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.
On Fri, 18 Jun 2010 18:13:00 +0000,
psaenger_at_muhlenberg_dot email@example.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.
On Jun 18, 12:26 pm, psaenger_at_muhlenberg_dot firstname.lastname@example.org
If you are of normal weight (under 250) your 800 lb estimate is
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.
Andy in Eureka, Texas
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.
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