2.4.3.2 Period

For use in Equation 2-4, the value of T shall be calculated using one of the following methods:

Method 1. The value of T may be taken to be equal to the approximate fundamental period of the building (T_a) determined as follows:

1. For buildings in which the lateral-force-resisting system consists of moment-resisting frames capable of resisting 100 percent of the required lateral force and such frames are not enclosed or adjoined by more rigid components tending to prevent the frames from deflecting when subjected to seismic forces:

   

   (2-6a)

   where:

   C_T = 0.035 for steel frames.

   C_T = 0.030 for concrete frames.

   h_n = the height in feet above the base to the highest level of the building.

2. As an alternate for concrete and steel moment-resisting-frame buildings of 12 stories or fewer with a minimum story height of 10 feet, the equation T_a = 0.10N, where N = the number of stories, may be used in lieu of Equation 2-6a.

3. For all other buildings,

   

   (2-6b)

   where:

   L = the overall length (in feet) of the building at the base in the direction under consideration.

Method 2. The fundamental period T may be estimated using the structural properties and deformational characteristics of the resisting elements in a properly substantiated analysis. This requirement may be satisfied by using the following equation:

(2-7)

The values of f_i represent any lateral force, associated with weights w_i, distributed approximately in accordance with the principles of Equations 2-8, 2-9 and 2-10 or any other rational distribution. The elastic deflections, d_i, should be calculated using the applied lateral forces, f_i. The period used for computation of C_s shall not exceed C_aT_a, where C_a is given in Table 2.4.3.2.