plywood lamination/bridge

Discussion in 'Materials' started by catenary, Feb 8, 2008.

  1. catenary
    Joined: Feb 2008
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    Location: west sussex

    catenary New Member


    Although I sail this thread is not about a boat, my apologies in advance if this is frowned upon. I'll get to the point...

    I'm planning to create a bridge over a river, and I want to fabricate it from ply lamination with an epoxy bonding, a technique I have seen in boatbuilding a while ago. Why ply? Well I want to achieve a minimal impact on the surrounding natural beauty, and an inverted cateneray curved blade of ply is the ultimate in bridge minimalism. Why this forum? Well I assumed there will be people out there who may offer an insight into the problems that I may encounter with laminating ply over these distances (a hull for example), and any thinking on this is most welcome, thanks in advance. Details?

    The span is 16 metres and the width only 1-1.5 metres. I'm trying to achieve at an inverted catenary curve, which will be held in place by abutments at each end. I'm thinking along the lines of marine ply laminated with overlaps and bonded together with an epoxy boatbuilding system. The final thickness of the ply blade and how many thickness/layers of ply I use is not fixed. I'm planning to use a top layer of (accredited) teak ply with non-slip rubber strips inlaid (like decking ply), and finish it with s/s yacht stantions and cables to stop people falling in.

    1. Am I bonkers?
    2. What thickness ply and how many do you reckon?
    3. Best epoxy system for ply? (no provision for baking etc, will be built on a hard landing outside, weather permitting)
    4. Should I lay up the ply inverted or right way up? I'm considering building the bridge inverted with temporary battens to keep the 2.4mX1m ply boards curved while the epoxy goes off.
    5. Will I get away without any further strengthening?

    Any ideas?

    Thanks and best wishes.

  2. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    My answers:
    1.Possibly,but you have shown the good sense to ask these questions,so possibly not.
    2.Consult a structural engineer.He will need to know what loads the structure might see.
    3.Talk to Gurit,Wessex Resins and MAS and follow their recommendations.
    4.Depends on whether you prefer lifting the ends into place while the mdidle flops down or vice versa.
    5.Depends on the load and how willing you are to expose family,friends or the public to the risk of an unexpected swim.If the public are dunked,expect to be contacted by an ambulance chasing lawyer.
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The concept sounds interesting, but some basic engineering needs to be worked out first, such as expected wind and traffic loads, etc. You may end up with a pretty thick laminate. If this is the case (suspected) then a thin box beam may be the answer.

    A 52' bridge of laminated plywood will be quite heavy, particularly when stout enough to hold anticipated loads, let alone it's own weight. The anchors (what you're calling abutments) will need to be particularly stout. The more I think about it, the more I like the idea of a box beam or sandwich construction. You can still have your "ribbon" of wood impact, but at a much higher strength to weight ratio.

    I'm also assuming that this is on privately held land (completely) and far enough away from the neighbors, that zoning officials wouldn't get wind of it. They dislike weird things and must be healthily convinced by a persuasive engineer of noted reputation, that it'll work with a substantial safety margin.
  4. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I can design you such a bridge, but that is not the most efficient way to build such a large wooden arch. I have designed similar structures (I do this kind of thing for a living), a segment of a circular or parabolic arch is fairly efficient, putting the arch is compression with little to no bending forces. Plywood has half the wood grain going in a direction that not useful, meaning you will need twice the amount of wood (and twice the weight) than if all of the wood grain goes in the primary stress direction. Using hand lay-ups of epoxy would add even more weight, and it is generally not a good way to go if exposed to sunlight and moisture for long term durability. Also with that large of a span you will be pretty high off the surface and you should have guard rails, the edge of the plywood does not offer a good attachment for a sturdy rail. If you have damp winters the surface will likely get pretty slimy unless you have some way to allow it to drain off, as they do on wood decks and boardwalks.

    The other issue with an arch it puts a very large amount of reaction loads in the foundation, which end up being buttresses. This means you will need fairly large and deep concrete footings on either side that must be designed by a competent engineer so it will not shift or sag the arch over time. In the USA there are also building code required "live loads" plus the dead load (estimated structural weight). For a residential application this would be 40 pounds per square foot (PSF) of bridge deck, or a public bridge in a retail store or public park it is more like 100 PSF. for a 4 foot wide and 52 foot span, that would be 8320 lbs total of distributed live load, plus perhaps another 4000 lbs of structure weight (it would take a small crane to place it). Not an insignificant amount of weight to hold up over a 52 foot span. There are similar building codes I am sure where you live, they are safety requirements to prevent people from building unsafe buildings and other structures in the communities that could kill or injure innocent by-standers.

    A much more cost effective way to do this would be either with two bolted wood trusses with a light deck between them, or with two glue or nail laminated arches with a bridge deck between them. The arch would be quite high which means the approach on each side up to the top would be quite steep. With two trusses or arches, you can suspend the the deck below it at a smaller, less steep arch.

    Two glue lams 4 feet apart, for instance, strong enough to meet USA building codes for residential applications would have to be about 5 inches wide by 22 inches deep (made of dough fir, and pressure treated to prevent rot). If the 4 foot wide deck was bolted to the bottom of the arches the beam (or a deigned trusses) could act as part of the side rail. It will still need a rail at about 3 feet above the deck to be considered safe. Two trusses would use much less material, but would need a lot of bolted joints (galvanized or stainless bolts and hardware are required for weather exposure). The deck material can be about 1.5 inch think planking. A solid plywood arch would have to be about 12 inches thick and 4 feet wide (a rough guess) and would take about twice the material as two glue laminated arches.

    All of the material must be pressure treated for rot resistance or be made of wood with a natural rot resistance (in the USA that would be cedar, redwood or cypress, I do not know what is common in your part of the world).

    It might be possible to have a local lamination beam manufacture build you the arches to your specs. but glu lams this big would have to be delivered by special oversized load carrying truck and craned into place. If you are intent on making it yourself, a much less expensive way would be to build two bolted wood trusses. It can have a nice graceful arch to it if you like, and you simply attach the walking deck to the lower chord of the truss, and the rest of the truss will be your hand rails. You can even build such a truss bridge "in place" with temporary support posts holding it in place until the full span is completed, and then you remove the temp posts. Since each member is relatively small and light, you would not even need a crane to build it this way. If you use all hand tools there would not even be any fuel or electricity consumed in the construction of your bridge, which would even be more "nature friendly".
  5. Pericles
    Joined: Sep 2006
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    Pericles Senior Member

  6. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Catenary, thats a terriffic post for you from Petros, thinking about it an aliminium curved "gangway" as seen at many marinas may be the go, they are similar as Petros described in are a pair truss with a deck attached between, the alu trusses form the hand rail & can have muliple staino wires for extra nautical aestetic & maybe you can have a nicely detailed clear finished kickboard/toerail to furthur represent your "blade of timber". As others mentioned dont build without pro advice on structure. All the best from Jeff.
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