The Seismic Design Category is either provided by the user or it is calculated based on chosen risk categories.

For Risk Categories I, II and III structures with spectral response acceleration at 1-s period, S₁ ≥ 0.75, the Seismic Design Category is set to Category E.

For Risk Category IV structures with spectral response acceleration at 1-s period, S₁ ≥ 0.75, the Seismic Design Category is set to Category F.

Otherwise, the Seismic Design Categories are determined according to Table 11.6-1 and 11.6-2 and the worst case category is chosen (worst category indicated with higher letter)