Roof Uplift forces on column footings

[I-beam]

I have a large pre-eng building foundation to design, and the "provided" column uplift forces are quite high...95 kips. I am planning to drop the top of the footings and add piers
to obtain the needed additional dead load from overburden. That is a lot of expense, and I'm sure the contractor did not plan on interior column piers, etc.

The typical interior footing needs to be 8'x8' for supporting the SL + DL. Has anyone ever considered the floor slab to act as a net in tension to hold the footing down. I would need about a 25ftx25ft section of slab to be activated .... I can't imagine the wind uplift pulling the footing up enough to crack the slab.... any thoughts.

Thanks

 

[r13]

I would double check the wing load, if the maximum uplift occurs at interior columns.

 

[BAretired]

Piles might be worth considering. They carry downward loads or uplift. Screw piles seem to be a popular choice nowadays.

BA

 

[Ron247]

If I used the slab for hold down weight, I would still make sure I have at least a 1.25 FOS without the slab working. With the slab would be my FOS of 1.5

I do not know how thick your slab is but I doubt you could get a 4" or 5" slab to work as 25'x25'.

Is the 95 kips an ultimate or working load reaction? I have used the slab before but I limited the distance to how far the slab could cantilever under its own weight. I then allowed my boundary to be that distance wider than the foundation below. For example, if it would cantilever 3' and my foundation was 7'x7', I used the slab dead load of 13'x13'.

I can't imagine the wind uplift pulling the footing up enough to crack the slab....

If you do not have enough weight holding the column down, I could easily see it cracking the slab. With enough uplift, you no longer have a slab on grade.

 

[JLNJ]

95 kips of wind could certainly pull up enough to crack the slab. Does this uplift actually happen? Seems a little unlikely, but I don't have a wind tunnel in my basement.

I would use the frustrum of a 60 degree prism from the top of the footing up. Whatever slab this intersects, I count on for uplift. Use the load combinations in the code.

If your contractor doesn't want a pier, you can always use a very thick footing. Concrete is cheap, they say.