NCEES Sample Exam (2008 ed) Geotech Question 517 (p75)

[brwone]

Can anyone explain how they are calculating the resisting overturning moment in the provided solution (p166)? The distance from the toe to the centroid of the soil above the footing should be 8 feet, but they seem to be using 1 foot. This seems to defy logic.

 

[Guest]

I don't have that publication, do you mind giving us a rundown of the problem and pertinent parameters?

Thanks.

JR

 

[IlPadrino]

I don't have that publication, do you mind giving us a rundown of the problem and pertinent parameters?
Thanks.

JR

I, too, don't understand why they used a moment arm of 1ft for the weight of the soil above the footing.

 

[brwone]

I don't have that publication, do you mind giving us a rundown of the problem and pertinent parameters?
Thanks.

JR

A simple problem. It is just an 18 ft x 5 ft retaining wall with a 9 ft x 2 ft footing. The backfill is 105 pcf, concrete is 155 pcf. They calc the resisting moment thus:

Resisting moment=(2)(9)(155)(4.5)+(5)(18)(155)(4.5)+(2)(18)(105)(1) = 79,110 ft-lb

I don't have a problem with the "driving moment." The 1 ft in the last term should probably be 8ft. Probably should be included in the errata if it is incorrect.

 

[Guest]

A simple problem. It is just an 18 ft x 5 ft retaining wall with a 9 ft x 2 ft footing. The backfill is 105 pcf, concrete is 155 pcf. They calc the resisting moment thus:
Resisting moment=(2)(9)(155)(4.5)+(5)(18)(155)(4.5)+(2)(18)(105)(1) = 79,110 ft-lb

I don't have a problem with the "driving moment." The 1 ft in the last term should probably be 8ft. Probably should be included in the errata if it is incorrect.

You are correct, the last term should be 8-ft (e.g. 2-ft + 5-ft + 2-ft/2-ft).

I think I can explain the error - it is a matter of notation.

If you look at each of the expressions in the resisting moment, they are expressed in ft-lb, meaning that that is the force per unit length of wall. Often times people will go ahead and include the (1-ft) unit length of wall in the expression so they end up ONLY with lbs of force.

In this case, it appears that the author was inconsistent with his approach throughout the expression - lb/unit length of wall or simply lbs (implicitly including unit length of wall).

Definitely an error.

JR

 

[LakersFreak]

You are correct, the last term should be 8-ft (e.g. 2-ft + 5-ft + 2-ft/2-ft).
I think I can explain the error - it is a matter of notation.

If you look at each of the expressions in the resisting moment, they are expressed in ft-lb, meaning that that is the force per unit length of wall. Often times people will go ahead and include the (1-ft) unit length of wall in the expression so they end up ONLY with lbs of force.

In this case, it appears that the author was inconsistent with his approach throughout the expression - lb/unit length of wall or simply lbs (implicitly including unit length of wall).

Definitely an error.

JR

What the author did was take the moment at the heal instead of the toe. Is this wrong????? Is the moment only on the toe???

Usually problems show the moment on the toe when asked such a question. Please advise.