Qnet vs. Qult

[maximus808]

Hey guys, I've been trying to do some problems from both PPI Practice Problems and Chelapati's Geo book.

When sizing a footing in Chelapait's the general method is

1. Set up your Qult equation based on your footing type.

2. Find your terzaghi factors. (another thing with this is that I get different results using Das Black Back and PPI's table 36.2.

3. Plug in your variables and divide by your FS. This will be Qallowable and Equation 1.

4. The force acting upon the footing divided by B (continous), B^2 (square) this will be Equation 2.

5. Equate Equations 1 and 2 and solve B.

However, in the CERM, it does not use Qult. but uses Qnet? Chelapati's do not even mention Qnet. Qnet is simply Qult - Df*Gamma.

When calculating the B, would you use Qnet or Qult? Maybe I should look at the NCEES books to see there approach. But tell me what you guys think.

Also what book or table do you refer to find your Terz. Factors? How do you interpolate if factor is not a listed number as it is curved interpolation? Thanks guys.

.

 

[Ambrug20]

Hey guys, I've been trying to do some problems from both PPI Practice Problems and Chelapati's Geo book.
When sizing a footing in Chelapait's the general method is

1. Set up your Qult equation based on your footing type.

2. Find your terzaghi factors. (another thing with this is that I get different results using Das Black Back and PPI's table 36.2.

3. Plug in your variables and divide by your FS. This will be Qallowable and Equation 1.

4. The force acting upon the footing divided by B (continous), B^2 (square) this will be Equation 2.

5. Equate Equations 1 and 2 and solve B.

However, in the CERM, it does not use Qult. but uses Qnet? Chelapati's do not even mention Qnet. Qnet is simply Qult - Df*Gamma.

When calculating the B, would you use Qnet or Qult? Maybe I should look at the NCEES books to see there approach. But tell me what you guys think.

Also what book or table do you refer to find your Terz. Factors?

To find Terz. Factors use Table 36.2 CERM page 36-3. I assume that no interpolation would be required on the test, but on my practice I do linear method. CERM page36-4 eq. 36.3 and read the text on top of formula. Qnet is corrected method by the overburden. Geotechnical people would probably explain it much better. I’ d like to know the answer.

How do you interpolate if factor is not a listed number as it is curved interpolation? Thanks guys.

.

 

[Ambrug20]

I repete myself. It wasn't clear reply before.

To find Terz. Factors use Table 36.2 CERM page 36-3. I assume that no interpolation would be required on the test, but on my practice I do linear method. CERM page36-4 eq. 36.3 and read the text on top of formula. Qnet is corrected method by the overburden. Geotechnical people would probably explain it much better. I’ d like to know the answer.

 

[boo]

Example 12.5

A square footing fails by general shear in a cohesionless soil under an ultimate load of

Qu= 1687.5 kips. The footing is placed at a depth of 6.5 ft below ground level. Given 0 = 35°, and

y=110 Ib/ft3, determine the size of the footing if the water table is at a great depth (Fig. Ex. 12.5).

Solution

see this help you

For a square footing [Eq. (12.17)] for c = 0, we have

For 0= 35°, Nq = 41.4, and Ny= 42.4 from Table 12.1.

Qu=y Df Nq+ .4y B Ny

qu=Qu /B^2=168.5 X 10^3 /B^2

168.5 X 10^3 /B^2=110 X 6.5 X 41.4 + .4 X 110 X 42.4 B

B^3 + 15.863 B^2 - 904.34 = 0 B=6.4

 

[chess5329]

Example 12.5A square footing fails by general shear in a cohesionless soil under an ultimate load of

Qu= 1687.5 kips. The footing is placed at a depth of 6.5 ft below ground level. Given 0 = 35°, and

y=110 Ib/ft3, determine the size of the footing if the water table is at a great depth (Fig. Ex. 12.5).

Solution

see this help you

For a square footing [Eq. (12.17)] for c = 0, we have

For 0= 35°, Nq = 41.4, and Ny= 42.4 from Table 12.1.

Qu=y Df Nq+ .4y B Ny

qu=Qu /B^2=168.5 X 10^3 /B^2

168.5 X 10^3 /B^2=110 X 6.5 X 41.4 + .4 X 110 X 42.4 B

B^3 + 15.863 B^2 - 904.34 = 0 B=6.4

What about depth and shape factors?.....what book is this example come from?...........a geotechnical guru might want to help?

Thanks