Using Oak Logs for a Wooden Bridge
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.
From contributor S:
The roof felt is a good idea. You could also get a roll of metal flashing and use that instead of the roof felt. White oak is best and if given the flashing/roof felt treatment, you may get 20 years or more if you can build some concrete or rock abutments to put the ends of the bridge on. The longevity is going to depend on the climate. In warmer areas you'll rot out faster. Red oak will also work, but won't last as long.
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.
From contributor A:
But just think of the chemicals leaching into that pretty little stream? (I am just messing with you, sorta.)
From contributor M:
If people can walk on it, then build it for the max load expected. 20' long, so room for 15 people x 200 lbs = 3000 lbs load. Add safety margin for deterioration over the years? Build to support 10,000 lbs.
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.
From the original questioner:
I live near Houston, TX. Mostly what I have on my property are live oaks. The property is heavily wooded. We are going to dig a pond with an island in the center. I was hoping to use a couple or three of the live oaks to build a bridge. I want to stay away from the treated lumber because of the chemicals. We plan on stocking the lake with catfish and bass. I appreciate all the responses - you've given me a lot of valuable information.
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