What dimension footing is required?

[CChris]

I have a poured foundation with a thickened slab that has a 5'x5', 18" cavity for an elevator shaft. Two of the corners support 28kips load each. Each load rests on a 4x4 steel column on the basement slab. Each has a 8"x8" base plate (baseplate corner hanging over the elevator shaft is cut out). My concern is too much stress is placed too close to the edge and will cause the slab to begin cracking or collapse into the shaft. The thickened slab is steel reinforce and approximately 16" thick at these points. My question is what dimension concrete foundation should be beneath each of these loads? The current foundation is 2'x2' the soil is shell and granite and previously undisturbed.

What options do I have if I under-designed the foundation:

I probably should have placed the columns on a foundation and poured the slab around them, but I still have a few options:
1. Leaving it and fix it if it cracks
2. Dig out from the elevator shaft a deeper foundation under these columns and support the columns with a vertical steel support welded to the column running down the side of the shaft and supported 16" below at the bottom of the elevator shaft with a new foundation.
3. Bore out a 6" vertical hole through the thickened slab to the dirt below, pour a new foundations and sink the 4x4 column in it
4. Move the colunns away from the edge by reengineering the structure around the shaft.

 

[Focht3]

Where is the site? What does your geotechnical consultant say? Your structural engineer?

The loads and dimensions scare me; I wouldn't leave this to fix later unless I was sure that everything is okay. You have at least 63 ksf bearing on your 8" x 8" steel plates, with no foundation beneath (excepting the elevator pit wall and foundation.) A structural engineer needs to weigh in on the 16" concrete slab; my initial thought is that the slab could be adequate if:

1. the 28 kip loads are "very live" (like wind, not household goods.)

2. there is no problem at the edge of the pit.

3. the subgrade can handle the stresses without excessive settlement.

But I don't have quite enough information to say much more.

Personally, I prefer option 4 at this point.

 

[CChris]

Thanks for your feedback. I am looking into option 4 and also have my structure engineer look into it.