How to design a reverse curve on a curve

[Will007]

I am designing a reverse curve on a curve with superelevation.

Does anyone have an equation or an idea of how to design this manually or on autocad?

Appreciate any info you can give.

Thanks

 

[civilperson]

The superelevation must runout to flat at the tangent section between the curves and then reverse to accomodate the reverse curve. Some DOTs use centerline as profile grade and raise/lower the edges, others use an edge for profile grade line. Spiral curves or a wider lane section during the curves makes a more driveable highway.

 

[eea]

I think you'll better off rough it out on paper / CAD. I’m still not clear what you creating, a reverse curve from a single existing curve or introducing an "S" into a larger, single curve? Post a sketch. You might be better slopping in a spiral.

As far as super-elevation between the reverse curves, I've always set 0% cross slope at the PCC (point of compound curvature) and transitioned to full super using 50% on and 50% off for the transition lengths. AASHTO suggests super transitions of 1/3 on and 2/3 off, but will allow 50/50. I do not believe a tangent segment is required if you are not going back to normal crown. You are dialing from full SE through 0% and back to full, i.e. -8% to 0% to 8%.

Ideally the reverse curves are the same radii and the same super. Run outs on either end (PC and PT) should get the normal 1/3 - 2/3 transitions.

 

[Will007]

Thanks eea.

Because of the Reverse Curve (RC) being on a curve, the two radii will not be the same. Are you saying the allowable ratio if not symmetrical is 1/3 - 2/3?

Or can the 2 curves on the RC be whatever radii that'll make it work?

Please advise.

Thanks

 

[lbassler]

Will:

The 1/3 2/3 is not a raduis ratio, it is how/where to locate the superelevation transition with respect to the PC or PT of a curve.

Eea is saying normally the superelevation transition is 1/3 in the curve and 2/3 outside the curve (usually on a tangent).

(eea also meant PRC - Point of Reverse Curvature, not PCC)

Because reverse curves don't have an adjacent tangent, anything that is outside one curve is inside the other. So splitting the transition 50/50 is about the best you can do.

To draw reverse curves in Autocad, Fillet Radius is a good tool in many cases, although it is not very scientific.