Bearing capacity equations 1

[chichuck]

greetings to all, from a "dumb structural type".

I've got a question for you. I thought of it when I reflected on many of the geothechnical reports I've seen and used over the years. It seems that whenever a bearing pressure is recommended, it almost never states explicitly whether this is a gross value (ie one that I have to add the weight of footing & surcharge to my applied loads) or a
"net" value (ie one that I can use directly with my applied loads. I know, for that I also need to use a correction for concrete density = 140 pcf vs soil density = (say) 110 pcf.)

I always had to ask.

Now, I've opened up a copy of Bowles 4th edition and I'm studying the section on bearing capacity. There, he discusses bearing capacity formulae by Terzaghi, Meyerhoff, Hansen and Vesic. In the text and examples, I can find no clue about whether these give values that are "gross" or "net".

Can anyone in the group enlighten me?

Thanks,

Regards,

chichuck

 

[euygar]

Of course, these equations are giving gross values. And if you're using a factor of safety you'll be calculating allowable values. To calculate net values, yes you're right, you have to subtract the weight of the footing plus the surcharge weight of the soil over the footing. Usually in shallow foundations the net allowable bearing capacity is very close to the gross values if you use a factor of safety of about 2.5-3 unless you have a very soft soil medium. That's why those reseaerchers did not stress on that point. The change caused by surcharge and concrete weight is usually negligible when compared to the change you've made by dividing with factor of safety. On the other hand, surcharge and concrete weight can also be assumed to take part as somewhat contributors to the factor of safety. If you subtract them and use net bearing pressure then you should use a bit higher factor of safety to be on the safe side.

Eris Uygar

 

[Focht3]

Good question!

Let me give you a slightly different, but complementary, answer. In most cases, the difference between the density of concrete and soil is not sufficient to justify the additional calculation effort. This assumption began in the days of the slide rule, and engineers at that time recognized that their ananyses weren't accurate enough to justify the additional refinement.

With the widespread use of computers, the effort to include the additional analyses is trivial and the calculations can be quite precise. But we should be careful to recognize the difference between precision and accuracy. Remember that most geotechnical engineers would be quite happy if their design values are within 30 percent of the actual values.

You aren't new to the design process. As a former Terzaghi Lecturer once said, "It is better to be approximate and correct than precise and wrong." While the issue is of academic interest, it isn't normally of practical importance.



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[BigH]

Just to reinforce a few points we've all made earlier.

For shear - the bearing capacity equations give gross bearing pressures and by using an appropriate factor of safety, you get the allowable bearing capacity. But, this is a pressure based on shear.

For settlement - the net bearing pressure rules. In this, you estimate the settlement under the net loading (assuming no contribution to unloading and loading). Then you compare this the permitted settlement. (hopefully, you are within 30% as Focht3 says!). If the settlement computed is higher than the net bearing capacity, you'll have to reduce the bearing pressure until you "compute" a tolerable settlement. Now, the resulting bearing pressure is usually lower than the net bearing capacity. Hence it is called the allowable bearing pressure. This pressure is based on settlement.

 

[chichuck]

thanks to all who responded.

euygar, I did recognize that for shallow footings, net values are close to gross.

focht3, you make a very good point. I tend to try and be precise, the Terzaghi lecturer's quote is a good thing for me to remember. But, that said, I don't see how being more precise (by accounting for difference in concrete weight vs soil weight) would make calculations using these values wrong.

BigH: your point about settlement is really good. In fact, you've answered my next question before I even got to ask it, now I know (and suspected before) that "bearing values" based on settlement are "net".

Thanks to all for the help.


regards,

chichuck

 

[Focht3]

It doesn't make them wrong. The point of the Lecture (1989) is to recognize that some approximate methods will give you realistic answers - so don't let a fancy printout "buffalo" you into accepting a "precise" answer that your "back of the envelope" estimate says is incorrect. If there's a discrepancy - take a hard look at the computer programs' assumptions and inputs before you accept them. And apply a generous dose of judgment as well.



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[ashjun]

A small footnote
In the bearing capacity equation recommend in the code here, part of the second factor that account for resistance along the upward sliding surface is q(Nq-1); where q is the total overburden and Nq is bearing capacity factor. If you see something like this, then the bearing capacity is the net value because q is subtracted from the total/gross equation.