Site‐dependent critical design spectra

A new type of seismic response spectrum is presented. It is based on the concept of a ‘ critical excitation’ which is defined here as an excitation among a certain class of excitations that will produce the largest response peak for a design variable of interest. Site conditions, namely rock, stiff soil and deep cohesionless soil, are taken into account through the definition of the class of allowable excitations. The response spectra thus produced are compared with others that have been proposed or used in the past. Results indicate that they lead to realistic if somewhat conservative assessment of structural earthquake resistance. On the other hand, they are derived under assumptions that are rather well supported by seismological observation and avoid other questionable assumptions, especially those regarding the statistics of ground motions. The critical response spectra may thus inspire greater confidence on the part of their users than those currently proposed or relied on.