What timber for bridge supports?

[Badgerland]

(Not sure if this is the right forum for this one, so if not can mod please move it to it's rightful place?)

 

I've been asked to quote for building and installing a bridge across a smallish pond to link the bank with an island. Only ones I've done before are relatively short (single) span, around 14–15 feet, so haven't needed a support in the centre. This one is around 30 feet, so will need a central support of some kind. The pond is clay lined but the owner doesn't know how deep the clay goes, so isn't keen on driving pilings in. I thought if I had the supports sitting on a flat base, akin to an inverted pi-shape, this should spread the weight and not sink into the base too much. In this part of the pond the water's only around 2-3 feet deep. (Or at least that's what he's said. Not got the chest-waders out yet!) Not sure how firm the base is, but it ought to be pretty good.

 

What do people think?

1. will this approach work? can you suggest a better solution?

2. would it require more than 1 pair of supports?

3. what would be the best wood to use, both for the supports and also the bridge itself?

 

Thoughts and suggestions welcome.

[Goaty]

For your sleeper at the bottom alder (alnus glutinosa) Is said to last indefinetly under water. Its used in piers etc. However I think the problem is where the water level is and you get air and moisture. Use a wide enough piece of 2 attached together I reckon it wont sink in that much.

[agg221]

For your sleeper at the bottom alder (alnus glutinosa) Is said to last indefinetly under water. Its used in piers etc. However I think the problem is where the water level is and you get air and moisture. Use a wide enough piece of 2 attached together I reckon it wont sink in that much.

 

Agreed. Elm is also good underwater, as is pitch pine.

 

The best at the waterline, ie your posts, would be greenheart or purpleheart. Rob D had some greenheart - not sure if he has any left though. British Waterways use both for lock gates, so I would start with them and find out if you can get hold of some reclaimed sections - they will be well oversize so you would need to get them resawn.

 

I would use oak for the bridge itself as it will last well and be suitably native. I wouldn't worry too much if it was green or seasoned, just design to cope. Alternatives would be robinia (which would also be OK for the posts) or sweet chestnut, but I would want to reduce the span for either of these.

 

A 15ft span sounds OK, but I would want to design the structural members carefully. It either needs to be a structural deck with side rails applied, or structural side frames with a deck, forming effectively a U-channel. The latter would allow you to use smaller, lighter timbers but would require more care in the design.

 

Alec

[ballibeg]

Why not a floating pontoon across. Common on the west coast here.

 

Daze

[Paul Jenks]

Greenheart is used for piles and you may be able to find some from folk who work on tidal rivers. The Hamble has hundreds of them. Be warned, it's almost as hard as concrete, just as heavy and will take many hours of cutting. I think the dust may be poisonous too

[delabodge]

you could use two rsj`s and box them in , attach an 8x2 too the top and build from there,,,thats how I did the last one,,,,was for a country park,so needed to be strong,,,

[aswales]

There are some nice examples of bridges in oak in Trada publications Green Oak in Construction. Might find some helpfull info via Trada

[se7enthdevil]

wood for outdoor structures are greenheart, purpleheart, azobe, iroko, english oak, massaranduba, ipe, jarrah, robinia, cumaru, curapay, teak, balau, opepe, cypress, larch, cedar (true only), douglas fir and believe it or not most specoes in the mahogany group.

[Ian Leach]

Ideally greenheart but I would use oak or elm. I've used a few things for bridges mainly for weighing down oil barrels filled with concrete with pins sticking out to attach posts to.

Take a look at my technical drawing, if anyone doesn't understand it I can simplify it for you.

[sandspider]

Ideally greenheart but I would use oak or elm. I've used a few things for bridges mainly for weighing down oil barrels filled with concrete with pins sticking out to attach posts to.

Take a look at my technical drawing, if anyone doesn't understand it I can simplify it for you.

 

Where do you get ducks bigger than a man?!!