Rotational spectra consistent with horizontal seismic hazard

The distinct contribution of rotational ground motion to the failure of structures has been observed in the past. A direct recommendation on considering rotational hazards in seismic design is not presently available. In this work, the construction of rocking and torsional spectra [collectively called rotational spectra (Rot)] for scenario- and intensity- based seismic performance assessment is proposed based on scaling of the associated horizontal spectra (H) through respective Rot/H spectra. Rot/H spectral shape is determined given the earthquake magnitude, epicentral distance, and average shear wave velocity. The procedure is based on the ratio of median rotational to median horizontal-spectral ordinates, which has been demonstrated to be a good representation of the median Rot/H spectral ratio. The median level recommendations are also demonstrated to apply to design level (of a particular facility located in a specific site, which is also governed by a scenario) under certain assumptions. Determination of Rot/H spectrum contingent on a scenario involves two steps: construction of normalized Rot/H spectra (denoted by alpha) with unity at zero period scaled by the ratio of median Rotational- to median Horizontal- PGA (denoted by beta). An estimate of alpha and beta for different soil types is also proposed for intensity-based assessment.