Quiz #7

[John QPE]

An equal tangent sag curve has an initial grade of -6.0% and a final grade of +3.0% with a length of 1,270 feet. An overpass is to be placed directly over the PVI of this curve. Determine the minimum required height of the proposed overpass.

 

[Monkey]

Is it 23'-10".

 

[John QPE]

Sorry it is not.

A. 7.87 ft

B. 14.5 ft

C. 17.2 ft

D. 8.67 ft

 

[Monkey]

14.5, or what is the solution and how did you get it.

 

[ptatohed]

Johnster-Monster, is design speed not needed to solve this one?

 

[John QPE]

Design speed is not given (you have an A and L)

Anyone else?

 

[ptatohed]

Ok, my friend and I just worked on it at lunch. I thought we were on the right track but we do not match the answers exactly. Close to answer A or D I guess.

A = 9

L = 1270

K = 1270/9 = 141.1

From GDHS Table 3-36, DS is between 60-65mph and SSD is between 570-645 ft

Using interpolation, SSD ~=600'

Using the S<L formula eqn 3-53, with h1 = 8 (or use eqn 3-55) and solving for clearance, C...... C = 8.2'.

Answer A? Or where did we go wrong?

 

[John QPE]

OK .... well our difference in solutions here is semantics ... my math came to 8.67' but I did not interpolate, I just used the lower design speed of 60 mph. HOWEVER!!!!!

You missed a key part of this problem. Both A and D are incorrect.

14.5 feet is the correct answer.

Remember, AASHTO requires a minimum overhead clearance of 14.5 feet, and desirable height of 16.5 feet.

Monkey is correct.

 

[ptatohed]

Very good. I see it now on page 10-21 of the 6th ed GDHS. Is that where you found it?

 

[John QPE]

It is located in the text on page 10-21.

However, I am cheating. I'm using Testmasters, Principles of Highway Engineerin and Traffic Analysis, and Mansour notes pretty much exclusively. These are doing a great job at summarizing everything.

 

[afraid]

I would like to take a second look at this problem. Let's assume that the height of the pvi was given. Could I have used equation 79.52 on the CERM 14TH., page 79-14. To solve for the actual height required to clear the obstruction?

 

[John QPE]

I don't have my CERM with me, but if the PVI elevation was given what I would do is solve for do (the offset between curve and PVI)

{ | g2-g1 | L } / 8 to get the elevation on the curve, then add in the required AASHTO offset

 

[afraid]

John,

Would you add the final value?, I was there thinking why not pick the greatest of the two?. I guess this can be a completely new question for on its own. Thanks for the response.

 

[John QPE]

If I understand your question correctly .... AASHTO requires a minimum clearance of 14.5 feet and a preferred of 16.5. My understanding of this, is basically you can go with 14.5, if there is another route somewhere that provides 16.5 feet clearance.

Now why would I not pick the greater answer choice?? We are given 14.5 feet or 17.2 feet as answer choices.

The problem asks for minimum required, and I think you have to use a little engineering judgment .... 17.2 feet would just cost more to build.