Superelevation

[civilized_naah]

Question

What is most nearly the superlevation (%) for a roadway with a (horizontal) curve radius = 1800 ft and a design speed of 50 mph?

Answer 1:

V^2/15R = 50^2/(15*1800) = 0.093

However, the recommended side friction factor for 50 mph is about 0.14

This implies that no superelevation is needed? Since 0.093 - 0.14 is negative.

Answer 2:

Lookup from AASHTO Green Book Exhibit 3.21: For 50 mph and R = 1800 ft, the recommended superelevation is 5.5%.My question is, why this HUGE difference? I would kind of understand it if the theoretical conclusion was around 1% and the recommended value was around 2-3%

 

[Dexman1349]

The two things to remember with the (V^2/15R)-fs equation is:

1) large R's causes the equation to go negative. Forcing you to use the greenbook tables.

2) the answer is a decimal. The greenbook's tables are shown as percentages.

I would personally use the Green Book. Why calc something if there is already a table with industry accepted standards?

 

[civilized_naah]

The two things to remember with the (V^2/15R)-fs equation is: 1) large R's causes the equation to go negative. Forcing you to use the greenbook tables.

2) the answer is a decimal. The greenbook's tables are shown as percentages.

I would personally use the Green Book. Why calc something if there is already a table with industry accepted standards?

What if the question shows up in the AM exam? You are not expected to use specific design Standards for the breadth exam (of course, NCEES never says that you need these standards for PM exam only). People would tend to use the physics equation rather than 'look up' from the Green Book. I do understand the difference between percent and decimal. My point was, the physics says 'no superelevation required. side friction is adequate' while Green Book says 'Use 5.5% (or 0.055) superelevation, which is nothing to sneeze at.

 

[Dexman1349]

I would not expect to see a question like this on the morning portion. If you do, it would be such that the calculation would be sufficient (ie small radius) and would match a table value from CERM.

 

[civilized_naah]

I would not expect to see a question like this on the morning portion. If you do, it would be such that the calculation would be sufficient (ie small radius) and would match a table value from CERM.

My curiosity still remains: why such a big difference? I haven't heard a convincing answer to that question? Anybody else venture an opinion?

 

[natee_dj]

I don't know if this makes a difference but, I just used this formula:

R = V^2/[15*(0.01e + f)] and solved for e.

 

[civilized_naah]

I don't know if this makes a difference but, I just used this formula:
R = V^2/[15*(0.01e + f)] and solved for e.

Yes, but that gives you a negative answer for e, implying that side friction is adequate to resist the lateral force V^2/15R.