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Spectral Response Accelerations Sa(T)
Damped spectral response accelerations Sa(T =0.2), Sa(T= 0.5), Sa((T=1.0), Sa(T= 2.0),Sa(T= 5.0) are input, where T is the period in seconds, along with the value PGA These values are specific to the geographic location of the building site.
The default values for new files are those appropriate to the city chosen from a list of dozens of Canadian cities in the Default Settings. When a city is chosen, the response factors are taken from Appendix C of the NBC and Table J-2 of the structural commentaries for that city.
These are measured in units of gravitational acceleration g.
Site Coefficients F(T)
Site co-efficients F(T) are determined from Tables 4.1.8.4.-B to -F for periods Ta from 0.2 to 5.0, based on the values of Ta and PGAref and the Site Class. For Site Classes A-E, these cannot be changed except under special circumstances, and are shown for your information. Linear interpolation is used on these tables as per 4.1.8.4.(5).
For Site Class F, and for other site classes under certain circumstances, the F(T) values are not found in the corresponding tables. Instead, according to the notes to these tables referring to NBC 4.1.8.4 (6) , site-specific geotechnical investigation and dynamic site response analysis shall be performed to determine their values. Therefore, these inputs become active when Site Class F is selected, or under those other circumstances, to allow you to enter the coefficients so determined.
PGAref
PGAref is determined from the PGA value you input by 4.1.8.4.(4) as 0.8PGA if Sa(0.2) < 2.0, otherwise it just equals PGA.