NCEES sample exam

[AJK]

I'm working the NCEES sample exam AM problems. I cannot figure out their solution for problem No. 110, which is to find the max. moment on a basement wall due to hydrostatic pressure.

My solution involved going to the CERM appendix 44.A, Case 10 - Simple beam with trangular load. The equation there for max. moment is M=0.0642wL^2, and I use 40 lb/ft for w and 8.5' for L. I get 185 ft-lb (per LF of wall), which is wrong.

Why can't this problem be solved this way? Thanks, Andrew

 

[Chris_TX]

I'm working the NCEES sample exam AM problems. I cannot figure out their solution for problem No. 110, which is to find the max. moment on a basement wall due to hydrostatic pressure.
My solution involved going to the CERM appendix 44.A, Case 10 - Simple beam with trangular load. The equation there for max. moment is M=0.0642wL^2, and I use 40 lb/ft for w and 8.5' for L. I get 185 ft-lb (per LF of wall), which is wrong.

Why can't this problem be solved this way? Thanks, Andrew

Andrew,

Take a look at the problem again. It states the load is a fluid pressure of 40 pcf not 40 plf. This is where your hang-up may be. If you resolve it into a linearly distributed load and then plug into the equation it comes out to 1577 lb*ft/ft.

Chris

 

[Cheese]

Andrew,

You are exactly correct in your approach.

The formula for a triangular distributed load (simply supported) can be found from appendix 44.a

You used w as the load per foot. w (for the equation mentioned above) is for the maximum load (which acts at the bottom end of the wall).

In this case w = 8.5 x 40 = 340

0.0642*340*8.5^2 = 1577ft-lb/ft

I'm really not sure yet where the equation on the solution page comes from.

Chris is essentially correct.

Cheese

 

[AJK]

Now I see what I did wrong - it seems so obvious now.

Thanks for your help.

 

[ferryg]

Now I see what I did wrong - it seems so obvious now.
Thanks for your help.

Be careful...these are the kinds of traps waiting for you. The exam is full of questions like that....don't be suprised on the exam if one of the answers is the incorrect one you got. When I was preparing the amount of simple mistakes I was making became very frustrating....eliminate the simple mistakes and you will pass.

Good luck.