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construction of constant radius spline?

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  • Member since
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  • From: Maryland
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Posted by ATLANTIC CENTRAL on Tuesday, May 10, 2022 8:01 AM

I think you are over thinking this. You need some sort of cross support everywhere to hold the longerons in place and parallel, so longer ones, and more of them, under the loop should support the foam just fine. That assumes reasonable sizing for these supports.

I think you are expecting the spline to require less support than it will require to be stable. Curved, straight, loop, whatever, I would expect the spline to need support every 2 feet more or less. And the relatively sharp curve you are planing will require more, you cannot expect to cantilever that curve very much, it will twist and sag.

Sheldon

    

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Posted by gregc on Tuesday, May 10, 2022 12:08 PM

ATLANTIC CENTRAL
You need some sort of cross support everywhere to hold the longerons in place and parallel

yes some cross pieces may be needed between the longerons to hold them parallel (they can also support the foam)

ATLANTIC CENTRAL
I would expect the spline to need support every 2 feet more or less

that's why i plan to add the cross pieces (blue)

greg - Philadelphia & Reading / Reading

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Posted by Doughless on Tuesday, May 10, 2022 12:18 PM

gregc

 

 
Doughless
Maybe this has been addressed, but how do you support the middle of a curved spline?  Wouldn't you still need some sort of riser near the middle?

 

i'm planning on the 1" thick foam and the spline sitting on the "girder" (red).   there are no cross supports

as i said, the ends of the spline will need supports where they meet the foam.   fascia shown in brown, track in orange

because the sides of the loop are unsupported, i can add cross pieces (blue) that are attached to the tops of the girders and attach to the inside of the spline

yes, the foam inside the loop will need to be cut to make room for the cross pieces

2225

 

I understand better.

I agree with Brent in that lap cuts in the blue/orange supports would keep things on the same plane.

IMO, the advantages of spline (a laminated beam really) is to make an extremely narrow yet rigid subroadbed for instances where you can't have or don't want a lot of support structure underneath.

Like a laminated beam home remodelers use to open up a space by eliminating the vertical supports for the ceiling.  

You can do the same thing with traditional benchwork, as long as the horizontal supports are robust enough as to not sag towards the edge.  

As depicted, the blue/orange grid structure looks very close to a traditional horizontal support structure for sheet plywood subroadbed. where there would be four legs in the middle of the peninsula where the blue/ornage intersect.

Is the issue about avoiding sag from the middle supports to the edges, more so than scenery, and wanting to keep the entire structure light enough to discourage the sag?

- Douglas

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Posted by gregc on Tuesday, May 10, 2022 3:05 PM

Doughless
I agree with Brent in that lap cuts in the blue/orange supports would keep things on the same plane.

would that be less work and significantly better structurally than just attaching the cross piece to the side of the spline?

Doughless
Like a laminated beam home remodelers use to open up a space by eliminating the vertical supports for the ceiling

i'm familiar with light-weight wing spar design and laminated beams when i built a garage

Doughless
As depicted, the blue/orange grid structure looks very close to a traditional horizontal support structure for sheet plywood subroadbed. where there would be four legs in the middle of the peninsula where the blue/ornage intersect.

this makes me wonder if such a lightweight structure would easily tip and if additional legs are needed farther out?

Doughless
Is the issue about avoiding sag from the middle supports to the edges, more so than scenery, and wanting to keep the entire structure light enough to discourage the sag?

not concerned about scenery which i believe shouldn't be a problem.   

agree with Sheldon that the spline needs to be supported and now wonder if additional support is needed when someone leans on the outer edge

 

 

greg - Philadelphia & Reading / Reading

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Posted by riogrande5761 on Tuesday, May 10, 2022 5:11 PM

Don't forget to have a straight section between the curve and the reverse curve!

Rio Grande.  The Action Road  - Focus 1977-1983

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  • From: Heart of Georgia
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Posted by Doughless on Tuesday, May 10, 2022 5:37 PM

gregc

 

 
Doughless
I agree with Brent in that lap cuts in the blue/orange supports would keep things on the same plane.

 

would that be less work and significantly better structurally than just attaching the cross piece to the side of the spline?

 

 

 
Doughless
Like a laminated beam home remodelers use to open up a space by eliminating the vertical supports for the ceiling

 

i'm familiar with light-weight wing spar design and laminated beams when i built a garage

 

 

 
Doughless
As depicted, the blue/orange grid structure looks very close to a traditional horizontal support structure for sheet plywood subroadbed. where there would be four legs in the middle of the peninsula where the blue/ornage intersect.

 

this makes me wonder if such a lightweight structure would easily tip and if additional legs are needed farther out?

 

 

 
Doughless
Is the issue about avoiding sag from the middle supports to the edges, more so than scenery, and wanting to keep the entire structure light enough to discourage the sag?

 

not concerned about scenery which i believe shouldn't be a problem.   

 

agree with Sheldon that the spline needs to be supported and now wonder if additional support is needed when someone leans on the outer edge

 

 

 

I don't want to get into a work/value or ethic type of topic...what might be more work to some might not be to others.  Its hard to say if either method would cause materially more work than the other.

I think the tipsy issue can be avoided if properly balanced, but any type of method is going to run the risk of tipping or sagging if leaned on or extra weight placed on one side, other than the traditional multiple vertical support method...primarily along the perimeter.

I had the same discussion with Randy Rinker in his build thread, where he was wrapping an upper deck around an existing stick built basement support wall.  He was fretting over what types of brackets to use to secure to each side of the wall and the clearance to the upper deck.  I suggested to just screw 4 foot long 2x4s horizontally into each stud of the wall to form sort of a series of Christian crosses 16 inches on center from each other (typical load bearing wall construction).  As long as the 2x4s were securely anchored to the studs (three or four screws), and the weight placed on either side of the deck was similar, it should be very sturdy to support trains and scenery, with plenty of clearance for the lower deck.  Even crowning the 2xs properly (in this case upside down) would eliminate any serious sagging over time.  Of course, the wall was attached to the ceiling so it kept the entire series of T crosses from tipping.

The upper deck turnback loop being a different challenge.

IMO, it still comes down to spline (like Lam beams or I beams) and cantilevering being the chosen method if you want to limit the amount of vertical supports.  There may be reasons to do that.

- Douglas

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Posted by gregc on Tuesday, May 10, 2022 7:12 PM

riogrande5761
Don't forget to have a straight section between the curve and the reverse curve!

greg - Philadelphia & Reading / Reading

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Posted by gregc on Wednesday, July 13, 2022 7:32 PM

how much length of spline do you glue at a time?

how far part do you typically clamp?

i'm working to get the first two layers glued together along the radius i want.   i assume after that, 2 layers can be added at a time to the outside of the existing spline.

greg - Philadelphia & Reading / Reading

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Posted by BATMAN on Wednesday, July 13, 2022 9:17 PM

Greg, I would glue about 2' and clamp being careful to make sure it was dead even with the other peace. Using a glue gun it set quickly and I proceeded to glue the rest. I would probably glue a couple of feet clamp and keep going. Once the first two strips are together nothing is going to change. If you get the first 2' lined up a set the rest will line up automatically. 

 

Fifteen years later it is still solid without any signs of delaminating. Remember to put a screw through every couple of feet in alternating directions. Drill a pilot hole or you will blow up the spline.

 

Brent

It's not the age honey, it's the mileage.

https://www.youtube.com/user/BATTRAIN1/videos 

You can never ever out-train poor nutrition.

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