W = effective seismic weight of the structure (dead loads plus applicable portions of some storage loads and snow loads, as specified in IBC 1617.4.1)

The importance factor is essentially an extra safety adjustment used to increase the calculated load on a structure based on its occupancy and/or function. Essential facilities (such as hospitals, fire and police stations, etc.) have the highest seismic importance factors (I_E= 1.5), while buildings where people congregate (such as schools, auditoriums, etc.) also have relatively high seismic importance factors (I_E = 1.25). Other structures have a seismic importance factor of unity (I_E = 1.0). Higher importance factors are intended to insure that structural integrity is not compromised and important facilities remain operational during emergencies and natural disasters. Based on typical occupancy classifications for most wood structures, wood buildings are frequently designed using an importance factor of unity (I_E = 1.0).

Designers should note that the seismic importance factor (I_E) is not identical to the importance factor for wind (I_w ) nor the importance factor for snow (I_s ).

T = fundamental (natural) period of vibration for a structure
The IBC provides the following simplified method for estimating T based on the height of the structure (h_n):

where C_t = 0.02 for wood structures
           h_n = height of the top level of a structure (ft)

For structures with flat roofs, h_n is the distance from the ground to the roof/ceiling system. For structures with sloped (pitched) roofs, h_nmay be taken as either the height of the ceiling system above the ground or as the mean roof height.