What am I doing wrong (vertical curve clearance problem, find L)?

[ptatohed]

I wasn't sure if I should start this thread in the Transpo or CA-Survey sub-forum, so I just started it here in the Civil forum.

Anyway, please see the attached problem #44. I keep getting answer A. The answer key lists the answer as D. There aren't any solutions, just answers (this is out of the CalTrans LS/LSIT Workbook).

What am I doing wrong? Or is the answer key perhaps wrong??

Thanks everyone!

 

[civilized_naah]

There is nothing wrong with the solution. An easier way would be to use the formula

-----------------

L + 2h / v - G1 h

--------- = / --------------

L - 2h v v - G2 h

where v and h are the vertical and horizontal distance of the point on the curve from the PVI

In this problem: h = 6.87 - 6.50 = +0.37 sta

v = 39.11 - 34.23 = +4.88

G1 = -1.65%

G2 = +4.10%

Solving, L = 6.064 sta (same as yours)

 

[ptatohed]

Thanks Dr. The reason why it is answer D hit me toward the end of the day. I thought at first when the problem statement said to determine L to the nearest 1/2 station, I figured L = 6.07 sta would round down to L = 6.0 sta. But, if we round down, then we'll have a smaller L for the VC and we won't have our 3' clearance above the pipe. So, we must round up to L = 6.5. This gives a STAbvc of 6+50-(650/2) = 3+25 (answer D).

It would be similar to, say, solving for the pipe size for a given Q and you end up calculating a 16.1" pipe. The answer would be the next pipe size up, not 16".

 

[civilized_naah]

Good catch! I missed that in the question.