Currently my 3 story RAM structural system model is calculating loads differently for each story when the roof diaphragm is switched from pseudo flexible to semi rigid. Additionally, when the roof diaphragm is semi-rigid, the load is applied to the perimeter of the building vs around the central CMU shear walls when the roof diaphragm is pseudo flexible. Is this correct or is there a way to fix this? Thanks!
That is correct, or at least what we do for wind on semi rigid diaphragms. Check out Ram SS Wind Load FAQ for more.
Hi Seth, thanks! One last thing is do you know why the lateral wind loads are calculated differently for lower level floors when the roof is set to a flexible diaphragm vs a pseudo-flexible diaphragm? When the roof diaphragm is set to flexible, one of the lower levels has a wind load story force of about 24 kips, but when it is changed to pseudo flexible, this story force load is increased to around 75 kips. It doesn't make sense why the load is increased so much when the diaphragm for this specific level hasn't been changed at all.
Check loads and applied forces report. Frequency and gust factor are affected. Also check assigned pseudo flexible diaphragm percentages. Use the sum of reactions in the analysis log.
Seth, we have a large building (100k sf) with a 2nd floor over about 1/3 of it. So 1/3 of the perimeter walls are 2 story and 2/3 are a tall 1-story pushing more load to the roof. Can we get Ram to do this? We were prepared to use Ram for gravity loads and do lateral analysis by hand if needed. Thanks!
You should use User Defined Story Forces for the wind load so that you can get the correct WW and LW distribution. See: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/2052/ram-frame---wind-loads-faq Seismic is problably fine. P-Delta is probably fine too.