...
Within the Response Spectrum Function Definition menu, shown in Figure 1, the function damping ratio represents the damping ratio for which the response spectrum was generated.
...
...
Figure 1 - Response-spectrum function damping ratio
The modal damping ratio of the structure must also be specified in the response-spectrum analysis case definition, which is shown in Figure 2:
Figure 2 - Response-spectrum modal damping ratio
During analysis, the response-spectrum curve will automatically adjust from the function damping value to that of the actual damping present in the model. The velocity formula (Newmark and Hall, 1982) used for computation is as follows:
...
- A1 = acceleration corresponding to damping ratio D1;
- A2 = acceleration corresponding to damping ratio D2;
- 0 < D1 < 100 (percentage);
- 0 < D2 < 100 (percentage); and
- log = natural log (base e).
For example, given an input acceleration (A1 = 0.4) at a particular period and function damping ratio (D1 = 0.05), the acceleration (A2) which correlates with the modal damping ratio (D2 = 0.08) would be computed as:
...