Meyerhof/Vesic Bearing Capacity Factors

[ktulu]

In CERM Table 36.3, they give you the bearing capacity factors developed by Meyerhof/Vesic. I understand and know how to use them and all; the question I have is this: When do you know when to use either value for N(gamma)? They sure do give you different answers (I just used the wrong one...)

 

[redrum]

I think they tell you which N to use for the most part, but I agree its confusing to know when to use either one.

 

[bigray76]

I've been using the table out of the CERM instead of out of my Das text from college. The answers in the CERM questions book and the 6 minutes books seem to be more consistent with the CERM (which I am assuming will also be more consistent with the NCEES).

-Ray

 

[Guest]

ktulu, bigray --

ce hopeful is correct. If you are presented with a problem that requires you to use shape factors, you will be:

1. Provided those factors in the problem statement,

2. Provided a table/figure as part of the problem statement, or

3. Told to use Terzaghi or Vesic-corrected factors.

Truthfully, it would be most likely 1. or 2. due to the lack of direction pointing to which set of factors are 'correct'.

JR

 

[ktulu]

I just wish that the CERM would have given some kind of explanation as to what type situation the different Meyerhof/Vesic factors should be used....

 

[Blacktop]

I just wish that the CERM would have given some kind of explanation as to what type situation the different Meyerhof/Vesic factors should be used....

Me too. Looked in my college Geotech book (Das) and haven't found anything. God that book is confusing, I wish I had the CERM in college as a supplement.

I had the same question as you ktulu, asked here:

http://engineerboards.com/index.php?showto...p;#entry6621363

So, on the actual PE test, if they specify that you should use the Meyerhof and Vesic table, will they also specify which N sub gamma to use between the two?

(to ktulu or jreg).

Thanks for any help.

 

[Guest]

So, on the actual PE test, if they specify that you should use the Meyerhof and Vesic table, will they also specify which N sub gamma to use between the two?(to ktulu or jreg).

I will offer my 'theoretical' take on this problem but I would like ktulu to offer his insights since he has tested for the geotechnical exam whereas I am completing a M.S. in Civil Eng with emphasis in Geotechnical Engineering.

First, a brief discussion of the problem as presented. Warning ... lots of theory!

The shallow foundation equation as first presented by Terzaghi (1943) yielded an approximation to the bearing capacity equation based on general shear failure. In simpler terms, Terzaghi evaluated failure conditions along known planes based on shear and developed a general relationship based on the THEORY. Fundamental assumptions (applicability) is based on level strip footings placed on (near) ground surface where the depth of the footing is less than the minimum width of the footing. Additionally, the footing is assumed to be in plastic equilibrium with defined failure surfaces that follow a theoretical pattern (boussinesq distribution).

The Meyerhof Model came along and included in the solution correction factors for eccentricity, load inclination, and foundation depth. This solution also varies from the basic Terzaghi solution in that the influence of shear strength of soil ABOVE the base of the foundation is INCLUDED. This means that beneficial effects of the foundation depth (e.g. surcharge) is included in the analysis. The failure plane analysis is slightly more complex as the soil is still in plastic equilibrium but has a log spiral failure surface that includes shear above the base of the foundation. Ngamma is based on upsilon = 45 + phi/2.

The Vesic Model closely follows the Meyerhoff and Hansen Models but incorporates analysis for Ngamma from Caquot and Kerisel (1953) to address the assumption that local shear failure leads to lower bound estimates of ultimate bearing capacity. It is the local shear that becomes the driver in the analysis.

So ... where does this all lead you?

In practical terms, Practioners of geotechnical analysis will typically evaluate the effects under the different shallow bearing capacity models and may even combine the results of some of the models in order to better understand how the bearing capacity will be affected by the loading.

In terms of the PE exam, I do not believe you will be given enough information in any given problem that will allow you to distinguish when to apply the Meyerhoff Model vs. the Vesic Model in terms of the correction factor. Therefore, it will be necessary for the correction factor to be provided in the problem statement as a 'known' value rather than asking you to derive it based on other information.

That is my take from my own perspective. :2cents:

JR

 

[Blacktop]

Thank you, that's very good info.

 

[Guest]

Thank you, that's very good info.

FWIW -

I just noted Problem #516 (Geotech Afternoon) in the 2008 NCEES Civil Practice Exam that required the use of shape factors to evaluate a shallow foundation. In the problem statement they provided a table of friction angles with corresponding values for each of the three shape factors.

I think the presentation of this problem would be consistent with what you would see on the exam.

JR

 

[sehad]

FWIW -
I just noted Problem #516 (Geotech Afternoon) in the 2008 NCEES Civil Practice Exam that required the use of shape factors to evaluate a shallow foundation. In the problem statement they provided a table of friction angles with corresponding values for each of the three shape factors.

I think the presentation of this problem would be consistent with what you would see on the exam.

JR

I know this is along time after the above post, but it may help someone on the October exam. In Geotech, there are several different ways to evaluate capacity. For shallow foundations, there are 3 or 4 atleast that I can think of off the top of my head. For piling bearing capacities, there are atleast 5. For skin friction values, another 2 or 3. DAS does a decent job of showing the different formulas then elaborates on the ones that he prefers. But none of them are technically wrong even though(especially in pile bearing capacity problems) there can be HUGE differences in the answer(sometimes as much as 5 times).

So, due to their being several different methods and none of them really wrong, they exam MUST give you the constants or what equation to use for the answers to be consistant between the examinees. It is sometimes harder to understand DAS's texts, but that is the only thing I stuided besides the CERM and I passed first try(Geotech depth). They may be tough reading, but they are very very good for the exam!