Foundations at Lightly Loaded Columns with Large Biaxial Moment

[enigma2]

I have an open structure with a fabric roof. The vertical supports are tube steel and they are to sit on a base plate anchored with epoxy anchors to a concrete slab. I have been charged with designing foundations for the 16 columns. It is a proprietary structure and the manufacturer has provided me with reactions at the base. Being open, it is lightweight, so the axial dead load at each column is no more than 1.5 kips. However, there is biaxial moment due to wind load at the base, 1.6 k-ft in one direction, 2.4 k-ft in the other. The contractor just wanted to embed sonotubes down to frost at the column locations and be done with it. But the e=M/P is so large here, that I'm not sure it would be adequate, even though the top would be braced by a 4" deep slab on grade. My thought was that an isolated footing would indeed be large, but what if I thickened and reinforced the slab under the entire structure to be like a giant footing.

Does this seem reasonable? How do I know if the sonotube could be braced enough by the slab to be adequate. It seems like there's too much potential for overturning.

Any thoughts would be appreciated

 

[hokie66]

This must be a small fabric roof, as the base moments you listed are not much. Most of these things have greater moments just due to the fabric tensioning, without adding the wind load, etc.

I wouldn't consider using epoxy anchors. Use cast in anchor bolts. As it is a proprietary structure, I would be sure that my footings are not the weakest link, but rather that the columns would fail first.

 

[geoman110]

Hi,
You can design cast-in-place concrete piles (caissons) in order to bear the horizontal and overturning moment. I can suggest the Brom’s method for this analysis and design. There have been several threads in regard to this design method. If your design does not result to considerable length (depth) for the caissons, it can be constructed with sonotube method (I personally do not like it as requires care backfill of the excavation around the sonotube to ensure lateral sopport).

The other option is to design your slab as mat foundation to take the lateral and overturning loads. I do not think that would be the most economic option and require carful structural design.