I'm working on reverse-engineering some standard drawings so that I can repurpose them for nonstandard conditions. This leads me to making sure I have everything plotted correctly. This leads me to analyzing the attributes of the transitions and all that.
So, here I am, looking at a back deflection given on the standard drawing as 8d 32m 29.65s. The radius is 2890'. The length is 430.837'. The tangent is 215.818'.
When I draw up a curve to represent this, I set the back transition as follows: type: Curve; method: Deflection; radius: 2890'; deflection: 8°32'29.65".
I change the method to length. The length field displays 430.8369'. Great.
From my calculations (and confirming with an offset line), the lateral displacement of the PT should be offset 32.05483' from the back tangent.
When I change the method to offset, the offset field displays 16.4156'. What does this 16.4156' represent? If I manually set it to 32.05483' as it should be, I get a much wider curve, as you might expect.
Hello Derek,
I can take a look into this if you'd like to send me your data to the link below:
https://bentley.sharefile.com/r-r288652c57ea48618
For more information about the Road and Site design tools, visit the Road and Site design WIKI at:
http://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki
I just uploaded it.
In the file, you'll see some baselines on WORKING_LV1.
On CONST_LV1 are the fillet between them and the relevant offsets. The fillet is set up exactly as described in the original post. Again, it reports the offset as 16.4156'. According to the further offset, I would expect the transition offset rule to report 44.0548' or 32.0548'.
On CONST_DCH are two offsets of 16.4156' from different elements that could conceivably be relevant. Neither one has any validity that I can see.
For the record, I'm using Geopak .878.
By changing the "Back Transition" = Curve, you are now telling the program to construct a second curve to act as a transition between the back tangent and curve. Is this what you are intending to do? Sounds like you are just wanting to construct a curve with radius = 2890' and delta (sweep angle) = 8d 32m 29.65s.
I place a curve using the Arc Between points command with those two inputs and measured the offset to the PT and did get 32.0548', so it sounds like the confusion here is adding a second curve as a transition.
Yes, I want to construct a curve with the radius and delta you're observing. But more than that, I want to position it in such a way that a 1410' curve will transition from there to an arbitrary baseline. See, the 1410' curve is the primary curve here, and the 2890' curve is transitional.
For the record, with the inputs we're looking at, the result of Arc Between Points and the result of Fillet with back transition should be identical with one another. Indeed, that's what I'm seeing on my own end.
And this is probably what you're seeing too, now that I see your specific wording "and measured the offset to the PT and did get 32.0548'". My concern is in the value of the Offset field--how it doesn't match the PCC's offset from the back tangent.
Please supply the offset 32.0548' that we're looking at here, and let me know if the transitional curve changes its delta. If this field is available for us to supply a design value, we ought to know what it's even doing with that value.
Let me know if this explanation helps... The offset value stored on the back transition is not the measure of the distance from the PCC to the tangent, but is a measure of how much we are offsetting (or "shifting") the primary curve away from its back tangent to allow room for the transition. For example, without any transition specified, the curve will become tangent to the back tangent at an offset of zero (or an offset of 12’ in your data). Since we desire to have a transition entering the curve, we need to add an offset to move (or shift) the curve away from the back tangent in order to provide room to build the transition. The larger the offset, the longer the transition. So the Offset stored on the Back Transition is basically the offset distance that the primary curve would become tangent with the specified back tangent (if there were no transition).
Are you saying that you have a known value (32.0548’) for the distance from the back tangent to the PCC?
>Are you saying that you have a known value (32.0548’) for the distance from the back tangent to the PCC?
Yes. Exactly. As you know, I can specify the curve using the delta or length too, of course.
>The offset value stored on the back transition ... is a measure of how much we are offsetting (or "shifting") the primary curve away from its back tangent to allow room for the transition.
So if we continue with my previous inputs, the primary curve has a radius of 1410'. When I specify an Offset of 30', that 1410' curve's back tangent is 30' off the transition's back tangent. So the center of the curve is 1410' + 30' off the transition's back tangent. Huh. Okay, you've made sense of that.
Might I suggest an improvement by labeling that field more clearly. I'm not necessarily sure how to do that, but maybe "Primary curve offset from back tangent" (as clunky as that is). And also maybe add another field for offset of primary curve's PC.
Because I associated it with the offset at the primary curve's PC. It may be for multiple reasons, but foremost in mind is that when I set a back taper, Offset is the offset of the PC from what would have been the back tangent. The center of the curve is not translated 30'. Clearly Offset means different things to different people.