Under the Missing Mass Combination drop down listing in an AutoPIPE model, only the SRSS and Absolute SUM method appear as choices. Where is Modal?
As seen in the online help:
The Modal method is only available for the Uniform Support Response Spectrum (USRS) option. If Modal is selected as the combination method while the MSRS option is disabled (unchecked), and then the MSRS option is enabled (checked) the combination method selection will automatically change to SRSS.
Dynamic "Response Spectrum" Analysis