Mean Roof Height vs modeling in RAM

[Samwise Gamgee]

I have a monolsope roof with varying heights on either side. (High side : 30'-4 1/2", Low side : 25'-2 1/2"). Mean roof height = 27'-9"

When modeling in structural RAM system, I modeled it as a mean roof height of 28'-6". Is that considered a conservative/safe approach for the application of wind etc as I already accounted for few more inches above the mean roof height ?

 

[andrews2]

For generation of wind loads, this is slightly conservative as you say. However this will not accurately capture the behavior of your building. Frames/walls on the left are shorter than those on the right and will therefore be stiffer and take a greater portion of the load, but RAM will not know this since it sees all the columns as being the same height. RAM thinks you have a perfectly torsionally regular building, but that is not actually the case. This is not as big of an issue if you have a flexible diaphragm.

Moments, capacities, and story drift for the columns on the right will be calculated for a shorter height than what is actually being built, which is unconservative.

My advice would be to offset the tops of the columns in the modeler and set the mean roof height to the value you stated in the wind load case generator in RAM Frame. Or use User Defined Wind Loads.

 

[Samwise Gamgee]

I have an untopped metal deck and I designed it to envelope both flexible and rigid behavior. Its a single story building.

 

[andrews2]

Flexible is probably a fair assumption then. Forces to the frames would be correct, but drifts and moments caused by those forces still aren't accurate if the column heights are not correct.

 

[JoshPlumSE]

I think you'll have to figure it out on your own.... I just don't think there is an answer that will always be true.

That being said, I would tend to use the mean height. But, I might back check my base shear (and total overturning moment) vs a hand calculation. If it was off a bit, I might scale up my load factor for wind.

Caveat: I am REALLY not very well versed with RAM. I've used many of the other programs, but I don't think I've ever really used RAM. Seen it used. Maybe played with a demo once. But, I'm basing my responses off of experience with other programs.

 

[Samwise Gamgee]

I did perform a hand calculation to confirm the base shears. My hand calculation was less than what RAM applies. So I am not really concerned about the base reactions in comparison to what RAM is applying. Also RAM applies load using Wind Load for all heights. (If I consider low residential building, the wind load is about 18% less than what the RAM applies). I am a bit concerned with the moment distribution per what andrews2 was saying. If the shorter columns drag more load, they will have more moment etc etc.

 

[Samwise Gamgee]

However, for a monoslope roof ASCE permits us to use mean roof height at the eave height which is 25'-4 1/2" (for slopes less than 10 degrees). So what I modeled is for a mean roof height of 28.5 is conservative anyway. Is that safe to say that?

 

[jerseyshore]

For something like this I would probably design the whole building at the higher height just to be conservative. If for some reason I can't get it to work/ sizes are too big, I'd model it accurately then. It is a bit of a pain to put in sloped columns in Ram, but just have to follow the tutorial.