CE 552 Problem Analysis - Due Thursday, May 2, 2002
Court Trial - Case of the bypass relocation.
Recall tort case: redesigned detour ; speed advisory of 25 mph;
20 degree curve, (Radius = 285 feet )
Design superelevation = 0.06; Actual superelevation = -0.04
From basic principles of dynamics, the centripetal force (mV2/R) along the roadway plane must be balanced by the component of Weight acting down the plane, plus the sliding side friction acting down the plane in the opposite direction of the centripetal force.
One of the CE 552 class members posed the question, "could the car have traversed safely, if the superelevation were -0.04 with a speed of 25 mph?". Let's take you to a trial deposition and check your ability to show the basics in the analysis. If you can demonstrate basic skills here, you are likely to have an easier time when in the court. Juries don't like to see too much badgering of expert witnesses.
1) A lawyer, taking your deposition, may ask you to provide the AASHTO data for minimum radius of curvature that should be used for a rural route situation in which the design speed is 25 mph and a 0.06 superelevation is to be provided. (She may even be so kind as to show you a Table from AASHTO so you can read it from the table, but at the deposition stage this is less likely than in the courtroom).
Your response would be ______(Also, for me, provide Reference and page number).
2) A second question - At that 25 mph speed, and with a superelevation of 0.06 foot per foot, what is the corresponding value of this term you call the "side friction factor", f. (of course she may first want you to explain what this side friction factor is and why this is different than the sliding friction factor we use in many other places when considering stopping distance - we'll skip those questions here)
Your response would be ______(Also provide Reference and page number).
3) A third question would be - Well, now if the contractor does not put a +0.06 superelevation on this curve, but uses a -0.04 instead, how large must this side friction be in order for my client's vehicle not to have slid off the roadway, even if by some miracle he was able to reduce his speed to 25 mph on this road which was generally signed as a 55 mph route.
Now you are in the dilemma zone. The Table doesn't provide any assistance. Negative superelevation isn't shown in the Table. Fortunately your engineering education kicks in and you say I can use the figure on page 1 to develop the basic relationship between V, R, e and f, then I can give you the answer.
On a separate sheet, show this development. [If you can't get the same relationship that is found in the AASHTO policies, say so and use the equation that is in AASHTO.]
Your response would be: Under that scenario the vehicle must be able to generate side friction equivalent to a factor of f = _____
4) Well then, Expert witness, what is the maximum f that AASHTO would recommend for a design speed of 25 mph? [I'll help with this one - See Exhibit 3-13 in 2001 AASHTO green book; f drops 0.01 per 10 mph from 0.17 at 20 mph to approximately 0.12 at 70 mph]
5) Have any studies ever reported that the f value you just calculated could be attained at 25 mph? [Answer is in AASHTO design book]
6) In your opinion, what was the greater contributor to the loss of control of the vehicles involved in the accidents? (careless drivers, failure to give adequate notice, adverse crown on the roadway, …)
Deposition is done: Question from Kannel: What strategies would you have recommended to have reduced potential hazards on the road. (beyond changing the location of the construction zone signs.