spectra in the European code provisions.