Net Foundation Bearing Pressure 1

[DEL2000]

In a soils report, when a foundation engineer gives a bearing pressure, say 3500 psf, and says its a net Bearing capacity, what does that mean exactly. I've always assumed that the geotechnical engineer has taken into account the self weight of the footing in the allowable bearing capacity, and that you would simply proportion the footing based on the superimposed column or wall load and neglect the weight of the footing. My boss and another engineer in our office always add a little extra load into the column reactions to size the footings, while me and my supervisor always neglect the self weight of the footing in sizing the footings.

I guess, my main question is on semantics. It's obviously more conservative to include that footing self weight in your design, but is it necessary, especially when your footings jump around in thicknesses which makes the footing sizing more tedious.

Thanks for any help.

 

[richardm]

In the meantime, can I expand on the quote given by bjb in his first contribution of 27 May? I have it on my wall in large letters as:

"Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance."

 

[BigH]

Focht3 - I found it in our Lateral Load Paper Discussion - with your permission, I attach it hereto:

"Soils are quite different from concrete and steel; they were placed by our Creator, and are not made by man. As such, soils must be evaluated differently. Karl Terzaghi very pointedly addressed the danger of treating soils in the same manner as concrete and steel in his May 1936 Presidential Address to the First International Conference on Soil Mechanics and Foundation Engineering (ICSMFE).

"The major part of the college training of civil engineers consists in the absorption of the laws and rules which apply to relatively simple and well-defined materials, such as steel or concrete. This type of education breeds the illusion that everything connected with engineering should and can be computed on the basis of a priori assumptions. As a consequence, engineers imagined that the future science of foundations would consist in carrying out the following program: Drill a hole into the ground. Send the soil samples obtained from the hole through a laboratory with standardized apparatus served by conscientious human automatons. Collect the figures, introduce them into the equations, and compute the result. Since the thinking was already done by the man who derived the equation, the brains are merely required to secure the contract and to invest the money. The last remnants of this period of unwarranted optimism are still found in attempts to prescribe simple formulas for computing the settlement of buildings or of the safety factor of dams against piping. No such formulas can possibly be obtained except by ignoring a considerable number of vital factors."

from: Focht and Fries, Discussion to Gupta's "Analysis and Design of Piles in a Group", Indian Concrete Journal, October, 2004.

 

[RVSWA]

There is another topic in the above threads for which I would like to comment/question. I love the quotes "Engineering is the art…" As a fairly new PE, I have spent considerable time trying to balance my responsibilities. I have safety, reality, mechanics, codes, liabilities, and pocket books to name a few and striking a balance between all of them can be at times difficult. It can be especially difficult to analyze a structure with all the precision and consideration I can muster only to arrive at the soil to use 2000 PSF for my bearing. But I suppose a construction crew can pour allot of concrete into footings over a single day, it really is money well spent. As for the use of "net bearing", I'm taking notes; I think I'll get to know my Geotech's a little better…

 

[SlideRuleEra]

Focht3 & BigH - Here is a link to the paper:

http://www.icjonline.com/forum/october_forum.pdf

 

[VoyageofDiscovery]

Hi Focht3 and Ron,

Sorry for taking so long.

I'd better start a new thread regarding Ultimate capacity as this is one for net.

Regards

VOD

 

[VoyageofDiscovery]

I started it in Foundation Engineering.

Sorry BigH for not addressing to you as well. Everyone else is also welcome.

Regards

VOD