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Spectral response acceleration values specific to the geographic location of the building site and the corresponding site co-efficients are input in this data group.
Spectral Response Accelerations SS and S1
You enter the short period and 1-second period spectral response accelerations S1 and Ss that are derived from maps that are published in ASCE 7 Chapter 22. A link to the website seismicmaps.org is also provided next to this input, where you can find these values either from interactive maps or by entering the address of your project. These are measured in units of gravitational acceleration.
Site Coefficients Fa and Fv
These inputs are for the acceleration-based and velocity-based site coefficents Fa and Fv. from ASCE 7 11.4.4.They are derived either from site-specific ground motion analysis that you must do using Chapter 21 of ASCE 7, or from values in Tables 11.4-1 and 11.4-2. For site class F, site specific-analysis is always required, so these inputs are always active, default to 0, and you must enter a value.
For other site classes A to E, the values that appear in the boxes by default are those in tables 11.4-1 and 11.4-2. If you check Use site-specific ground motion procedures, described belo
w, the boxes become enabled, still showing the default value, but you can change them. The operation of the input also depends on the rules regarding use of ground motion procedures in 11.4.8, and the exceptions to those rules also given in 11.4.8, as described below.
Use site-specific ground motion procedures
As 11.4.8 permits a ground motion analysis for any structure, two check boxes called Use site-specific ground motion procedures allow you to override the tabulated values for Fa and for Fv. They also control whether the Exceptions to the rules in 11.4.8 are applied, as described below.
For Site Class F, they are always inactive and checked. For Site Classes A-E, the they are active and unchecked by default, and Fa and Fv are inactive. If they are checked, then the Fa and Fv values become active and you may modify these values.
Seismically Isolated or Damped Structures with S1 >= 0.6
The first rule in 11.4.8 says that ground motion analysis is required for seismically isolated or damped structures with S1 >= 0.6. If you have such a structure, check the Use site-specific ground motion procedures box for Fv and enter the result of your site-specific analysis.
Site Class E, SS >= 1.0
The second rule in 11.4.8 says that ground motion analysis is required for site class E and SS >= 1.0, and there are no values in Table 11.4-1 for these cases. Exception 1 says that you may use the Fa values for Site Class C to avoid ground motion analysis. For this reason, the values that appear in in the Fa input for Site Class E , SS >= 1.0 are those from Site Class C. If you choose to perform ground motion analysis instead, check the Use site-specific ground motion procedures box for Fv and enter the result of your site-specific analysis.
Site Class D, S1 >= 0.2
The third rule in 11.4.8 says that ground motion analysis is required for site class D and S1 >= 0.2, but the Exception 2 gives rules regarding the calculation of the seismic response coefficient CS that must be applied to avoid site specific analysis. If the Use site-specific ground motion procedures box is not checked upon the load generation, the values from table 11.4.2 are used for Fv and the rules from the Exception are applied to the calculation of CS .
Site Class E, S1 >= 0.2
The third rule in 11.4.8 says that ground motion analysis is required for site class E and S1 >= 0.2, but Exception 3 says that site specific analysis is avoided if the period T is less or equal to TS , defined as SD1/SDS . T is usually much less than Ts for structures designed in Shearwalls, but in the rare case that T > Ts the Use site-specific ground motion procedures box is checked and disabled, and you must enter Fv . There are no values in Table 11.4.2 to use if the Exception is applied; ASCE supplied us with the appropriate values, and they will appear in a revised ASCE 7. These values are Fv (0.2) = 3.3, Fv (0.3) = 2.8, Fv (0.4) = 2.4, Fv (0.5) = 2.2, Fv (0.6) = 2.0.