Spiral Geometry

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Spiral Geometry


I have a question and a possible bug (or I'm just inputting things wrong). I am trying to input a spiral curve as the horizontal alignment. The initial direction is S 77 15 43 E and the final direction is S 86 12 52 E. The program crashes every time I try to input the final direction, so I am putting in the Delta Angle. The PI of the entire spiral is 841+24.24 and the radius is 400 ft with entry and exit spirals of 125 ft. The central angle of the entire spiral is 50 05 04 L. When I input all this data, the program shows an exit direction of S 52 39 13 E. As stated above, this direction should be S 86 12 52 E. I'm not sure what I am doing wrong. Should I be inputting the central angle and PI of just the circular portion? Is there a way to tell where the P(S)T is in the program? It doesn't appear, graphically, that this is in the right location either. Also, as I stated, every time I've tried to input the Forward Tangent Direction, the program boots me out. I'm not sure if this is a bug or it's confused because it's not the direction the program calculates (i.e. user error).

Any direction you could provide would be greatly appreciated.


Rick Brice
I'm running the geometry you

I'm running the geometry you have described through PGSuper in a debugger and it is indicating the that entry and exit spirals overlap. Can you please send a sketch of the curve you are trying to model? This will help me determine if it is an input issue or an issue in the program.


Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office

Rick, I started to send you


I started to send you the info, and I think I may have figured it out. I have two sets of data for the curve - one is for the entire curve and doesn't include the in/out bearings and the second is curve by curve data and does include that info. I think I am using the Tangent Direction after the circular curve (at the CS) as the initial tangent, rather than the tangent direction into the the spiral (at the TS).

That fixed it. Amazing how you just telling me to sketch it clarified the whole thing! Thanks so much for your quick response! I really do appreciate it!


Jennifer L. Whiteside
RTD FasTracks

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