Lawn tractor bridge design


I need to span a small creek in my back yard and am building a 16' bridge with 2" x 12" x 16' pressure treated lumber; making it 4' wide with four 2" x 12"s set on a little less than 2' centers.
Can anyone give me an idea of about how much weight this should hold? I want to drive a small garden style tractor over it; I need to check the actual weight, but I am thinking about 800lbs or so.
------------------------------------- Peter
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responding to http://www.homeownershub.com/construction/Lawn-tractor-bridge-design-19138-.htm Petersaenger wrote:
Petersaenger wrote:

Already made a mistake; the 2 x 12s will be on about 14.5" centers
------------------------------------- Peter
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On Fri, 18 Jun 2010 18:13:00 +0000, psaenger_at_muhlenberg_dot snipped-for-privacy@foo.com (Petersaenger) wrote:

Regardless, I suspect 4 ft may be too narrow for comfort, you may want to go five feet wide.
How much weight? Hard to compute, depends on the quality of the boards, what end bracing you are going to use, a lot of factors, but I'd double the two outside boards if this were my project.
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On Jun 18, 12:26 pm, psaenger_at_muhlenberg_dot snipped-for-privacy@foo.com (Petersaenger) wrote:

Andy comments: If you are of normal weight (under 250) your 800 lb estimate is fine.,
However, your width is not. Many small lawn tractors have mower widths of 42 inches (plus housing). Even tho the wheels are a little less, I suggest going to 5 feet to make it easier to drive across when you have your half empty beer cooler with you.
As far as strength, I would use something heavier. For instance, doubling up on the 2x12's will probably do the trick. Be sure to uses treated wood, and, if it were I, I would paint the wood before assembling it. Paint is an excellent weather seal. Actually, a truss design of sorts might have more ambiance, and would add strength. Personal preference, tho.
Be sure to use adequate footings or piers at the ends.
Good luck,
Andy in Eureka, Texas
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Hi Peter,

According to an old carpentry book I have, a single 2x12 will support somewhere around 1500-2000 pounds (depending on the wood species) for a 16' span. Of course, this is a distributed load over the full length, not a point load like your mower.
A standard house floor rated at 40lbs per sq ft would need 2x12's at 16" OC for a 16' span. If you think of that 800 pound mower as four 200 pound guys standing in the middle of your living room, it's about the same load.
If it were me, I would use five 2x12 joists at 16" OC. This would make the bridge about 5.3 feet wide. For added side-to-side stability, I would double up the two outside joists, for a total of seven 2x12's. Install solid blocking at the ends and every 4-6 feet across the span to prevent the joists from twisting side to side. Then I would use 2x6 decking on top to minimize flex between the joists.
Obviously, you'll need a solid foundation on each side of the creek to support your bridge. Not only to support your lawn tractor, but to withstand potential flooding in the future.
Good luck!
Anthony
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