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.

 

[SlideRuleEra]

In my opinion the soil pressure exerted by the self weight of the footing must be deducted from the net allowable soil bearing pressure. Using your example, assume footing 2 feet thick concrete:
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

 

[DEL2000]

Thanks for the speedy response,

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.

 

[SlideRuleEra]

DEL2000 - I just took a look at Thread256-64702 that you refered to. As Focht3 says, engineering is not precise. Anyway you need to consider other factors. For example, it is common practice (for constructability) to "round up" your calculated footing dimensions in 3 inch increments. Your 5.51 feet would become 5 feet, 9 inches.
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.

 

[haynewp]

I find soils interesting because geotechs go through all these many correction factors and huge equations for settlement and bearing capacity and at the end, they say well I'll put a safety factor of 3 or 4 or 5, or whatever they feel like that day.

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.

 

[Focht3]

I find soils interesting because geotechs go through all these many correction factors and huge equations for settlement and bearing capacity and at the end, they say well I'll put a safety factor of 3 or 4 or 5, or whatever they feel like that day.

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?"

 

[JAE]

So let me throw my two cents in and hopefully Focht3 or another geotech will confirm or reject it.

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.

 

[haynewp]

I figured I would get slammed for that one.

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.

 

[VTPE]

If you consider a footing without a column load, the total weight of the soil and footing is balanced by an upward pressure from the soil. Therefore, there are no moments or shears in the footing. When the column load is added, the total prssure on the soil is the original balanced pressure plus a pressure equal to the load divided by the area of the footing (P/A). This is the GROSS SOIL PRESSURE, and must not exceed the allowable soil pressure provided by the Geotech. When the foundation is designed for reinforcement and shear, The NET SOIL PRESSURE is used, which is the load of the column divided by area (P/A). The original upward and downward pressures cancel out.

 

[Ron]

Call the geotech and ask him. Most likely he means it as JAE described. I work with Geotechs every day....they know little about the structural interaction ( I don't mean that as a slam on Geotechs....it just isn't an area of great consideration to them...they concentrate on the REACTION of the soil to the imposed loads, of which the weight of the footing is one).

 

[BigH]

We have had many discussions on this in the geotechnical threads - where we talk about
(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.

 

[bjb]

I have asked the senior geotechnical engineer at my company about this issue. He replied that when he gives the net allowable soil bearing pressure, he has already factored in the presence of a few feet of overburden. The net allowable bearing pressure that he gives can then be used to calculate the required footing area based on the column load, as long as the footings are not unusually deep. It is not necessary to deduct the self weight of footing, or the weight of the overburden. In most cases the net allowable soil bearing pressure is governed by limiting settlement, and the ultimate soil bearing pressure is usually at least 3 times the net allowable soil bearing pressure. When designing footings I never take it right to the limit anyway, and always leave some "cushion". The extra concrete required to do this is negligible, and given the fact that soil properties can vary widely on the same site, it is a good idea to provide some fat.
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."

 

[VoyageofDiscovery]

What exactly defines ultimate soil bearing pressure?

VOD

 

[bjb]

the ultimate soil bearing load is the load that causes a general shear failure of the soil. A triangular shaped wedge of soil under the footing is pushed down, and the adjacent soil is pushed out horizontally and upward. At this failure load you would see soil bulging above the surface.

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.

 

[VoyageofDiscovery]

I understand net 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

 

[Focht3]

Plastic deformation governs the soil behavior at/near the ultimate bearing capacity. It's hard to define a movement associated with this condition; most of our data has to do with "elastic" deformations.

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?"

 

[Ron]

VOD...bearing capacity and settlement are distinctly different soil parameters. Settlement can be influenced by soils significantly deeper than those controlling the bearing capacity. Bearing capacity is a function of the soils in the immediate proximity of the footing bearing including just below, to the side and above the footing bearing level. The shape of the failure area for each is significantly different as well. Settlement influence occurs vertically within an approximate 30 to 45 degree angle downward from the edge of the footing. Bearing capacity failures occur in almost a circular cross section immediately beneath and to the side of the footing, much like a slope stability failure plane.

 

[SperlingPE]

The soils reports I receive (from local geotechnical engineers) always report the allowable soil bearing pressure as a "net" allowable soil bearing. It then goes on to explain that the weight of the footing can be neglected or has been accounted for in the value reported. However, if I have footings that are thicker than 12", I will verify the "net" allowable bearing pressure with the geotechnical engineer or acount for any extra concrete over 12" thick in my calculations.

 

[BigH]

Focht3 - where is your Terzaghi quote? - to augment bjp!!! To me, it is pretty clear - the net bearing pressure is that pressure over and above the existing effective stress at the particular level. Yes, for practical purposes, the difference between the weight of concrete and the weight of soil is usually neglected - but this might always be the case if you have very heavy compressor raft foundations. Ron is quite right that the depth of influence on the settlement issue is much greater than that relative to the bearing capacity depth of influence (typically in the order of 0.7xfooting width) - so any correlation of settlement at the ultimate capacity is difficult to determine.

 

[Focht3]

My Terzaghi 'quote' file was lost in January, along with over 1 Gb of other documents...


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?"