Foundation Design and Resisting Uplift

[KCEngineer]

I am designing a metal building foundation and uplift is by far the controlling factor. I am considering using part of the grade beams between column footings, which are square, to assist in the gravity load resisting force. Are there any limitations as to how much of the grade beam can be used?

 

[kslee1000]

In doing overturning/uplift analysis, you may relying on all of the grade beams.

However, uplift rarely a problem for properly sized concrete foundations, rather, the bearing capacity shall be checked to ensure the soil would not fail (excessive settlement) under the added bearing pressure due to effect of overturning.

 

[msquared48]

kslee1000:

For metal building structures I deal with, uplift IS a major problem - they want to become airfoils and sail away - literally - because the amount of dead load is so small - usually 3 to 5 psf. I rarely have soil pressure problems with the size of footings I have to use to resist the uplift.

The type of metal building I am speaking about are the ones with sidewall girts, purlins, and mainframes at 10 to 25 foot spacing, up to 20 foot eaves, and 2/12 roof pitch. If you are talking multi-story metal buildings, then your comment I can agree with. But not the animal I am talking about.

Cheers...

Mike McCann
McCann Engineering

 

[oneintheeye]

i agree i design foundations for indutrial buildings and uplift is major problem. Sometimes adding massive amounts of concrete to the foundation to resist.

 

[Begone]

By far and large, adding concrete weight to the foundation is usualy the easier answer.
The only major issues come up if the soil is contaminated and over-excavation is discouraged. Also, if the bearing of the soil is relatively limited, and the new load will induce too much settlements, you can always use piles to resist the uplift. In some cases mini-piles anchoring to rock has also been used.

 

[kslee1000]

The MAJOR problem can be easily solved by answer provided by Bgone above, by tying concrete floor to the foundation/grade beams. I think it is more often to see the roof has flew away, and the columns have pulled out the piers after a major wind storm, rather than the foundation been lifted to the surface.

 

[JAE]

We do include some of the adjoining grade beams - but only use a portion based on a 45 degree downward angle from the edge of the main footing - so if you have a 36" deep grade beam, we'd only use 3 feet - or maybe a little more.

If the uplift overcomes the main footing, the grade beam of course will be engaged and also participate in the dead weight downward hold. But it will flex a bit and you don't want the main footing coming up and the grade beam bending to engage in the resisting load.

 

[msquared48]

kslee:

There is no MAJOR problem of tying the slab to the footings/grade beam system when the slab is poured integral with the footing and grade beam system. Been doing that for years and that works great.

No worries.

Mike McCann
McCann Engineering