Advice for an Arched Bridge

[Barron Shaw]

Inspired by the cedarworks website, I've spent all summer building my kids a cedar swingset with fancy joinery, a copper cupola, and lots of extras. The "fort" is next to a small hill, and I'm thinking about building an arched bridge into the fort from the hill.

I haven't done any laminate jobs before but this looks like fun. Has anyone ever built an arched bridge using a bent wood laminate technique, and if so, do you have any advice? I'm guessing that it would be stronger in an arch than flat, especially with all the glue. Moreover, once the arch was built, I could use Trex (no expansion) for the floor and insert material between each step to form a very solid arch (like they used to build stone bridges).

It would be about 10ft long; no more than 32" wide. I'm planning to use a plastic resin glue like David Marks uses on his show.

Do laminates work well outside in the elements?

Thanks.

[Mark Ball]

I am going to build this bridge. http://www.woodstore.net/argarfoot.html

It features laminated beams from 5/4" decking. I need a few more clamps, though.

[Peter Quinn]

Do you have a long wall in your shop, say 11' for a 10' arch? You can fix a few sheets of plywood (CDX or something cheap) to the wall and make L brackets (with a plywood brace to stiffen the L) screwed to the plywood on the wall to act as both a form and a clamping surface. Kind of tough to build forms from MDF like DJ Marks, and not really neccessary for what you are doing either. We do this at work all the time to make arched jamb heads.

Plastic resin glue is water resistant but not water proof. Resorcinol is the water proof cousin used in wet or marine applications. Not sure which is appropriate for your application. I'm also not sure if Plastic resin glue is suitable for structural applications like yours or not. Resorcinol is, pretty sure PL glue is too, as are many epoxies.

[Barron Shaw]

Thanks for the replies guys.

Mark, do your plans state which kind of glue should be used? I like the look of that bridge and the copper spindles would match my cupola well, and may be less expensive than ripping up some more cedar.

[Jamie Buxton]

I haven't built a bridge with bent lamination, but I have built outside furniture which used it. I used western red cedar and Unibond 800. Unibond 800 is a ureaformaldehyde glue, and is waterproof. I've understood that David Marks' "plastic resin glue" is a ureaformaldehyde too. The furniture is still in use, some ten years later.

I was pretty careful to avoid places where water can pool. For instance, I wouldn't drive mortises into the beam for the lower end of balusters. I'd be concerned that water would get into the mortise and stay there, eventually leading to rot. I'd look to the design of fences. There, the balusters secure sideways against a rail. The water drips out of there.

[Mark Ball]

I believe they used titebond III, I have the magazine with that article at home, I could check if you want. The plans are really nice, with pictures of the lamination in progress. Worth a thousand words!!

I don't think the copper would be less expensive than cedar, though. At least not around here.

[Barron Shaw]

Well I hope Titebond III is good stuff - that is what I used for all my mortises and biscuit work. Pics are here...

http://picasaweb.google.com/BarronShaw/2008SwingSet#