NCEES Morning 523

[patioshep]

Vertical steel Channel (3 bolts) with 500 lb force on the end. When calculating the secondary shear force , where does NCEES's formula come from ?

F = 9500 (6) / 2 (36). I know the 9500 is the moment , but I'm guessing on the rest: the 2 is the number of bolts ?(the 3rd bolt is actually the centriod of the bolt pattern), the 6 is the distance from the centriod ? , what about the 36 ? 6x6 ?

MERM 51-16 has eccentric loaded arms & critical fasteners , but do not see this equation.

You would think a professional publication would include the equation, units, and NO ERRORS !

Any help would be greatly appreciated.

Thanks

 

[Matt-NM]

I think this is it. (I have a note in my NCEES book that says "Dont Get" for this problem, so I must have had similar issues when studying for the test in April).

1. MERM formula (I am using MERM 11th ed.) 51.57 gives shear stress = Tr/J.

2. If you equate shear stress = F/A, and substitute this into the equation above you will get F/A = Tr/J. (This F is the shear force, not the external

applied force that equates to 500lb in this case).

3. Move the area (A) to the other side of the equation, thus you get F = TrA/J

4. From MERM eqn. 51.58 J = Summation of r^2*A, where r is the distance from the centroid point to the point in question. Thus, with two fasteners this equates to (2)(6)(6)A = (2)(36)A

5. Now substitute J from eqn. 51.58 above into the equation from step 3 above. Thus you get F = TrA/(2)(36)A.

6. The A terms cancel and you are left with F = Tr/(2)(36), which is the formula in the NCEES solution.

I agree that the NCEES solutions are sketchy in some cases.

Hope this helps.

 

[patioshep]

Ok... Thanks Matt-NM ! I follow it now.. Is it reasonable to assume one should be able to derive an equation like this on the fly during the exam? are the PM MD questions on this level ? thanks

 

[Matt-NM]

From what I remember, I don't think I had to derive any equations on the exam. If you get to one where you think some type of derivation may be necessary, it would probably be best to skip it until the end unless you think it is going to be relatively simple.

I personally thought that the afternoon machine design section was very difficult. There are just so many variations of problems that they seem to ask. They aren't going to simply ask the same NCEES sample questions with different numbers. Make sure you understand the concept of each problem. This increases the liklihood that you can apply it to a slightly different situation.

I believe the key to the exam is doing well in the morning. Then all you have to do in the afternoon is ok and you still have a chance to pass. I did far better in the morning than the afternoon. There didn't seem to be many machine design problems in the morning section. Also be sure to take many references with you, even if you haven't looked through them. Some say not to do this but I personally found the answers to at least two questions in books that I hadn't opened before. Make sure you look at the new specifications for the October ME exam. I believe they have changed.

Hope this helps some. Good luck.

 

[patioshep]

Thanks for the advice.....

What type of references do you suggest to have on hand? So far I have Marks Standard Hanbook, MERM, and Mechanism Design Textbook (not sure of Author?) Are the MERM steam property tables sufficient or should I have a Thermo Book. ?

 

[Matt-NM]

Those are probably the most important ones. Any books you still have from college won't hurt either, just in case. I don't think I used any steam tables except those in MERM. Now that I think of it, though, i'm not sure if MERM has complete tables for the refrigerants. If not, then I would suggest getting a thermo book that would have the complete refrigerant properties. (R-12 and R-134). I wouldn't put it past them to suddenly have three R-134 problems in a row! Expect the unexpected with this test.

Also, be sure to read my post in "Anything about the PE exam" on engineering economics. It could be a life-saver.