Determining Vertical Curve Length???

[Lndsn]

My basic question is: What determines the curve length that you need to use for two intersection tangents. Also does wheel base of the design vehicle relate at all to this?

Here is my problem in more detail:
I am designing and entrance to a parking lot at a park. The entrance comes off of a state highway and has an acceleration and deceleration lane. It is a pretty steep entrance unfortunately but our only option. My tangents coming in go from 3%(cross-slope of the accel/decel lanes) to a 12.5% then back to 8 then 5 then 2 (drive goes around a curve then to handicapped accessible parking area). I currently have and unequal tangent curve at the entrance because I only have about 15' max before I have to drop to a 12.5% tangent from the 3%. I am concerned that the curve length is two short. The drive will also be the construction entrance that I am going to need to get cement trucks and other machinery down(and garbage trucks upon project completion). How can I ensure that the drive is accessible to these vehicles? Redesign of the horizontal alignment & layout is not an option.

ANY help woudl be much appreciated!!!!!

 

[Lndsn]

Some additional info: I basically need to drop 18 feet in 200 ft. the last 80 or 90 feet are also a 90 degree turn so I want to make that a bit shallower. the way I have it now I have 13'2" of 3% off of the edge of the accel/decel lane and 36'10" of 12.5%. So my total curve length (unequal tangent) is 50 feet. Would it be better to make it more equal and make the secont tangent steeper?

 

[francesca]

Vertical curve lengths are determined by sight distance. L = KA where A is the algebraic difference in grades and K is a constant determined by the design speed and ultimately sight distance.

Guidelines for vertical curves are pretty clear in the Green Book, and which ever authority will be taking ownership of the road will have their own set of regulations. Usually maximum grades in very low speed subdivision roads are 10% or 12%, with variances sometimes allowed up to 15%, however steep grades are discouraged with horizontal curves. If you have an intersection in there, then typical maximum grades for very low speed roads are 5% and the subdivision I'm working on now it is 2% for 100'.

 

[Maury]

The Green Book is "Geometric Design of Streets and Highways", published by AASHTO. They also have a publication "Geometric Design of Very Low-Volume Local Roads (ADT < 400)". The State highway department shoul have geometric design guidelines you could use, but make sure they are for local roads, not state highways.

 

[TerryScan]

Lndsn

The Green Book addresses vertical curves in terms of design speed and sight distance while traveling along the curve. This does not really apply at a site entrance, since there is usually a stop condition leaving, and a low speed entering.

While having a transition that is as smooth as possible is desirable, I believe in the situation described, ground clearance is the main design factor.

I suggest you plot the profiles of the roads...then create a block of your design vehicle and check clearances on the profile.

Some places do have guidance on maximum grade differences and distances at entrances, so check entrance guidelines.

 

[Lndsn]

I'm sorry I am pretty new to this. What is the Green Book?

Also, TerryScan - you are right, because of the accel/ decel lanes, I'm not very concerned about sight distances. I just want to ensure easy of access. And I don't want any vehicles bottoming-out. I have done what you suggested with the profile, but cannot find any data on ground clearance for certain vehicles only wheel base. Any suggestions?

Also, is there anywhat to get plain old AutoCAD to produce a parabola through specific points? The best that I can think of is to use spline and the adjust it so the actual line goes through my points offset from my tangent.

Thanks!!

 

[civilperson]

You have over specified the problem which makes a solution unfindable. Either give yourself more distance for the vertical curve or limit the access to short whellbase vehicles. A graphical solution of the limits of wheelbases able to access your drive is a easily developed when a section of the 15" VC goes from -3% to -12.5% is shown. Make a template of truck wheelbase with 11" clear at the center between axles and place on VC.

 

[TerryScan]

Lndsn,

I recently did a similar analysis. I thought of a vehicle that I have know to have issues with ground clearance - the corvette. I found specs including ground clearance using a web search (ended up at

http://www.caranddriver.com).

To draw the VC, I use a civil add-on to Autocad (Carlson Civil 2007 - a Land Desktop alternative).

One could calculate points every 5-10' along the VC and connect them..or find another add-on that will draw it.

 

[CarlB]

For drawing vertical curves (parabolas) in AutoCAD I found the following tip which appears to be correct:

"Vertical curves are parabolas, you can draw them by drawing a polyline from endpoint to VPI to endpoint, then PEDIT>>Spline. For it to be a proper parabola, set SPLINETYPE to 5 before pediting, and to get a smooth curve set SPLINESEGS to 15 or higher. (Thanks to rc McSwain for the Spline/parobola info)."