I am doing a case study comparing the fundamental period of a 20-story reinforced concrete building with dual system in both directions and compared the results from SAP 2000 and Midas Gen. The reference building is from the book of SK Ghosh Seismic Design using Structural Dynamics. here is the link of the book http://secure.skghoshassociates.com/product/show_book.php?isbn=9781580017485.
The example problem in the book is solve by the author manually and the modal analysis was done using SAP 2000 and I wanted to verify the accuracy of the result by comparing them with midas gen and staadpro, Midas Gen result is very close with the book's output within 1 to 2 % difference however, staadpro output is way off, Here is the tabulated result,
In Section 1.18.2.1.3 of the Technical reference manual, there is an example shown for considering the PDELTA effect during dynamic analysis. It says that if you provide the commands in that sequence, the frequency will be calculated using the [K+Kg] matrix.
Also, I don't have that book that you are referring to. Can you attach the pages of the book describing that example, and the STAAD model?