Uplift on Foundation for PEMB

[cmbyrd77]

I am doing the foundation design for a Pre-Engineered Building and as any of you know that has done this type of work, the shear/thrust and uplift are always killers. Before, I used to just increase the size (perimeter and/or thickness) of the pad footing until I exceeded the uplift reaction (after taking the 40% reduction, of course).
However, I'm starting to wonder why I never considered part of the wall footing that is tying into the pad footing. At first I thought maybe I could consider the portion of the wall footing from bay to bay (so if I have 20' col spacing, i would using 20' of wall footing) but that seems like too much. Is there any sort of rule-of-thumb to go by as to how much of that wall footing (and for that matter the grade wall also) you can consider to counteract the uplift?

Thank you in advance for the help.

 

[slickdeals]

For the building to *fly away*, it must lift the footing and anything that ties to it. If you have a continuous wall footing, tying the footings together, I do not see any reason to neglect that.

In some cases in Florida, I have used 5-8' of slab on grade weight (16' diameter max) in interior column locations as well, and weight of soil resting above the footing and slab on grade.

 

[slickdeals]

An example here

http://web.archive.org/web/20110426004436/http://aaof.org/documents/Concrete Slab Rupture.pdf

 

[charliealphabravo]

Rebar hairpins are sometimes used to engage a larger area of slab. The slab is thickened around the hairpin to conform to cover requirements.
I have also considered using the shear capacity of the soil above and next to the footing.

 

[Eldorado]

I design for uplift resistance taking the stiffness of the foundation into consideration. I use a length of perimeter grade beam equal to 3 time the beam depth as ballast. I also include a 3' wide strip of slab of the same length (basically an L beam). This is in addition to the weight of my footing, which is obviously dependent on the footing shape. Intuitively, a dock high foundation should provide more resistance to uplift than one that has a 2' deep perimeter grade beam.

 

[steellion]

You can also use grade beam "ties" parallel to the PEMB's if you have excessive thrust.

I wouldn't use the entire 20' length of wall footing weight to counteract uplift, but I would take advantage of it. Maybe 5 feet in each direction, plus soil/slab-on-grade weight over the footings. It's whatever you feel comfortable with and can rationalize with sound engineering, realizing that it's unlikely the building footings are going to actually pull out of the ground and fly away.

 

[haynewp]

Also, when using the weight of the footing and walls etc., the 0.6 factor applies to the self weight under service combinations. Some engineers seem to think they can use the entire weight of the footing to counteract uplift.