Curved baseboards

[Alan Turner]

Missing from all the discussion so far is how the diagonal bracing works. Most of the examples shown in photos look like they are expected to withstand compression loads. In contrast, something under tension could be extremely light - bicycle spoke for example.


- Nigel

The bracing works in Torsion. It's there to add to the torsional stiffness of the baseboard.

regards

Alan

[grovenor-2685]

Missing from all the discussion so far is how the diagonal bracing works. Most of the examples shown in photos look like they are expected to withstand compression loads. In contrast, something under tension could be extremely light - bicycle spoke for example.
- Nigel

The diagonal acts to prevent the rectangle from becoming a lozenge, a lozenge has one long diagonal and one short diagonal. the bracing having a fixed length the lozenging action is prevented. With just one diagonal it may have to act in tension or compression depending on the forces applied to the frame. If you fit two diagonals, ie a crossbrace then one of them would always be in tension so you could use bicycle spokes.

[bobwallison]

And out of interest what are the effects of creatng a box girder from ply and thin strips of solid timber top and botton rather than an I gider?

Our basebords will of course have beams on both sides with either a solid top surface spanning the space between the two, or just the track bed supported on cross beams. What effect does that have on the stiffness of the whole structure?

"Stiffness" for various shapes attached. Ixx relates to vertical load and deflection, Iyy relates to horizontal effects and J relates to the twist. For a given load and span, doubling "I" halves the deflection, doubling J halves the angle of twist, and so on. The huge increase in "J" for the closed box is not an error - they really are very much stiffer in torsion than thin, open cross sections of similar size.

A track bed that is not fixed firmly to the side beams contributes virtually nothing to the vertical or torsional stiffness of the assembly (Ixx and J are very low). If it was 600mm wide rather than 200mm as tabulated, Ixx and J would be increased by a factor of 3 - so the contribution is still negligible. But if a trackbed were fixed to the top edge of the side beams there are two significant benefits - each side beam becomes an inverted "L" beam which increases Ixx, and the horizontal leg will prevent the vertical leg from buckling sideways, allowing thinner sections to be used. (Linn Westcott discovered this half a century ago.) I am fairly sure this is how the laser cut boards manage to get away with main beams just 6mm wide spanning 1200mm or more.

Bob

Section properties 001.pdf

[Terry Bendall]

Thanks for the information Bob. Interesting outcomes.

Terry Bendall

[CDGFife]

Late to this interesting thread as just back from holiday. I see Nigel has already mentioned the Cadhay boards.

Here's a pic of the first of the recent two curved boards before landform and scenics. Trackbed radius is 1215mm.

Curved Board.jpg

As you can see I'm with Stephen and Bob on the merits of box section (maybe it's the civil engineer in me!). Even without the trackbed shown in this photo this board is totally rigid, weighs less than 5kg and can support my weight when spanning it's length. Mostly made from 4mm Laserply but with ends of 9mm. From the kit of parts that comes off the laser cutter it took about 15 minutes to assemble and glue.

As usual with railway modelling there are many different ways to skin the cat and I agree entirely with Terry's reference to rule 1. I went down this route to try to produce lightweight strong exhibition boards that allowed my track bed to sit in the landscape on a mixture of embankments and cuttings rather than on a flat top. All Cadhay's boards are made this way but now I have the big laser cutter I can produce kits to almost any reasonable size and shape. I will have a range of kits available soon from 300mm wide to 750mm wide, curved and straight.

Cheers

CDG

[bobwallison]

Here's a pic of the first of the recent two curved boards before landform and scenics. Trackbed radius is 1215mm.

CDG

Tempting, very tempting. It looks like a sound piece of aeronautical engineering - entirely appropriate for a portable layout. And elegant with it.

Bob

[allanferguson]

If I had produced a work of art like that I don't think I'd be testing it with my weight -- or even yours!
How are the pieces fastened together -- ordinary woodglue?

Allan F

[philip-griffiths]

The Kendon Viaduct curved baseboards I have are based on a curved I section beam. Two ply boards were cut then routed with a channel and another board put into the channel. These are attached to Barry Norman style ends to butt up the ends to each other.

On holiday otherwise I’d be able to dig up photos.

[CDGFife]

If I had produced a work of art like that I don't think I'd be testing it with my weight -- or even yours!
How are the pieces fastened together -- ordinary woodglue?

Allan F

Allan - yes Screwfix No Nonsense PVA in this case! (other PVA is available).

CDG