Blue Book Article 4.03.010
ASCE 7-05 (ASCE 2006) addresses determination of building periods in section 12.8.2. The approximate period,Ta, is intended to provide a conservative estimate of the elastic fundamental period of a building. This period depends on the mass and the stiffness of the building. Until the building is designed, its period cannot be determined using the principles of structural mechanics. However, a period is needed to commence the seismic design of the building, because the design force is dependent on the period.