Net Foundation Bearing Pressure
Net Foundation Bearing Pressure
(OP)
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.
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.






RE: Net Foundation Bearing Pressure
Allowable Bearing Capacity For Loads = 3500 psf - (2 Ft)x(150 Lb^Ft3) = 3200 psf.
One reason for this: When the geotechnical engineer is performing his/her calculations & making written recommendations, is the final footings thickness known? Probably not.
Another thread touches on this subject: Thread256-91191
RE: Net Foundation Bearing Pressure
I guess I was looking at thread 256-64702 and the person asking the question seems to be looking at the net bearing pressure the same way that I normally look at them. That the net pressure assumes that you remove a set thickness of soil and replace it with the same thickness of concrete, although I assume that the weight differences between the soil and the concrete are negligable. Basically, you have unloaded the soil by 200 psf before you put on that 200 psf pressure from the footing self weight.
In essence, I have a 100 kip reaction that would require a 5.35' square footing at 3500 psf net bearing pressure. Other engineers seem like they would require a 5.51' square footing for this same reaction. I guess which one is correct probably depends on who you ask.
Keep suggestions coming.
RE: Net Foundation Bearing Pressure
Be conservative on foundation design - the diffential costs for a larger fondation are minor, but the cost of finding out later that your were wrong are major.
RE: Net Foundation Bearing Pressure
I've always used net to mean basically don't worry about the weight of the footing if the top of it is below the original top of grade.
RE: Net Foundation Bearing Pressure
Judgment. And settlement. Like structural engineering, many problems in geotechnical engineering aren't controlled by the bearing capacity or soil strength, but rather by allowable movements.
Decisions are largely made on the basis of risk - real and perceived - and the engineer's perception of the owner's (and his own) tolerance for risk. It's important to remember that soils are made by God, not man. Variability in properties is a given - not an exception. And no building code can possibly address all the variations in conditions that geotechnical engineers face on a regular basis.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Net Foundation Bearing Pressure
A geotech will provide a NET allowable pressure. What this means to me as a structural is that, wherever my footing is placed vertically, the "piece" of soil directly below that footing will either feel an increase or decrease in vertical pressure due to my new structure occurring.
If the structure is of such a configuration and size that the "piece" of soil feels an increase in pressure from what it ORIGINALLY FELT, then that increase is the net difference that is used to compare with the net allowable.
So if a footing is placed in a basement, where the original soil above my "piece" of soil was, say, 10 feet (10' x 110 pcf = 1100 psf less pressure) and the footing delivers, say, 3,800 psf (which would include self-weight of the footing), then the net pressure = 3,800 - 1,100 = 2,700 psf and that is the design pressure.
The above, to me, is the correct method to use. Now whether you put extra safety factors on it, or round it up, is per the discretion of the engineer.
RE: Net Foundation Bearing Pressure
Yeah it is structural as well (and nothing against geotech's), I guess I just find it frustrating that I am expected to get things down to the gnat's ass with finite elements or the increasingly difficult codes and equations I use, but then in the end, I apply a safety factor that makes all the exact analysis I have done up to that point seem a little silly. And the assumed loads will probably be off 10% anyway. I guess it's good to have as close as exact number as possible to apply whatever safety factor to it. Sorry for the off topic.
RE: Net Foundation Bearing Pressure
RE: Net Foundation Bearing Pressure
RE: Net Foundation Bearing Pressure
(1) allowable bearing capacity
(2) allowable bearing pressure
(3) net allowable bearing pressure,
etc.
As for the structural frustrations - I empathize but do not wholly sympathize (or is it the other way round?). In structures, you are usually dealing with steel (pretty well consistent in strength (for given grade), modulus of elasticity, unit weight - or with concrete (also rather well known - with some fuzziness due to workmanship, poor control, etc). But with soils - we have huge variations in strength from place to place in the same stratum (and whether to use undrained strengths or effective strengths), similar with unit weights. We also have problems with variations of strata thickness, past history of site, groundwater levels and variations in such from time to time, etc. This, too, given that formulations for such things as settlement give varying results (which method to use), etc. - soils is not so neatly tied up. Let Focht3 give his Terzaghi quote!! But, do check out several threads in the geotechnical. We have also touched upon this in threads discussing Euro-code style LRFD (hope I have that right) vs the traditional geotechnical approaches.
RE: Net Foundation Bearing Pressure
This is one of my favorite quotes:
"Structural engineering is the art of molding materials we do not really understand into shapes we cannot really analyze so as to withstand forces we cannot really measure."
RE: Net Foundation Bearing Pressure
VOD
RE: Net Foundation Bearing Pressure
To get back to the earlier topic, according to my soils book the net allowable soil bearing capacity is the allowable load per unit area of the foundation in excess of the existing vertical effective stress at the level of the foundation, and divided by a factor of safety.
q(allowable)=(qu-q)/safety factor
q=soil unit weight multiplied by the depth of the footing.
RE: Net Foundation Bearing Pressure
Can any geotechs venture to say what kind of settlement to expect at ultimate bearing capacity. If it's beyond 1 inch what is the use of "ultimate bearing capacity" from a structural perspective?
VOD
RE: Net Foundation Bearing Pressure
Why do you ask?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Net Foundation Bearing Pressure
RE: Net Foundation Bearing Pressure
RE: Net Foundation Bearing Pressure
RE: Net Foundation Bearing Pressure
I'll find it in a few days and post it in this thread -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Net Foundation Bearing Pressure
"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."
RE: Net Foundation Bearing Pressure
"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.
RE: Net Foundation Bearing Pressure
RE: Net Foundation Bearing Pressure
http://www.icjonline.com/forum/october_forum.pdf
RE: Net Foundation Bearing Pressure
Sorry for taking so long.
I'd better start a new thread regarding Ultimate capacity as this is one for net.
Regards
VOD
RE: Net Foundation Bearing Pressure
Sorry BigH for not addressing to you as well. Everyone else is also welcome.
Regards
VOD