construction of constant radius spline?

[ATLANTIC_CENTRAL]

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

[gregc]

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)

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)

[Doughless]

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/orna

[gregc]

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?

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

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?

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

[riogrande5761]

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

[Doughless]

[quote user=“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.&

[gregc]

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

[gregc]

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.

[BATMAN]

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.