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This spectrum is defined using several parameters including Ss, S1, soil class, and is typically used in codes derived from or adopting ASCE 7 standard document or the International Building Code (IBC). This spectrum has an idealized simple form, but is very widely used in the US and other countries. The SS and Sl values are usually obtained from published maps or online tools (e.g. USGS online tools).
The user defines the spectrum parameters (SS, Sl, TL,), soil class, damping ratio, and target dispersion (LogSigma) if needed. When the user defines the input parameters, the values in the "Data" table will be automatically updated.
In order to define an existing IBC spectrum, the user has to modify the following parameters:
1.Set the Damping Ratio
2.Input the spectral accelerations at the short and long periods (SS, S1)
3.Input the Long-Period Transition (TL); 12s is the default
4.Modify the Site Soil Class (A, B, C, D or E)
5.Input the scale factor
6.Modify the standard deviation if required
The data table and spectrum curve will update automatically when all input parameters are defined. Users may also select to plot to the Standard Deviation and Target Mean +/- Standard Deviation in a Linear or Logarithmic scale on the same plot.
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Figure: IBC Design Response Spectrum Dialog
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Figure: IBC Design Response Spectrum Dialog showing the effect of dispersion (LogSigma) on the spectrum
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