Lindberg Sample Exams Problem 164

[Predgw]

4th edition- 2nd printing

Problem ask to recomend a rate of superelevation. Because all the sample material out there and reference books use the 2004 green book I want to double check my method.

Question ( not word for word)

Circular curve is designed as part of a 2 lane rural highway. Highway will have 12' lanes and 6 foot shoulders. Degree of curve is 2 degrees. Design speed is 40MPH. Level surroundings

A 0.02 ft/ft

B 0.03 ft/ft

C .04 ft/ft

D .05 ft/ft

I quickly cauclated the radius as 2,864.79' I then turned to CERM 79-7 and calculated fs using equation 79-38 (13th edition cerm) I got 0.15

I then solved for e based on equation 79.37(b) e= v^2/15r - fs I got a negative value which is possible on a larger curve so I used 0.02 ft/ft

The answer is B. The solution refernces the 2004 Green book and list exch. which are now tables and NOTHING matches up.

However, I belive the correct solution is to use the green book page 3-45, e max of 6% ( table 3-9) Because e max of 4% can only be used on urban conditions I picked the next e max up. Then I went into the table at 40 MPH and looked for my radius. At 2.6 % the radius is 2740 feet. At 2.4% the radius is 3030 feet. So , 2.6% would work.

Given the choices, clearly 2% is to small, so answer B is picked.

The solutions assumes a max rate of superelevation of 0.08 to 0.10 and then enters the emax 8 and emax of 10 and comes up with 2.8 to 2.9% First, I cant find where it is recommended that rural highways have an assumed .08 to .10 superlevation. I do find langauge about being mindful of areas that can receive snow and ice. Where snow and ice can be expected, the superelevation should not be above 8%. Am I overthinking this? I assume there will not be such an open question on the exam.

Any thoughts?

 

[ptatohed]

Pred,

It seems like you have a pretty good understanding of the concepts.

First - I'd recommend that you try not to use two sources for one problem. Sometimes you may not be able to help it, but in this case you should stick with the GDHS exclusively. No need to go to the CERM. This will save you time on the exam.

Before I continue, I apologize, I only have GDHS5 here with me at home, as my GDHS6 is on loan to my friend who is studying for the April exam. But, I don't think there is any difference in these se calcs.

In my opinion, you need not waste time calculating the coefficient of friction, f. Use the GDHS defaults. In this case, for 40mph, f = 0.16

Like you, I would then go to eqn 3-9 (GDHS5) which is f = V2/15R - .01e. When you rearrange and solve for e, you get -12.3% (-11.3% if you use your f = 0.15)

I would then go to the 12% emax exhibit/table (Exh 3-29 in GDHS5). Like you did, I'd look for my 2865' radius under the 40mph column. I see that R = 2865' falls between R = 2700' and R = 2910' so the e falls between 2.8% and 3.0%. Answer B.

I do not know why the solution uses an emax of 8% and 10%. ??

 

[Predgw]

Thanks for the reply and the tip. Jumping around for no reason will kill my time. I dont know why they assumed 0.08 to .10. The green book does state on page 3-30 that the highest superelevation rate for higways in common use is 10 percent, although 12 can be used. It then states that 8% is the max when snow is anticipated. This is where I get a little tied up. I have never known a winter with out snow. It is the way it is, so it is natural for me to assume for snow conditions.

I like your way of staying in the green book. Again, you went to emax of 12%, where that is not allowed here in New England.

Am I correct in assuming the actual exam will be a little more specific on some of the conditions? The next question is, for driver comfort one should always enter the table at the highest emax allowable and then select appropriately. You agree?

Thanks

This was my break time by the way. Today is another practice exam day. I did pretty well in the am considering the questions dont always match the NCEES outline. For example the masonry question and earthquake question...... I dont see those as an am type question.

Thanks again.

 

[ptatohed]

Thanks for the reply and the tip. Jumping around for no reason will kill my time. I dont know why they assumed 0.08 to .10. The green book does state on page 3-30 that the highest superelevation rate for higways in common use is 10 percent, although 12 can be used. It then states that 8% is the max when snow is anticipated. This is where I get a little tied up. I have never known a winter with out snow. It is the way it is, so it is natural for me to assume for snow conditions.

I like your way of staying in the green book. Again, you went to emax of 12%, where that is not allowed here in New England.

Am I correct in assuming the actual exam will be a little more specific on some of the conditions? The next question is, for driver comfort one should always enter the table at the highest emax allowable and then select appropriately. You agree?

Thanks

This was my break time by the way. Today is another practice exam day. I did pretty well in the am considering the questions dont always match the NCEES outline. For example the masonry question and earthquake question...... I dont see those as an am type question.

Thanks again.

LOL, I assure you, we get no snow here in southern CA. (Actually, I drove my 3 yo son 1.5 hours up into the mountains last weekend to go sledding)

Don't think of the NCEES exam as local. The N stands for National. Think country-wide and figure if it's in the GDHS, HCM, MUTCD, etc., it's fair game on the exam. Don't rule out 12% just because your locality does not allow it.

Yeah, I think you'll find that the actual exam will provide more specifics in the problem statement and require less assumptions of you. Or, if it is a question where assumptions need to be made, they'll make the answer choices spread further apart.

Regarding your question about comfort and emax, I am not entirely sure. To 100% honest with you, I never totally understood the emax tables. My understanding of how to use them is much like the way I solved this particular problem (see post #2 above). Using one of the formulas including f, V, R, and e, solve for e. This tells me that your calculated e is the highest e that will work with your V, R, and f. In other words, your e 'max'. Thus, use the corresponding table for the e 'max' but then pick your e "ideal" based on the radius and speed. I hope that makes sense and I hope I am right!

Nope, no masonry and no earthquake in the AM, you are correct.

It sounds to me like you have this exam wrapped up. Let me give you a "pre-congrats".

 

[Predgw]

I like the pre-congrats. My goal was is to walk out of the exam and know that I probably over studied, but will not need to take it again. My first practice exam with Goswami exposed areas of weakeness, and I have since corrected those. I took another Goswami morning practice test and completed slighly early and scored above 30. Based on what I'm hearing the actual exam will not be as hard as these.

I'm saving the 2011 NCEES practice exam as my last practice the weekend before. The wild card will be the HCM and finding information if needed but I think I have a handle on it. Going through the prep with out any reference material for the new HCM was tough. In fact , I started tabbing the Guide for Planning, Design, and operation of pedestrion facilities and noticed this book makes refernce to chapters of the old HCM. But, at any rate, I think I'm in good shape.

Thanks for the help.

 

[ptatohed]

No problem. My pleasure.

PS. In post #4, I meant to say 1.5 hours up the mountain, not 1.5 miles.