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
Seth, I rechecked the model. The deck is correctly applied to the entire diaphragm, and is not missing short of edges. The two pics shown above are the same roofs, with no modifications. One is subjected to auto generated wind forces. The other is the user defined story force. [ the values for the user defined story force is taken from the output of the auto generated wind force analysis]
Try changing the WF and LF force percentages so that they add up to 1.
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?
I tried varying the percentages based on the WW and LW wind pressures as shown in the output, but with no effect. Also, the LW load distribution is to be uniform. But, for user defined story force, the LW wind distribution does not seem to be uniform and varies along the length of the building.