I want to build a bridge of about 20 feet. We will be removing some oak trees with a trunk diameter of 12 to 14 inches. I was wondering how these would work for my beams for the bridge. This would be a people bridge to walk across. Would I need to remove the bark? How long would they last? Also would I need to treat them with a water preservative?
(Sawing and Drying Forum)
From contributor A:
It would need to be white oak, and yes, remove the bark. Also make sure the ends do not sit on the ground. If you can, flatten the top and nail your deck down. Get some 30 lb felt on top of the logs under the decking so the water will shed off or put roof coating on the top third of the log. The logs would do better if they are not sealed with any type of waterproofing, so they can dry out.
If you need a load rating, use the square timber the log would saw out. So a 12" top log would make an 8x8 timber, and that would be your load rating for that log.
For a foot bridge, you can purchase 20' pressure treated 2x10 for about $35. I haven't priced them in a while but that should be close. 3 2x10's should do the job (plus deck boards) and they'd be easier to handle than 20' 14" logs. My two cents: get the bridge done the easy way (with 2x10's) and then go fishing or take the Mrs. out to dinner.
You need to alternate butts and tops so one end is not weaker than the other. Build nice level abutments and lay out your logs on them. Take a chalk line across the ends leveling the larger butts with the smallest top. Then rotate the logs 180 degrees and all you will have to level up for the decking is the odd knot. The logs also need to be tied together one third of the way across, so two tie points. This is usually done to ensure that no one stringer bears the entire load.