I am attempting to find the natural frequency of my staircase model as shown below using the total weight of the structure distributed evenly over every node as a nodal mass in the Ty direction. My issue is, I am able to get a frequency running the dynamic analysis but any changes made in the earthquake acceleration inputs has zero affect on the frequency results. I need to ensure that my damping is being taken into consideration but changing the scale factor and/or the damping percentage has not changed the frequency whatsoever. Any insight would be helpful as to why my inputs may not be being taken into consideration. Thank you.
That's logical, the mode shapes are based on the masses and the stiffness matrix only, not on the response spectrum curve. See also: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/8162/ram-elements-dynamic-modal-analysis-faq