The approximate fundamental period,
Ta is
determined from the following equation:
where
| Ct and
x | = | are given in Table 12.8-2 |
| hn | = | the
structural height (given in feet) |
Alternatively, it is permitted to use the following equation (subjected
to some limitations, see Section 12.8.2.1):
where
| N | = | the number
of stories above the base |
It should be noted that Ta is calculated
separately for each loading direction. In addition,
Ta can be provided directly (user input).
The equation 12.8-10 is provided for masonry or concrete shear wall
structures. This equation is not implemented.
Table 1. Values of Approximate Period Parameters
Ct and x (Table 12.8-2)
| Lateral Resisting System
|
Ct
|
x
|
| Moment Resisting Frames
|
Steel Moment Resisting Frame
|
0.028
|
0.80
|
| Concrete Moment Resisting Frame
|
0.016
|
0.90
|
| Steel eccentrically braced
frames in accordance with Table 12.2-1 lines B1 or D1
|
0.030
|
0.75
|
| Steel buckling-restrained braced
frames
|
0.030
|
0.75
|
| All other structural systems
|
0.020
|
0.75
|
