Spirals - again

[rade_tomovic]

Need to create alignment with 3 arcs and spirals between them. But, condition is not to move circular arcs and to have specific lenght of spiral curves. I've attached something like my problem. Ofc, radii of circular arcs also have specific values. I can also use tangents (not drawn in the attachment) if that is easier. Final solution need to be curve - reverse spiral - curve - reverse spiral - curve

[sboon]

Seems simple enough.  Three point curves for the arcs, then free Reverse S-L-S between them.

Steve
Please use the Accept as Solution or Kudo buttons when appropriate

[rade_tomovic]

Yes, but I don't need any lines between curves, only spirals

[sboon]

---

@wabafet wrote:

I can also use tangents (not drawn in the attachment) if that is easier.

---

OK - I read the original post to assume that I could use a tangent line floating between the spirals of specified length.  If you want to just use spirals then could use a free reverse spiral spiral component.  The issue with this is that the only way to control the length of the spirals is to play with the A1/A2 ratio in the alignment grid view.  You probably will not be able to get both spirals to the exact length that you're looking for.

Steve
Please use the Accept as Solution or Kudo buttons when appropriate

[onsionsi1]

Can you give me the lengths of spirals in order to know what you want exactly.

Thank you.

[rade_tomovic]

I will attach exactly what I need. 3 arcs in the dwg must, after inserting spirals, be the same as before according to center point and radius, but they lenght can be different. What I need is to insert reverse spiral between right arc and middle arc with A value of 160 and also reverse spiral between middle arc and left arc with A value od 180. Type of spiral is clothoid and I need A1=A2 (lenght of spirals can be different). I know that R*L=A^2, so it's ok to me to find solution with input spirals by they lenghts. Tnx

[onsionsi1]

But let me tell you that if you need these criteria the middle curve must be floating in order to get A value as you mentioned.

[rade_tomovic]

OK, 1st and 3rd can be fix, middle floating. Another solution is to have first fixed, other two floating, right?

[onsionsi1]

Let me know where you are working now because these criteria i didn't see like it before.

And i'm trying to find a solution to you.

[rade_tomovic]

I'm not working anywhere, just learning Civil, I'm a student.

[sboon · Accepted solution]

It isn't possible to connect these curves using the specified geometry and constraints.  I added spirals to all three curves using the specified A values and as you can see the spirals would overlap.  The only way to correct this is to move the middle curve so that it fits between the required spiral lengths.

Check the attached drawing for a second alignment that may be a viable solution.

Steve
Please use the Accept as Solution or Kudo buttons when appropriate

[rade_tomovic]

Ok, that is what I have to know - can't have both spirals and geometry specified, something must translate/rotate to meet conditions. Thanks 

[Pointdump]

Steve,

I'm glad it wasn't just me that couldn't figure out how to make it work without moving the center curve. But is there a better way to find out without a guessing game?

• 100'--No Solution
• 50'--No Solution
• 20'--No Solution
• 150'--No Solution

Do you just have to develop a feel for things, or is there a systematic approach?

Dave

[onsionsi1]

Dear Mr.Steve,

Let this question to be more difficult if i have the same case but the middle curve is more than 180 degree like a loop on interchange so can i get a specific A value for this case like folowing example:

First curve 2500m radius

second reverse curve 90m radius

third curve 2500m radius

and as you konw the radius 2500 don't have a spiral so it will be A=1 as the length will be almost zero length

but the radius 90m will have a spiral like A=30

For your advice

Thanks in advance.