Clothoid spiral
David Gibbs
I have a question regarding the clothoid spiral,in particular, the
terminology. The prompts in question are "Degree of Curvature",both
initial and final, and, "Direction of Tangent". I have asked a number of
people in various areas inclunding Bentley Systems in Australia but have
not received an answer which has been clear to me. It would be
appreciated if you could clarify this for me please, possibly including
an example.
Regards
David Gibbs
bridge branch
ph 011 61 2 96625946
fax 011 61 2 96625825
Brian Peters
David Gibbs wrote:
>
> I have a question regarding the clothoid spiral,in particular, the
David,
The phrase "Degree of Curvature" refers to the reciprocal of the radius
of a circle that matches the spiral. Clothoid spirals transition between
circles with differeing radii. Civil engineers typically want one of
these radii to be equal to inifinity. This is used, for instance, when
creating the alignment for an exit ramp from a highway. The highway is
straight, the main part of the ramp is a circle of finite radius, the
spiral provides the transition between them.
A straight line can be thought of as a circle with an infinite radius.
The problem is how to enter "infinity" in the numeric field. Well, if we
use the reciprocal, this is no problem. One over infinity equals zero.
This technique and vocabulary come from a civil engineering text book
recommended to me, _Route_Surveys_and_Design_ by Thomas F. Hickerson.
The "Direction of Tangent" is simply the direction of the curve at the
start. In the above example it would be along the straight highway. The
clothoid is represented in MicroStation using a B-spline. If you turn on
the control polygon you will see that the first leg of the control
polygon lieas along this direction. (As a matter of fact, that holds
true for all B-splines in MicroStation.)
A little experimentation, snapping to some existing points, lines, and
circles should make all this clear. I hope this answers your questions.
thanks,
bfp
---------------------------------------------------------------
Brian Peters Brian....@Bentley.com
Senior Development Engineer
Bentley Systems, Inc. http://www.bentley.com
---------------------------------------------------------------
Jim Weisgram
Brian Peters <Brian....@Bentley.COM> wrote:
>David Gibbs wrote:
>>
>> I have a question regarding the clothoid spiral,in particular, the
>> terminology....
>
>David,
>
>The phrase "Degree of Curvature" refers to the reciprocal of the radius
>of a circle that matches the spiral. Clothoid spirals transition between
>circles with differeing radii. Civil engineers typically want one of
>these radii to be equal to inifinity. This is used, for instance, when
>creating the alignment for an exit ramp from a highway. The highway is
>straight, the main part of the ramp is a circle of finite radius, the
>spiral provides the transition between them.
>
[...snip...]
>thanks,
>bfp
>---------------------------------------------------------------
>Brian Peters Brian....@Bentley.com
>Senior Development Engineer
>Bentley Systems, Inc. http://www.bentley.com
>---------------------------------------------------------------
Brian,
I am uncertain of general industry practice, but at ODOT our decision has been
to forego the "Degree of Curve" definition on metric units. Degree of Curve,
after all, is based on a 100' arc, and under metric that works out to an arc of
length, ... well, never mind.
For metric projects we have adopted the standard that we will use radius to
define all curves. Hence the built in spiral command, and the curve calculator,
are less useful to us than they could be.
Our solution has been to develop our own MDL tool for spiral curves. The
procedures are a bit different than with your tool, but they match our typical
workflow better. (Or we just have InRoads draw the silly things.)
------------------------------
All opinions expressed are mine and not my employers
Jim Weisgram
Oregon Department of Transportation