2011 GDHS - Spiral Curve Lengths - Minimum, Recommended, Maximum

[PeonPE]

Hello,

Again, I am working these somewhat sktechy PPi Exam Cafe problems and I need input.

I am working a problem about Spiral Curve transition, Horizontal Curves.

Given: the Radius (1200ft.) and Design Speed (60MPH) , perpendicualr tangents( i.e. I = 90 degrees), two lane highway: 10 foot lanes, 4 foot shoulders, located in an area that has snow & ice removal. They want to know what the "recommended" length of the spiral transition should be.

My Solution: I am using the guidance in the 2011 GDHS as my NCEES Design Standard. The CERM has the older guidance from the 2004 GDHS and the equations have changed. So per pp. 3-70 thru 3-73 GDHS you can calculate the three values for

1) Minimum Length = Greater of the two equations

a) SQRT (24*pmin*R) = 138 ft.

b) 3.15 * (V^3 / (RC)) = 142 ft.

2) Recommended Length @ 60MPH from Table 3-20 = 176 ft.

3) Maximum Length = SQRT (24*pmax*R) = 308 ft.

Answers:

A) 140 ft.

B) 180 ft.

C) 230 ft.

D) 310 ft.

I myself picked B) the Recommended Length (180 ft.) since it was greater than the minimum and less than the maximum.

As usual, according to PPi I am always wrong and the correct Answer D) was the Maximum Length

I guess I have read the guidance several times and I see no reason why you should ever choose the maximum length over the recommended length unless you need it for turning at roadway terminals to adequately develop the desired superelevation. None of that information was given in the problem. The maximum is just listed to avoid driver navigation issues.

Which would you choose and why?

 

[MikeMcC215]

Because the problem stated there is snow and ice on this roadway, the transition length should be maximum. i would choose D.

 

[PeonPE]

Hey Mike, the guidance seems to indicate the opposite - curves that are too long tend to mislead the driver leading them to overcorrect on a curve. Instead the recommended curve length seeks to give the driver a length that is proportional to the radius. I think overcorrection in icy conditions would be worse than over correcting in clear and sunny conditions.

Not trying to be arguementative - just stating what the book says.

 

[MikeMcC215]

Seems like you are overthinking this, gotta be conservative.

 

[ptatohed]

I am only 95% sure on this (but it is a "solid" 95% ) but I do not believe the exam tests on spiral curves. I skipped any and all spiral curve material during my review. I never saw spiral questions on the exam. It's only my opinion but I'd recommend that you spend your study time on other topics.

 

[MWC PE]

I agree with ptatohed but to answer your question this is how I think they got 310. However if I'm correct I don't really think this is a fair 6 min long question because of the multiple areas to reference.

pg 3-70 last sentence of the paragrpah under formula 3-25

para "a more practical limit is to make the length equal to the e runoff length..."

Since snow is present max e = 8% (see page 3-30 4th pargh)

Lr (see table 3-17b) @ e=8, 2 lanes rotated, V = 60

Lr= 328' (note this is for 12' lanes.)

adjust for 10' lanes = 328*10/12 = 266'

then round up to best answer 310'

If that is indeed how they wanted you to solve the question, I would say that is not representative of a typical PE problem. To me this is a bit of a "trick" question.