Pre-Engineered Foundation Design

[dangnm]

I am designing a foundation for a 80'x120' pre-engineered building in the northeast. I am using 4' frost walls except for the rear wall that has a loading dock with an 8' wall. I orginally designed the foundation with so that the piers/footing would independantly resist the thrust and cooresponding moment. The footings turned out to be very large especially on the loading dock side with the taller wall, 10'-6 square by 2'-0". The contractor wants to try using hairpins with a slab tension design instead of independant piers/footings. Any comments on this design type and references would be helpful. Thanks.

 

[fndn]

What is the site soil profile & allowable bearing capacity?
The contractor won't sign it off, besides a 10 ft square is not large for your building.

 

[jheidt2543]

Using hair pins is pretty standard in the metal building industry and they do permit a smaller footing, however there are some things to keep in mind:

1. Hair pins only work after the floor slab is poured and since they are the primary wind load resisting element, there is little wind load resistance until the floor is poured and up to strength. If you are in a snow load area, that also adds to the horizontal thrust. It might be good to get the horizontal loads for wind and snow separately from the building manufacturer to see how big each load is.

2. If the floor slab is cut out over a hair pin, you lose that resisting force. One thought might be to provide "redundant" hair pins, an extra set at a different angle to the column pier. The rebar is pretty cheap, so an extra set is cheap insurance.

3. Some engineers and contractors require that the floor slab be poured BEFORE the building is erected, then the hair pins are fully engaged. However, this can cause problems with erecting equipment on the slab and potential for damage or oil leaks on the new slab. Some protection of the slab is usually required.

4. Depending on the width of the building, you might consider a continuous tie rod between piers.

Just some things to think about!

 

[stiffener]

Thanks for the input. The allowable soil bearing pressure is 2500 PSF. I have assumed that I would need to use tension ties between the columns consisting of a thickened slab or grade beam with reinforcing, most likely consisting of large diameter bars with long splice lengths. I am not convinced that the additional labor and materials for the ties is cheaper than placing a larger independant footing.

 

[jheidt2543]

I should have commented on the cost and reliability factors of the available methods, at least as I see them.

Hair pins are the least costly, but have the most POTENTIAL problems as noted in my first post.

A continous tie-rod is the next cheapest. I've put them in using a solid 1-1/2" diameter steel rod continous across the building without concrete cover. The size is based on the load obviously and the concrete cover can sometimes be omitted under the right soil conditions. However, there are some problems, like welded connections, turnbuckle connections, poor soil etc.

Usually, the most expensive, designing the column pier/footings to take the moment and horizontal loads is also the most reliable since it voids all the problems mentioned above.