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Shallow Foundations- Bearing Capacity


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#1 terzaghi83

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Posted 04 February 2012 - 06:35 PM

There is a slight difference in the ultimate bearing capacity equations for square and circular foundations as listed in variosu Braja Das and V.N.S Murthy texts versus the Equations in the CERM.

In the Das and Murthy texts, the terzaghi bearing capacity equation is modified for square foundations and circular foundations with the following changes:
For square foudnations:
c*Nc is multiplied by 1.3
the Ngamma term is multiplied by 0.4

compare this to the use of shape factors listed in CERM, for a square foundation the nc term is multipled by 1.25 and the Ngamma term is multiplied by 0.425 (.5 in strip footing equation multiplied by the shape factor of 0.85 in CERM.)

Similar differences are present in the circular foundation equation.

As far as I know, these are both terzaghi bearing capacity equations, although they are just listed as terzaghi equations in Das and murthy but the one with the shape factors is listed as terzaghi-meyerhoff in the CERM so perhaps this is the difference? Both are different from the general bearing capcity equation which can account for shape, depth, inclination factors from meyerhoff, hansen or Vesic.

If the exam asks you to solve a bearing capacity problem using terzaghi bearing capacity equations and factors what is one to do (method with shape factors in CERM or slightly different equations with shape factors "built in" as listed in Murthy and Das?

Using the Das equation to solve problem 15 of six minute solutions, the answer is 3553 psf. If you use the equation in CERM with shape factors you get 3857 psf. In some instances this may lead you to the wrong answer selection from multiple choice perhaps?


I'm looking for input from anyone who has taken the exam (or anyone else) about how to handle this situation.

thanks, and back to another wonderful weekend of studying!

#2 Badger

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Posted 05 February 2012 - 06:42 AM

I believe you should generally use the shape and depth factors from the CERM for square and circular foundations.If other shape factors are needed they will usually be given or specified on the exam.

I would be familar with the various factors. Pages 138-140 of Das' Principles of Foundation Engineering 5th edition, cover most of them.

Problem 516 in the 2008 NCEES Sample Exam originally had the same answer whether you used shape & depth factors or didn't. The they changed it to use Vesic's shape and depth factors, then NCEES replaced the problem with a strip foundation question requiring no depth or shape factors. So I think they NCEES know there could be confusion, if they don't provide or specify the factors.

On this forum somebody recommended when in doubt use the CERM as your reference. I think that advice was very elpful to me.

I found the CERM and Das' Principles of Foundation Engineering most helpful for the afternoon geotech depth questions. I also answered a few questions with an old edition of Garber's Transportation Engineering. I hope this helps.

Good Luck.

I

#3 IlPadrino

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Posted 05 February 2012 - 02:43 PM

On this forum somebody recommended when in doubt use the CERM as your reference. I think that advice was very elpful to me.


If I had to choose for a conflict between the CERM and any other recognized reference, I'd go with the *other* reference. In general, the CERM is excellent but it's not as if NCEES is using it to generate exam questions.

#4 geo pe

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Posted 12 February 2012 - 04:12 AM

If the exam asks you to solve a bearing capacity problem using terzaghi bearing capacity equations and factors what is one to do (method with shape factors in CERM or slightly different equations with shape factors "built in" as listed in Murthy and Das?

Based on my experience with the exam, shape and B.C factors are always given with the question, if in doubt, I would use Das. Moreover I think at such times, go by negating the other options!




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