I am trying to model a 4 story building with the first story having no wind exposure. Using the concept described in the following RAM wiki, https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/18422/ram-frame---criteria---ground-level, I tried to define the user defined story forces by using the output from the building model with wind exposure to only the top three storys.
Applying the loads in user defined wind story forces seemed simple enough for Rigid diaphragms. The Roof in the model is defined as semi-rigid diaphragm. For semi-rigid diaphragms, when I applied the force, the distribution of applied story forces did not match with that of the auto generated wind forces from the wind load case. See below.
Fig-1: Auto generated wind-load distribution on semi-rigid diaphragm.
Fig-2: User-defined wind load distribution on the semi-rigid diaphragm.
I applied the same force as displayed in the "loads and applied forces" report output. If the eccentricities come into play for the wind load distribution, then it is definitely not given in the output of the report. Is there a procedure I can define the appropriate user-defined wind forces for semi-rigid diaphragm? Or a better way of modeling a 4 story structure with no exposure to the first story?
Could anyone please help me out with this?
Thanks in advance!
It looks like the deck does not quite extend to the slab edge causing the program to miss some of the edges. See this for details. https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/2052/ram-frame---wind-loads-faq
Or to be more precise, how should I define the use defined story forces to be applied to a semi-rigid diaphragm, to mimic the force distribution as seen on a auto-generated wind load case? Like, how to know what percentage of WW or LF force to be defined, or the eccentricities to be defined, to replicate the results from the auto-generated wind load case?
You may not be able to replicate it perfectly. From the loads and applied force report you can check the windward pressure vs the leeward pressure in order to establish the relative percentages. But for wind with eccentricity you would have to compare the location of the applied load to the center of rigidity.
So, how accurate would the results from user defined story forces would be in comparison to the auto-generated wind forces? I already see some difference in the roof nodal displacement & story drift. I just want to know if I could apply the user defined story forces to model a 4 story building with no wind exposure to 1st story, or if I should employ other methods!
Good enough in my opinion. The whole concept of semi-rigid diaphragm analysis, including the effects of eccentricity is far beyond what anyone would have considered 20 years ago. Most of the buildings in New York and Chicago that have been standing for 100 years were simply analyzed with a constant pressure wind load full height.
Ok. Thanks for your timely response Seth! I really appreciate your help!