ASCE 7 minimum wind speeds on vertical projection

[Schambach]

The pre-2010 ASCE 7 used a minimum wind pressures of 10psf on the vertical projection of the entire building and then in 2010 and thereafter the minimum pressures are 16psf on walls and 8psf on the roof. This thread is not about ASD vs LRFD, I found posts on that, this question is about the legitimacy of the minimum pressures acting on the vertical roof projection found in Section 28.4.4. For buildings with a roof slope under 30 degrees (Fig 28.4-1) the net force acting on the roof in the direction of the wind is NEGATIVE. Note 6 (Page 301) says the total MWFRS shear shall not be less than that determined by neglecting the wind forces on the roof.

Can somebody explain why with laboratory-defined pressure coefficients we have to ignore the effects of the roof that actually "pulls" the roof back towards the direction of the wind, thus lessening the overall shear on the MWFRS? And if that truly is the case, isn't neglecting the roof pressures for roof angle degrees < 30 conservative? Isn't throwing an 8psf pressure against the vertical projection that is POSITIVE, when in actuality it is negative REALLY conservative? Is that really what ASCE 7 wants the engineer to do?

I read the ASCE 7 such that both pressures need to be applied to both vertical projections........no questions asked. Some engineers I know choose to disregard the minimum roof pressures on the vertical projection because in reality they've already conservatively estimated the roof shear. Am I looking at this wrong?

Thanks.

 

[Schambach]

Quick correction. Engineers I know choose to disregard the ROOF vertical projection and only use the wall vertical projects in determining their total shear forces. Is that acceptable is my question?

 

[phamENG]

To do wind analysis "right" you have to run about 15 different cases for each direction. (I'm exaggerating, but only a little.)

It's important to realize that the wind design parameters put forward in ASCE 7 from chapters 26 to 30 are VERY simplified (even as they get more and more complex). It's attempting to take a random and chaotic force and dumb it down into semi-uniform pressures that can be conveniently included in an analysis of a structure. No one element of the code completely covers every eventuality. So yes, wind flowing over low sloped roof will induce suction. Will that suction really be perfectly orthogonal to the axis of the rafters? Probably not, though it may be. So when we zoom in on the roof, we say that it is because it causes the worst case bending in the rafters. Good, we've captured one 'worst case' scenario. Now zoom out to look at the whole lateral force resisting system. That assumption that the suction is perfectly orthogonal is no longer worst case. The assumption that the wind flow is perfectly parallel to the surface of the ground (and our reference plane for roof slope) and laminar is also not conservative here. Ever ridden through turbulence in an airplane? Same thing can happen lower to the ground. I almost crashed a Cessna once when I was hit by a random downdraft on final approach to the runway. So a positive pressure on the projected surface is very possible. Indeed probable within the scope of the code if we're required to apply that pressure to the projected vertical surface of the roof (which we are).

 

[phamENG]

To the simplified question in your correction: No. It goes directly against ASCE 7 and, by reference, the building code.

 

[RPMG]

The new code is less than the old code:

ASCE 7-05 (ASD): 10 PSF * 30'-0" max roof ridge @ 15'-0"

ASCE 7-10 (ASD): 0.6 * 16 PSF * 20'-0" eaves @ 10'-0" + 0.6 * 8 PSF * 10'-0" projected roof @ 25'-0".

This is also a minimum. It's not based on lab test coeff's. The other CH 28 load cases are, but not this case.

 

[Schambach]

Thanks phamENG. That makes sense........kind of. I hadn't considered a downdraft acting on a building and exerting a positive pressure on the windward side of the roof, unlike that which is tested in a wind tunnel. I like your Cessna example.........I get it. Couldn't one argue though that if that were the direction of the wind, the load acting simultaneously on the sidewall wouldn't be near what it would be had the wind direction been normal to the sidewall? So where does that leave us?? I don't know. Probably just gives credence to the fact that since nobody knows for sure..........throw some minimums on there and "call it good!"

RPMG - I'm with you on the new code being less than the old code.....which is good.

Thanks

 

[phamENG]

Schambach - another thing to think about is drag. We don't talk about it much, but it's there. As the wind runs up and over the roof, even if there is suction there is also going to be drag parallel to the surface. So the resultant vector is going to shift and end up positive. You also have the suction on the backside of the roof. So when you add all of that up together...a net positive lateral pressure on the roof projection is quite likely in some instances.