Wind pressure and suction on roof

[Lion06]

I would like to get an idea of who adds wind pressure and suction to the roof for lateral frame strength checks and drift checks. I don't mean for the roof infill beams, but when checking the LFRS (a odd shaped moment fram in my case), would add the resultant roof suction and pressure in with the lateral wind forces for the wind load case?

I don't believe the program that we use does this (RAM), but it seems to me that it should be added.

What does everyone else do?

 

[csd72]

re:
"I don't believe the program that we use does this (RAM), but it seems to me that it should be added."

That is a scary statement coming from you, dont you think you should know what the program accounts for. Easy to check by checking the sum of the reactions.

Anyway, the suction on the roof will increase the moment at the windward eave connection.

 

[Lion06]

I guess a more approriate statement would be that it does not use suction and pressure on teh roof in conjuncion with the lateral wind cases. The question I was getting at is "Is this apropriate?". It seems to me that the roof suction/pressure should be included in the MWFRS strength check in conjunction with the lateral cases.

 

[civilperson]

It must be checked in all quadrants of possible wind! Throw in another load case or get different software.

 

[COEngineeer]

structuralEIT, just call RAM support. They are all PEs and should be able to answer questions like that. I guess it is a little hard to prove it whether it does or not. How do you know it doesnt do it? We usually run the lateral and look at the storey base shear and compare it with our spreadsheet calc. They are usually pretty close (both wind and seismic)

Never, but never question engineer's judgement

 

[Lion06]

I know that it doesn't because I made a very simple model that had only 4 lateral beams (2- 2bay moment frames) and ran RAM's wind load generated load cases. I then looked at the moment diagram for each of the wind load cases and they are all linear with M+ at one end and an equal M- at the other. If there were an uplift added, the moment diagram would be slightly curved with different end moments.

 

[WillisV]

I am not aware of any software that automatically adds roof MWRS load cases for lateral force resisting system checking. Not saying its not out there...

For taller buildings the effect is minimal, and in my opinion can be safely ignored.

For one or two-story buildings, yes it is appropriate to use MWRS roof pressures when checking the lateral force resisting system as the effect could be significant. This can be performed in RAM by creating snow load zones that allow negative pressures to be input to approximate the effect of the wind and creating custom load combinations that combines this effect properly with the other wind cases.

Alternately, just create a simple 2-d model of one of your frames and see how much a difference there is with and without the roof pressures added to determine if its worth the trouble of applying to the entire model.

 

[Lion06]

I am most worried about roof beams. We often use wind moment connections for our LRFS and if the program ignores those roof suction forces, our wind moments will be lower than what what we should actually be designing for (since we design for the actual moment required by the wind cases instead of the full capacity of the member).

 

[tngolfer]

I'm with WillisV.

As far as RAM support goes, we have got some very lame answers from their support staff when we have had questions. Good luck with them. Their support is so terrible that my firm has considered looking for a new program.