Floor Slab for Pre-engineered Building

[wvtech76]

I am designing a foundation and floor slab for a 113'x80' pre-engineered metal building. The geotech report recommends 24" drilled piers and grade beams for the column loads and perimeter walls, but use a structurally isolated slab on grade.

This building has two garage bays and the floor slab will be receiving heavy truck traffic (36K axle load).

Normally, I use a turned down slab with the slab poured monolithic with the turn down. But this is my first pre-engineered building using deep foundations. With needing the grade beams between the drilled piers, does anyone have any thoughts on how to do the slab to prevent settlement from the truck loads, but still be isolated from the grade beams? Thanks.

 

[Ron]

Have you considered a mat foundation instead of the deep foundation? 24-inch drilled piers seem excessive for a very lightly loaded pre-engineered, steel building. Further, the exterior walls present very little vertical load to the slab or to grade beams if they are installed. Unless they are being used to support the slab and its vehicle loads, the grade beams would be useless.

I realize you haven't provided much info on the geotechnical conditions, but based on your given info, I would re-think this one.

 

[msquared48]

Depending on a lot of things here such as the snow load and bay spacing, I figure that the mainframe vertical reaction could be anywhere from 20 to 50 kips. That being said, with an axle load of 36 kips, wheel at 18, seems like the max pile load would be in the same range for the slab. For a 20' grid of pile, the dead load to the pile would be 20 to 26 kips (6 to 8" slab), and the live load anywhere from 20 to 30 kips per pile - total 40 to 50 kips max.

As Ron says, I'd go either Mat footing or pile the slab too.

Mike McCann
MMC Engineering

 

[wvtech76]

Bedrock is 20' deep with an allowable loading of 20 ksf. Due to poor soil, the geotech recommended GeoPiers, drilled shafts, or preload the site with 15' to 20' of fill and let set for 3 months. Due to time frame, preload is out of the question.

Max. column load is 60 kips. All are exterior columns, no interior columns.

Sounds like you are saying to pretty much do it as I have in the past.

 

[hokie66]

If you use a pavement or slab on ground rather than supporting a structural slab on piles, you would need the grade beams to span horizontally, resisting the column thrust by passive pressure. But the grade beams need not be right up under the slab. The 24' piles could cantilever a bit, and the passive pressure is better as you go down.