Simplified r-factor relationships for strong ground motions

Recent studies have demonstrated the need to consider the ground motion frequency content in the development and use of R-µ-T relationships. Results from two different approaches to determining these relationships are unified in the present paper. Two bilinear R-µ-T/Tg relationships are recommended for most strong ground motions and structural systems. One is more accurate, while the other, more conservative relationship is used in FEMA 273, ATC-32, and the simple version of the N2 method. Both relationships are indexed by the characteristic period of the ground motion, Tg. Simple methods to determine Tg from smoothed design spectra and recorded ground motions are provided. Neither recommended relationships are applicable to the nearly harmonic ground motions that may be generated at sites containing soft lakebed deposits. An example illustrates the application of these relationships to a code design spectrum in both the acceleration-displacement and yield point spectra formats.