Allowable Soil Bearing Problem Question

[nafta]

The CERM, which I think is a bad reference for the PE, has q allowable set equal to q net/FS.

The CERM defines q net as q ultimate - depth of footing * soil unit weight.

The CERM's logic, which kind of makes sense, is that q net is the additional load the soil can take beyond the overburden.

I am confused as to what equation to use on the exam for q allowable: Qult/FS or Qnet/FS.

Wikipedia states Qult/FS and so does Engineering Videos. Engineering videos places the overburden load, the applied load to the foundation and the concrete load in the q allowable term...which is what I would do if I did not use the CERM.

I don't trust the CERM because it really is awful for how long this guy has had to tweak it. It keeps reinventing the wheel with seldom used notations, it goes into essays on topics that will never be covered on the exam, it does a bad job of wholistically explaining concepts and just breaks them up into several sections when they belong in one....heck, the FE Exam reference is sometimes better. The most annoying thing is how Lindenberg keeps writing things in SI then US and always keeps using lbf...it drives me up a wall. Just write one equation you doofus and say gamma = density * g....he must waste 50 pages with nonsense US vs. SI and lbf, vs lb, vs slugs; omg, he makes it so stupidly complicated.

He completely butchers hydraulic equations.... Anyway, I'm just getting frustrated with the CERM, but I hope one of you can shed some light on my question.

 

[IlPadrino]

Not sure where the hatred for the CERM is coming from... I have never found a better one-stop for everything Civil Engineering. It's far from perfect, but I don't think there's any issue of "trust" because as you note, it's been around for a long time. I also don't think it's fair to consider the CERM as written for the PE exam. It's probably the most common use, but I find myself pulling it out every month or so in the course of my work.

Anyway, to the question: There's no answer to "what equation to use on the exam?" It depends on what an exam problem asks. It sounds like you understand the difference between "ultimate" and "net [ultimate]" is the difference between the overburden pressure at the depth of the base of the foundation.

I wouldn't get caught up on "what to do for the exam", especially if you really understand the material. The exam isn't trying to trick anyone!

 

[civilized_naah]

If the column load is P and the footing area is A, then the soil pressure produced by the column load is P/A. The additional load due to soil overburden (and footing selfweight) is estimated as gamma Df

Therefore, the total soil pressure is P/A + gamma x Df

If this is to be less than the allowable stress (qult/FS), then the equation is

P/A + gamma x Df < qult/FS

This is a little bit different from

P/A < (qult - gamma x Df)/FS

To me, the first approach makes more sense, since the factor of safety indicated should not be applied to the gamma x Df term, which does not have a great deal of uncertainty.

Also, typical value of gamma x Df is about 400 psf, so the difference of 2/3 gamma x Df or about 250 psf is not a big deal when other terms (such as qult/FS) are in the neighborhood of 6000 or so.

You can’t really blame Lindeberg for this. You will see this duality in many respected and widely used Geotechnical texts as well.

 

[nafta]

That is true, and after venting, I came back to try and delete my CERM bash but you can't delete anything here.

Thanks for the answer Naah.

As for the CERM; it's a good reference book, but I feel like it could be much better considering how long it has been getting published for. I really feel that Lindenberg could make many topics easier to understand, and also use much less paper. Is there really a point in rewriting Bernoulli's equation in terms of unit weight and density.

Also, his appendices should have built in examples, there should be a TOC for tables, etc... I feel like he gets away with his inneficiencies because nobody can write something as dense but more understandable.