W = total seismic dead load of the structure (dead loads plus applicable portions of some storage loads and snow loads, as specified in UBC 1630.1.1)

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.) and facilities that house toxic or explosive substances have higher seismic importance factors (I = 1.25) than other structures (I = 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 = 1.0).

Designers should note that the seismic importance factor (I) is not identical to the importance factor for wind (I_w). Futhermore, the UBC provides two distinct seismic importance factors - one (I) for the design of typical structural systems/assemblies such as shear walls and diaphragms, and another (I_p) for the design of critical elements that are attached to structures and may need to resist large concentrated seismic forces (see UBC Section 1632 for more details).

T = fundamental (natural) period of vibration for a structure
The UBC 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_n may be taken as either the height of the ceiling system above the ground or as the mean roof height.