Any solid technical basis for exceeding the default SB allowable for footing in place for 12 years?

[Engineerataltitude]

I have a retrofit addition to a commercial building in Southern California that has been in place since 2000 where we are adding load to a TS columnn/pad footing which causes the soil bearing allowable to go to 2092 psf. The footing was originally designed with the '97 UBC/CBC permitted SB allowable of 2000 psf without a soils report, so no soils report was ever done. Both footing and TS column calc out great otherwise. Building shows no signs of settlement anywhere and there are no indicators of soil concerns (such as moisture, drainage, fill, etc.) anywhere on the site. This footing is on the interior of the building under a concrete slab floor and would cause significant disruption of use to underpin. I'd hate to tell my client he has to initiate a soils investigation for this small of an overload (+4.6%). My gut tells me that it will be just fine, that the actual soil bearing capacity is in excess of 2092 psf, but I suspect I should show up with something better than that for a plan check at the building department. Is there any kind of geotechnical basis for the benefit of soil compaction under existing construction that has been in place that long with no problems? I'd be much happier to be able to reference something technical if anything exists. Any help would be greatly appreciated.

 

[hokie66]

I don't know about in California, but where I have worked, it would be a no brainer. Just move on to something more important. But what does "TS" mean?

 

[Engineerataltitude]

In the absence of any brilliant technical references, that's just what I'll do.

TS is the old notation for structural Tube Steel, what is now called rectangular HSS.

 

[Ron]

I think hokie66 just provided that "brilliant technical reference"...I agree.

 

[JedClampett]

Your conclusion in your calculation should say, "2092 psf OK vs. 2000 psf allowable per hokie66."
Soil allowables are among the most mysterious, magical numbers we use in engineering. It's not worth getting too precise with them. Your 5% overage vs. this mandated allowable is not an issue.

 

[Engineerataltitude]

Thanks all!

 

[Ron]

Brings to mind an old saw....

Measure it with a micrometer
Mark it with a crayon
Cut it with an ax

Bearing capacity of soils is just as JC described....magical mystery. Most geotechs will tell you their numbers have a FS>3 on such.