Spiral Curves Blah
I've got a project where a farmer owns 80 acres and a state highway crosses the parcel; the right of way is an easement. He wants to split the parcel in such a way that the highway centerline is the new boundary. As luck would have it the highway goes into a long, sweeping curve right in the middle of his parcel and it has a 220' spiral curve that I need to deal with.
My crew was able to find a bunch of ROW monuments and they fit the record well so I have the tangent and the horizontal curve nailed down no problem. I'm at a bit of a loss as how to handle the spiral curve though. When I've worked with horizontal curves in the past I get the tangents set and then list my measured chord and the record curve data between the PC and PT. How would you do the equivalent of this with a spiral?
If the plans give you all of the information that you need, you should easily be able to calculate the spiral. I have come across this situation several times in NJDOT highway situations and NJ is good at setting up the monuments and giving you all the info that you need with baseline stationing and offsets, as well as the curve data.
Right, but I guess my question is what element of a spiral curve is typically held?
I found a handy resource that I'm reading through at the moment and I think it'll get me going the right direction, but I'm always curious to hear how the vets have handled them too. I think the last time I ran into a spiral curve at work was 3 years ago and then 4 years before that... so just long enough to forget everything I knew about them. 🙄😆
The elements most important are the length, radius, and delta that are given with a solid check in being the baseline station and offset. It took me hours sitting with my lead tech to explain the whole thing with him, and, at the end of the day, the light bulb went off in his head. Will he remember it three or four years down the line? I think so because we work along numerous county roads that use the DOT format, and they have lots of jug handles.
How does a spiral curve have a radius with it being a parabola? The length and delta make sense, but my measured values won't match those exactly since I'm holding found monuments. It seems like what I need is a way to proportion the record degree of curve to fit my measured spiral delta?
I've never had the nerve to do it, but it might be more realistic to calculate the R/W plan spiral (the one time it's great to work at an engineering firm) then calculate a spiral that'll fit the found monuments and show both or, more likely, represent it with an obnoxiously thick lineweight. I tend to favor the position that monuments hold over the intangible centerline, but I'm aware of instances where the opposite view prevailed.
then calculate a spiral that’ll fit the found monuments and show both
That's basically what I was planning to do, show record data and measured spiral + data. I'm not quite sure how to come up with the measured spiral at the moment though.
The given radius of a spiral is equal to the radius of the of full curve at one end and infinity at the tangent end. The spiral equation then creates curve (not a parabola) of varying radius beginning at infinity and ending tangent tot the full curve.
They are pretty easy to establish in 3CD but take a bit to do the math on paper. I normally hold the tangents, the radius (or degree of curve) of the full curve and the length of spiral.
Dealt with spirals so much in AK. I would give the length of the spirals, sometimes not equal, the overall spiral tangent, the "a" value (rate of change, be careful, Civil3D gives some "A" value that is not the same thing, should be a number <1) and I'd also show the chord of the spirals on the centerline and the sides if the R/W needs to be defined, since a spiral can't be offset, but it sounds like you are just dealing with a defined centerline and just showing a width, and I would include the usual circular curve data
Since a spiral can't be offset I'm curious how others would define the R/W, on US Surveys BLM would define with equally divided short chords. I'd considered, but didn't get the opportunity to try defining a separate spiral for the sides of the R/W.
Hope this helps.
I normally hold the tangents, the radius (or degree of curve) of the full curve and the length of spiral.
I'm using Carlson + autocad so maybe you wouldn't be familiar with the process, but any idea if there's a way to hold the TS and the SC and have the software calculate a degree of curve and a spiral length based on equal chords between these points?
