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Length of Spiral Curve


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#1 winner9459

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Posted 28 March 2011 - 12:08 AM

Hello group,

Would some one please clarify my question about when to use what formula for determining length of the spiral curve. CERM on page 78-18 gives (1.6*V^3/R) formula where as Green book on page starting 188 gives different criteria for calculating minimum lengths for the spiral curve. Any clarification would be much appreciated.

I encountered while solving PPI problems that the solutions made use of both the green book and CERM for 2 different problems of the same kind, namely, finding the length of the spiral curve.

Thanks.

#2 Ambrug20

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Posted 28 March 2011 - 03:20 PM

QUOTE (winner9459 @ Mar 27 2011, 07:08 PM) <{POST_SNAPBACK}>
Hello group,

Would some one please clarify my question about when to use what formula for determining length of the spiral curve. CERM on page 78-18 gives (1.6*V^3/R) formula where as Green book on page starting 188 gives different criteria for calculating minimum lengths for the spiral curve. Any clarification would be much appreciated.

I encountered while solving PPI problems that the solutions made use of both the green book and CERM for 2 different problems of the same kind, namely, finding the length of the spiral curve.

Thanks.


Green book described spiral curves on page 185. When you see the difference of spiral curve explanations in Green book - table 3-37 p.189, its Spiral Curve Transition and father described Spiral curve length for different acceleration and super elevations. Hopefully, we wouldn't go that deep or at least would be able to use the table for needed data.
Good luck on test.

#3 winner9459

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Posted 28 March 2011 - 03:52 PM

Thanks Tanya for the reply. It is my understanding that spiral curve transition and spiral curve length are one and the same thing. The thing I don't understand is the use of the formula's, the usage of the different formulas from CERM and Green book results in 2 different solutions.

I too hope that they won't go into such depths, but again, there is no way to tell to what extent they want to test on the exam. Basically, it is a simple formula application, but the concept behind and the knowledge when to use what is important. After all, I am taking transportation depth, so it might be useful to know.

Good luck to you too for the exam.


#4 lady_j

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Posted 28 March 2011 - 08:26 PM

I could be WAY off, but here goes...

The equation in the Green Book is: L = 3.15V^3/RC
CERM equation is : L = 1.6V^3/R

That "C" constant is described as an empirical value representing comfort and safety and is often taken as a value between 1 ft/s3 to 3 ft/s3......so I *think* that the CERM is using a value close to "2" for C.

Any thoughts? Thanks again winner for posting this stuff!

#5 winner9459

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Posted 28 March 2011 - 08:54 PM

CERM is a modification of 1909 Shortt equation, and the CERM books goes on to say that there are different methods to calculate the lengths of the spiral curves. I assume that PE exam is going to specifically mention what formula to be used, the way I encountered while solving some problems in the test series.

Sorry for the confusion, but I hope it gave you guys some new info.

Thanks.

#6 ptatohed

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Posted 04 April 2011 - 09:04 AM

Why are you guys studying spiral curves? They are not on the outline syllabus for the CA-Specific Survey (which I took and passed last time, without studying spiral) nor the NCEES 8-hour Transpo depth (which I took last time and failed but believe I will pass this time). I did not see spiral questions on the last CA Survey nor the last 8 hours. My suggestion is to not study anything which isn't on the syllabus.

#7 lady_j

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Posted 04 April 2011 - 04:11 PM

QUOTE (ptatohed @ Apr 4 2011, 02:04 AM) <{POST_SNAPBACK}>
Why are you guys studying spiral curves? They are not on the outline syllabus for the CA-Specific Survey (which I took and passed last time, without studying spiral) nor the NCEES 8-hour Transpo depth (which I took last time and failed but believe I will pass this time). I did not see spiral questions on the last CA Survey nor the last 8 hours. My suggestion is to not study anything which isn't on the syllabus.


Spirals are generally included under superelevation problems. I've come across a few spiral problems in my studying - there's a section in CERM about spirals and it's covered in Green Book (to name another souce). Check out this info, probably would make a good cheat sheet!

http://www.wikiengin...on/SpiralCurves

#8 ptatohed

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Posted 04 April 2011 - 06:32 PM

QUOTE (lady_j @ Apr 4 2011, 09:11 AM) <{POST_SNAPBACK}>
QUOTE (ptatohed @ Apr 4 2011, 02:04 AM) <{POST_SNAPBACK}>
Why are you guys studying spiral curves? They are not on the outline syllabus for the CA-Specific Survey (which I took and passed last time, without studying spiral) nor the NCEES 8-hour Transpo depth (which I took last time and failed but believe I will pass this time). I did not see spiral questions on the last CA Survey nor the last 8 hours. My suggestion is to not study anything which isn't on the syllabus.


Spirals are generally included under superelevation problems. I've come across a few spiral problems in my studying - there's a section in CERM about spirals and it's covered in Green Book (to name another souce). Check out this info, probably would make a good cheat sheet!

http://www.wikiengin...on/SpiralCurves



lady i (lady j?), I completely understand and agree with you that you'll find spiral problems, that the topic is covered in the CERM, that the topic is in the ASSHTO GDHS, etc. The point I am trying to get across to you is that the topic is not on the exam and that you should not waste precious time studying topics that you won't be tested on. My (very strong) suggestion is to adhere to the test outline/syllibus. Good luck.

#9 sac_engineer

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Posted 04 April 2011 - 06:58 PM

Agreed with ptatohed. Spiral curve calculations are very involved and have not typically been on PE exams. If it's not specified in the test plan, then don't spin your wheels on it. With regards to geometry, be a master at horizontal and vertical curves, plus superelevation.

Good luck!




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