Alternate for pile supports for slab foundation for metal building

[Engineerataltitude]

Owner wants to put a 50'x62' steel building on a slab for a quasi-industrial use. Slab designed for HS20-44 vehicle loading. Normally no problem, but geotech report says soil is poor and is recommending piles to support the building. Before I launch into that design, wanted see if anyone had any other recommedations as to how to support that slab...

Problem is that the steel frame in the metal building has pretty good sized concentrated loads (26K) at the perimeter of the slab. Would normally put pad footings there but since the site won't support them, didn't know if I should perhaps look at a combination of a perimter gade beam with pile supports underneath or something else?

Any suggestions?

Also, how much should I be concerned about settlement? Building should be pretty flexible.

 

[DRC1]

I would look to the building manufacterer for guidence on settlement.

As for the foundation, it depends on the soil condition. If only the top 4-5 feet are bad, you may want to excavate and replace. If it is the top 10 or 15 feet, rammed aggergate piers may make sense. Geopiers is a popular name out there. If the bad soils extend 20 feet or so, you are probly looking at piles. If the bad stuff is in defined pockets, you may be able to bridge.

 

[msquared48]

You could also preload the site to induce settlement, then use a raft flundation design.

Mike McCann
MMC Engineering

 

[Engineerataltitude]

To DRC1: Thanks. Don't have report yet, so don't know how deep I'll need to go. (only had a verbal exchange with the geotech guy, wanted to start thinking about options...)

To msquared48: Haven't done a raft foundation before. Any reference for designing one you'd recommend?

 

[geoffdale]

Helical piers? Same principle, but cheaper than piers?

 

[kslee1000]

The load (26k) does not sound excessive, if the soil property is quite uniform throughout the site, I will look into the possibility to design a slab on grade with thickened edges on a properly prepared subgrade.

 

[Engineerataltitude]

kslee1000: That's actually what I was thinking originally, instead of piles. Thought I'd design a slab with perimeter grade beams sized to take the moment from the column loads. Grade beams as beams on elastic foundation and slab as plate on elastic foundation, as long as I get any kind of fairly reasonable sub-grade modulus from the geotech report. Space other grade beams in a grid to provide support for the slab in the case of loss of bearing under the slab between grade beams. I think I can model that with RISA-2D now. I think they have the capability of adding a sping constant under plate elements now.

Guess I'll have to wait for the report to see if the geotech will spec any kind of sub-grade modulus I can use.

 

[kslee1000]

Engineerataltitude:

Yes, just to confirm your original thinking. Anyway, the soil report may alter the scenario, good luck.