Evaluating Existing Spread Footings

[ToadJones]

I am evaluating some existing spread footings for possible increased capacity.
The original drawings show a few locations where the bearing pressure is 9000 psf.
This seems extremely high.

When I check the existing footings for increased loads I am getting numbers in the 10,000 psf area. The footings would have to be on bedrock to satisfy these conditions and I am quite certain the building is not on bedrock which makes the original value of 9000 psf on the drawings seem absurd as well.

What were engineers using for allowable bearing values in the 1940's?
Is there any chance a soils test was done as this time?

Without a soils test now, am I ok to use the presumptive values of IBC Table 1806.2?

Does a soils test even help in evaluation of existing footings?

 

[JAE]

ToadJones - I once worked on a major upgrade of a large craneway building - typical "A" shaped columns trussed together to form a crane runway.

The footings were fairly thin concrete just below the slab. The soil was really bad - in fact in some areas it was less dense than water. When shooting some elevations down the length, a truck would drive by outside on the street and the level would shake like crazy from the vibrations.

We ended up drilling through the footings and installing (4) auger cast piling with mushroomed heads just below the footing to engage and support the existing.

The craneways were way out of alignment - roller coaster to the extreme. We had to shim and reset a lot of the rails as well as replace a lot of the crane beams as well since we were going from a 10 ton capacity to a 20 ton capacity.

 

[Splitrings]

The 20% increase is directly referred to in ASAbE EP486, "Shallow Post Design" but the IBC. I am being told you can also use the bearing capacity formulas. A quick web search turns up a Terzahi formula for bearing capacity that does include a depth variable.

http://www.ce-ref.com/terzaghi.htm

 

[dhengr]

To add to JAE’s example or suggestion of auger cast piles... in some instances injection grouting under and around the footing can increase the bearing area and capacity. More needs to be known about the existing soils conditions at these footings to make any determination of footing capacity or best improvement methods.