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Bearing Capacity of Shallow Foundations 1

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AidenJJ

Structural
Jan 12, 2018
3
US
I understand that the allowable bearing cacity of a shallow foundation is a function of footing width. The ultimate bearing capacity is a function of width since as per the classical bearing capacity equation,
1_ulaux9.jpg
, the width of footing comes into the equation. Additionally, the settlement of the foundation is also a function of footing width (both immediate and consolidation).
My question is that given above, how come the geotechnical reports prepared by geotechnical consulting firms, report a single number as the allowable bearing capacity of the soil (e.g., 3000 psf) without mentioning the width of footing? And then based on this number, structural engineers calculate the dimension of the foundation.

Thanks,
AJ
 
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The reports I have seen do mention the width of the footing. Many times different bearing values will be given for strip vs spread footings. The bearing capacity isn't super sensitive to footing width. Also, geotechs can use notoriously high factors of safety when it comes to bearing capacity that it encompasses a wide range of footing widths. When we prepare a geotechnical RFP, we will provide preliminary column loads and foundation types/sizes we would like to use so that the geotech can tailor the foundation design parameters to our requirements and preferred foundation type.
 
If you'll notice, most of those reports are marked preliminary somewhere. They always caution that the geotechnical engineer should have an opportunity to review the foundation design. That's because they need some input from us to do those initial bearing capacity calculations, we need those to get an initial foundation size, and then they need that to fine tune the numbers for settlement/shear/etc. Then iterate as required. It's a back and forth that, in my experience, never actually happens.
 
Also they normally provide a minimum footing size (and there is a minimum footing size in building code). They typically also indicate the style of construction and put something in there about the anticipated loading that needs to be verified by the geotech like phamENG is saying it rarely occurs. Also worth echoing MotorCity about the high factors of safety. Most of the reports I get seem to just pick bearing pressure from the IBC table depending on the tested soil type, this table accounts for mostly limiting settlement and has a high FOS against bearing failure in most cases.
 
From my perspective: I never gave in geotechnical reports the allowable bearing capacity unless shear governed. In almost all cases, settlement/serviceability governs the allowable bearing pressure[/],

For many projects of a routine nature, the footing widths would be in the order of 600 mm to 2 m - as a result, the geotechnical engineer will give a single value as a matter of practicality. phamENG is right that most geotechnical reports are identified as preliminary for the reason he states. There will be cases, say in a large industrial plant/refinery where footing sizes can be significantly larger than "routine" . . . in such cases our company would actually provide a graph of allowable bearing pressure vs footing size.

You can find a "similar" curve in most geotechnical text books where, for various N values, an allowable bearing capacity (although in my mind it should pressure and not capacity unless it is a very narrow footing) is shown vs footing width.

capacity_vs_width_m2mxbe.png
 
Since driftLimiter brought it up and we have a geotech in here...

driftLimiter said:
this table accounts for mostly limiting settlement

I didn't think this was the case. As the prescriptive codes in the US are so quiet on the point of serviceability and the factors that impact settlement are so much more complex than the surface soils identified during footing excavation, I always thought these were conservative values to preclude bearing failure without much of a consideration for settlement. That's one reason so many prescriptively built homes (in my clayey area, anyway) have so many differential settlement issues.

What say you, BigH? (Or anyone else who may be watching...)
 
@PhamENG Was reading commentary yesterday and thought back on this post.

See ASCE7-16 Commentary Section C12.13.1
 
driftLimiter: fair enough, but it still doesn't address the case of prescriptive values directly. I agree that a geotechnical report will account for it ("In traditional geotechnical engineering practice..."). But predicted settlement is going to be far more variable with reference to compressible soil layer thickness, moisture content, water table elevation, etc. These things simply can't be accounted for in a simple table like that - can they?

I recall one site in particular that, without a geotech, the AHJ would have accepted 1500psf for clay soils. But the geotech determined that we could only do 1000psf because settlement was expected to be severe.
 
Agreed the prescriptive values are just a table lookup and in reality have no actual correlation to the site conditions. One would hope that the lowest value in the table provides a certain level of safety for most sites. Would still be nice for a Geotech to come in here and help us understand a bit more about how the Allowable Bearing Pressure is determined for a typical project. Could also be interesting to see the difference between Allowable bearing and Ultimate bearing strength.
 
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