In the current implementation, the fundamental period of the structure, T, can be either calculated from an Eigenvalue analysis or it can be a user input. Alternatively, it is also permitted to use T = T_α, where T_α is approximate fundamental period defined per Section 12.8.2.1.

The following upper limit is enforced on the fundamental period of the structure:

T ≤ CuTα

The term C_u is given in Table 12.8.1.

It should be noted that the fundamental period, T, is calculated separately for each loading direction. In addition, the limit imposted on T according to the above equation is removed if analysis is carried out for story drift calculations.