ASCE 7 - Flat roof wind uplift

[JAE]

Just taking a poll here of what others typically do.

This is speciically geared toward wind analysis of simple one-story buildings with flat roofs.

By the specific provisions of ASCE 7, the main wind force resisting system (MWFRS) wind loads for buildings provide for the following pressures (Method 2):
1. Windward wall
2. Leeward wall
3. Parapets
4. Rooftop equipment
5. Uplift on the flat roof

It seems that a lot of engineers sort of ignore item 5 in design. If so is this correct?
The issue is that if you have an interior frame with an X-brace (two columns, a roof beam and an X) wouldn't the roof uplift be somewhat important to include in the calculattions involving the brace...specifically for the footings under the two columns?



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[JedClampett]

You are correct. But it's a pretty unusual case. Most (not all) one story buildings have CMU walls and the upward case is not a problem with MWFRS. And even for the X braced case, for the uplift to be a real problem, you'd have to overcome the dead load (actually .9 times the DL).

 

[JAE]

Well actually 0.6 dead load under service checks.

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[wannabeSE]

JAE,
With 2010 it's 0.6D + 0.6W
Do you think roof uplift is neglected more often than rooftop equipment's effect on the MWFRS, particularly mechanical screens.

 

[JAE]

The 0.6W is due to the fact that ASCE 7-10 uses ultimate wind loads. Same thing just different place for the SF.

Not sure about screens - We include them all the time.

I think the vertical elements tend to get counted in while my question is more about the horizontal flat roof wind uplift that I think doesn't.

My concern is with any interior wind braces where the roof uplift drastically affects the design of the brace footings.

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[steellion]

For a lightweight system like a one-story steel building, wind uplift could control your footing design. In that case, I will design for 0.6D + 0.6W, but will include everything that I reasonably can to hold that footing down (soil cone above footing, slab on grade above footing, weight of the wall, wall footing extending several feet in each direction) because I don't believe that a column footing pulling right out of the ground simply due to roof uplift is a realistic real-world scenario in most cases. But I do check for it and design for it.

 

[Ron]

JAE...I agree that it should be checked. Whether it has great influence is almost irrelevant, however checking its influence is necessary in today's litigious climate!

Can easily be controlling factor in my area! Steel frame with infill light gage is one example.

 

[KootK]

I'm certainly guilty of missing this on some early projects. I agree that it's important. In addition to the footing concerns, there's:

1) Anchorage capacity at the tension column base plate and;

2) Frame beam design for a case with high axial load and uplift bending.

During a QC review, I once got challenged for assuming that I could use the entire self weight of the roof assembly to resist uplift. My reviewer felt that, for un-ballasted roofs, I should assume that the roofing assembly above the metal deck might blow off of the roof altogether. He argued that there was no way to guarantee that this wouldn't happen unless I specifically designed the connection of the roofing to the metal deck.

How do others normally handle this? I've always assumed that someone designs the roofing not to fly away. However, that someone has never been me and I've yet to review a roofing submittal where the associated engineering has been presented. My region does tend to be pretty lax about stuff like this.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

The 0.6W doesn't apply in this case as the building is under ASCE 7-05.


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