f1 
Percentage of critical damping for the 1^{st} response spectrum (Default 5%) 
f2 
Optional percentage of critical damping for the 2^{nd} response spectrum (Default is zero) 
f3 
Optional percentage of critical damping for the 3^{rd} response spectrum (Default is zero) 
f4 
Optional percentage of critical damping for the 4^{th} response spectrum (Default is zero) 
f5 
An integer corresponding to the site class as described in the following table. The default value is 4 (i.e., site class D):
Site Class 
Value of f_{5} 
A 
1 
B 
2 
C 
3 
D 
4 
E 
5 

f6 
Spectral acceleration at shortperiod, S_{s} 
f7 
Spectral acceleration at onesecond period, S_{1} 