Shallow Foundations

[geno34]

For shallow foundations:

I have noticed that the CERM uses the q allow=(q net/FS).

However, I have found many example problems that use q allow = (q ult/FS).

If we are given a problem like this in the morning, which equation is correct?

Also, the CERM gives a couple of tables on shape factors; however, I have found in the NCEES practive test they use different values than the CERM.

Please help.

 

[K=1.0]

For shallow foundations:
I have noticed that the CERM uses the q allow=(q net/FS).

However, I have found many example problems that use q allow = (q ult/FS).

If we are given a problem like this in the morning, which equation is correct?

Also, the CERM gives a couple of tables on shape factors; however, I have found in the NCEES practive test they use different values than the CERM.

Please help.

The q_allow=q_ult/FS is the correct one.

 

[Dano_PE]

Be careful with footings and the water table.

It gets tricky depending the water table is in the zone below the footing and depth B(width of footing)

or the water table is directly as the base of the footing, at grade or somewhere between.

Certain terms are converted to Boyant unit weight and some are not. Read page 36-7 and 36-8 in the cerm and your good.

But yeah Qallow = Qult/FS (find what soil is capable of and apply a FS to it)

similar to structural engineering (find strength of beam and apply a FS to it)

Same Idea

 

[Jayman_PE]

Dano - I think what K1.0 is asking is the inconsistency of the texts equating qallow to qult/FS or qnet/FS. That is a VERY good question. I would suggest on the exam, if not specified in the question, apply it both ways (you're only subtracting the vertical soil pressure for qnet), and see if the answer shows up both ways. If it does you're hooped. Since it's more conservative I would use qnet for finding qallow.

7 days to go....

 

[terzaghi83]

unless problem asks for net allowable, im not using net. If it asks for footing dimensions without specifying to base it on net allowable bearing capacity, Im not using net.

I have no idea if this is the correct approach, I guess we will see!

Also, I think they will give us shape factors and bearing capcity factors as needed because there is too much variation between references.

Good luck everyone!

 

[ketanco]

shape factors were slightly different in the "All in One" book by goswami. I agree with Terzaghi. because there is differences they should give those - i hope... i didnt look at DAS books yet or geotech engineer's handbook. I suggest look at those too and compare the factors. if you selected geotech you should have those