Lately I've been running into issues with my lateral analysis on a large, square steel framed building. The roof is an 1-1/2" metal deck that slopes in one direction and drops 10' over the length of the building, which I've modeled as a semirigid diaphragm. No errors or warnings appear during the analysis but I notice some extreme shear magnitudes that are in the thousands of kips. These particular values only occur in the 'Y' Wind Load cases and only in the Fx component.
I reset my roof slope in the modeler to a flat surface to the respective level and the extreme values disappeared, or rather became more reasonable and comparable to the other direction. Any insight for why this phenomenon is occurring or suggestions of better modeling methods would be appreciated, thanks.
The increased exposure due to sloping is not considered in the total forces (See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/2052/ram-frame---wind-loads-faq )
Having sloped semi-rigid diaphragm is problematic because the lateral loads can cause extreme deflection a lot out-of-plane. See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/18616/ram-frame---criteria---diaphragms
I did verify total forces did not change with the slope. But including the slope does affect the loads on the diaphragm (see below). So when the extreme deflection is occurring, it must generate that internal story shear which prevents any realistic frame design to be possible.