Wind load distribution on semi-rigid diaphragm in a user-defined story forces

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Nik — Fri, 25 Oct 2019 16:04:49 GMT

Ok. Thanks for your timely response Seth! I really appreciate your help!

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Seth Guthrie — Fri, 25 Oct 2019 15:56:18 GMT

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.

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Nik — Fri, 25 Oct 2019 15:52:42 GMT

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!

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Seth Guthrie — Fri, 25 Oct 2019 15:47:06 GMT

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.

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Nik — Fri, 25 Oct 2019 15:34:03 GMT

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.

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Nik — Fri, 25 Oct 2019 15:31:25 GMT

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?

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Seth Guthrie — Fri, 25 Oct 2019 15:29:06 GMT

Try changing the WF and LF force percentages so that they add up to 1.

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Nik — Fri, 25 Oct 2019 15:21:22 GMT

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]

RE: Wind load distribution on semi-rigid diaphragm in a user-defined story forces

Seth Guthrie — Fri, 25 Oct 2019 15:15:44 GMT

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