bridge using cedar poles

[ger]

Anyone with experience with eastern cedar poles as bridge girders?
We're building a simple bridge across a creek on the property for ATV and motorcycles. We built a railway tie crib in the middle of the creek and placed white cedar poles as girders. we'll deck it with oak slabs or maybe pressure treated decking. At the moment we have three poles per span. The poles about 12-14" big end sharply tapering to 6 to 8" at the small end. The span is about 18 feet. anyone with an idea of what weight poles like this can support?

 

[beenthere]

I'd say you are coming to the limit of the strength of the poles, by their size.
Consider the risk of a pole breaking, and dumping 'whatever' in the water. If the risk is negligable, then such poles might do the job.
Problem with cedar poles is the branch knots that have very little strength in tension, and they usually whorl around the tree, so are on all sides, and hard to tell from the surface what they really look like underneath.

If it doesn't feel too springy (elastic) when you walk on them, then they may be strong enough to hold an ATV or snowmobile. However, you may be tempted to cross over with the Ford (8N?) in the background, and it wouldn't hold.

Just some things to think about.

Are the poles new trees? Or are they line poles that have been removed from line service?

Hard to tell what the quality of the poles are from the picture.

 

[ger]

Thanks for the input! and you've gone right to the heart of the question; I say make the bridge 3 feet wide-just enough for a motorcycle, brother says- may as well do it right - make it wide enough for a... err... Ford 8n right there in the background... I say it won't hold the tractor.
These poles are swamp grown cedar trees cut a few months ago- chosen for their longevity. A couple of white oak logs squared up would have been better- but supply is a problem.
thanks again! ger

 

[mboulais]

I built a very similar bridge about 15 years ago. I used cedar log cribbing and filled it with stones a little larger than a softball. My poles were about the same size as yours. The bridge was safe for ATV's up to 400 - 450 lbs and lasted about 12 yrs. It was getting pretty weak in the last year and did not much use that year.

 

[beenthere]

If me, I would find a mill to rip the cedar poles in 2" thick flitches or 'slabs'. Then I would bolt them to gether, end for end, and make laminated timbers out of them. That way, you will mix up the knots that lower the strength in each pole, and also gain thickness on both ends. The load will be 'shared' by each laminate, and will be more uniform end to end.
One could use a chainsaw to make the bolted beam somewhat leveled off for the oak decking.
You then have, IMO, a much stronger 'beam' and support with the exact same material.

 

[ger]

Sawing would not be a problem as we have a bandsaw mill but wouldn't a "laminated beam" be subject to rot? I left the poles as is due to the severe taper but I can see where bolting the "slabs", reversing every other one might make a stronger more symetrical beam.
Thanks; Ger

 

[beenthere]

The cedar is about as rot-resistant as one can get, and I don't think the laminate beams would rot any faster than the poles. Both will be subject to water (might consider putting a rubber roofing cap over them before the oak deck).

 

[Deerlope]

If you have or can find some Tamrack or sometimes called Larch it makes for excellent bridge stringers.

 

[beenthere]

I was driving on a long trip today, and was thinking about your poles and bridge needs, along with your mill capabilities.

If you saw faces (two 180° from each) that are parallel to the surface (so it would make a tapered timber with two flat faces) on each pole, so each face is about 4" wide, then slap some roofing tar on these faces, turn every other pole end for end, and drill and bolt them together, seems you might have a pretty solid and strong 'beam' for the bridge decking. A waterproof covering, or holes drilled at the low spots to keep pools of water forming along the 'tarred' lines might keep it pretty dry as well.

I'm thinking the 4" faces would line up pretty well (going to try to draw what the ends would looks like) and give you good sharing of the load. Limited by the length of bolts you have and the depth of holes you can drill.

Make any sense?