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I recently built a bridge over a small ditch for my 1026r. I used 12ft 6x6 with 1x6 for decking. The 6x6 are roughly 45in apart set on concrete footers with anchor bolts. The actual span is around 10 feet or so. I've driven over it several times with no issues at all but I'm just wondering what the horizontal breaking strength of your average 12ft 6x6 would be. Here are a few picks. Thanks
 

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Without crunching any numbers, it appears to me that the weak point of your bridge will be the weakness of the deck members rather than the 6 x 6 stringers. If it were mine, I'd stiffen the deck somehow, along with adding felloe guards (curbs) on the edges to keep the tractor out of the creek.
 

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I'll just add..........

........that if your stringer width, matches you tread (I.E. tire width spacing) I think your good to go. But I agree with the side 'curb board' suggestion.........~Scotty
 

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This is a cool file. I have to get back in the swing of things to make some calculations from the formulas presented.

http://www.awc.org/pdf/WSDD/wsdd.pdf

Assuming a middle value for the compressive force of the 6x6 lumber you used, and going to page 79 of the file, a 6x6 with a 10' span looks like it is rated for 2400 lbs. Note that is an actual 6x6, not a 5.25x5.25. Also, that is for just one of the spans. You need to figure out how much your tractor weighs, what you'll be hauling with it, and how much the decking weighs. You're probably safe driving a 1026r over it, but I wouldn't drive a pickup over it. :)
 

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This is a cool file. I have to get back in the swing of things to make some calculations from the formulas presented.

http://www.awc.org/pdf/WSDD/wsdd.pdf

Assuming a middle value for the compressive force of the 6x6 lumber you used, and going to page 79 of the file, a 6x6 with a 10' span looks like it is rated for 2400 lbs. Note that is an actual 6x6, not a 5.25x5.25. Also, that is for just one of the spans. You need to figure out how much your tractor weighs, what you'll be hauling with it, and how much the decking weighs. You're probably safe driving a 1026r over it, but I wouldn't drive a pickup over it. :)
Andy - you missed a very important part in that document... Go back and read the very first page again - it states that the load is EVENLY DISTRIBUTED. When you drive a four-wheeled vehicle of any sort over that bridge, the load is focused at two points on each side.

If it were up to me to build that bridge, I would have used the same essential design I'm using for the floor of my new shed - 2x12 pressure treated set on 12" centers for the joists / stringers. On top of that, I would use 2x6 or 2x8 pressure treated boards to create the deck.
 

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It appears to me that the pressure treated lumber that one would logically expect the intended use would be floor joists have no stamping of suitability for any purpose and many (more so at Home Depot and Lowes than my favorite independent) have knots in places you never would see them if you were a carpenter building a new house and using a pile of lumber delivered to the site. The expected use of a 6x6 probably would be an upright. The 6x6 and 4x4 and the stuff thicker than 1 1/2 inches generally is stamped ground contact where the other stuff is stamped above ground.

So my answer would be that without evaluating the size and location of the knots a useful answer is not possible. Round treated poles from yards owned by either the phone or electric company were what we used to build those bridges in the state forest with the sims (maybe syms?) act funds I mentioned recently. They had three poles on the bottom and one pole at each edge of the decking to keep the snowmobile from going off the edge. If I am not mistaken some engineer had drawn up little sketches. It is also a good idea to tie a cable to one end and around a tree so that when a flood comes along it will end up in a predictable place.
 

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Andy - you missed a very important part in that document... Go back and read the very first page again - it states that the load is EVENLY DISTRIBUTED. When you drive a four-wheeled vehicle of any sort over that bridge, the load is focused at two points on each side.
I used the W number which assumes a worst case point load at the center of the span. The distributed load would be the w (little w versus big W). Don't worry, I've played with load strengths before. :lol:

Fran...k does bring up a good point though, if there are any knots in the 6x6, it will most certainly decrease the load rating.
 

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Making a few assumptions, such as the 6 x 6s being sound enough to have a section modulus of 36.0 C.I., an allowable bending stress of 1200 PSI, and all knots being located above the neutral axes (in the top half of the members), each 6 x 6 can safely (repeatedly) support a point load of 1440 lb. That number comes from equating the resisting moment provided by the members (product of section modulus and stress) to the statics bending moment formula of M = P x L / 4, and solving for P. No deduction for deadload, which would be minimal.

This means a pickup truck could be driven across the bridge, unless loaded with tons of stuff (my trusty 3/4 ton Dodge V-10 has carried more than 6000 lbs. of gravel on a few occasions, scaled weight at the plant--air springs help, of course).
 

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Very cool project!

I'd mention we engineers use saftey factor for a reason. Mathematically, the stress analysis may say you can do it... I wouldn't try drivng more than the tractor unless you have a few factors of headroom. Wood properties are generalizations for the variety of wood used and should not be considered exact when constructing load bearing structures.

I very much appreciate the challenge as I have 35 ft to span over a river someday (half my land is on the other side). I can only imagine how nice it is to cross directly using the bridge as opposed to less convenient routes to the otherside - 2 miles for me. :thumbsdown:

Kudos! :thumbup1gif:

Matt
 

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Very cool project!

I'd mention we engineers use saftey factor for a reason. Mathematically, the stress analysis may say you can do it... I wouldn't try drivng more than the tractor unless you have a few factors of headroom. Wood properties are generalizations for the variety of wood used and should not be considered exact when constructing load bearing structures.

I very much appreciate the challenge as I have 35 ft to span over a river someday (half my land is on the other side). I can only imagine how nice it is to cross directly using the bridge as opposed to less convenient routes to the otherside - 2 miles for me. :thumbsdown:

Kudos! :thumbup1gif:

Matt
I would also suspect that there's a loss of load bearing capability as the wood ages (deteriorates), too.
 

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Actually, wood is not the only bridge-building product that deteriorates over time. The vast majority of the structurally deficient bridges in the U.S. (more than 60,000 at last count, of 605,000 total) are composed of steel and concrete. Steel corrodes, while concrete cracks and spalls, or develops a host of other deficiencies. And the beauty of timber that's often overlooked is its ability to absorb a certain amount of energy when impacted, something steel and concrete can't do.

MattF, you should give some serious thought to using a recycled railroad flatcar for your bridge project. An outfit called Skip Gibbs Company in California has been a specialist in that area, and did respectable work on the several projects that used them I was involved with a few years back. They'll even install a substantial railing system prior to shipping the flatcar out to you; all you need to provide is a means of swinging it into place on your abutments.
 

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Looks Great!! I would add "running planks" on the deck. Spaced at the width of your wheel base. Running Planks take the brunt of any wear and can be replaced as needed. Running planks will also help "tie" the bridge timbers to the deck. I also would add some curbing- just cuz


Thanks for the post
 

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I did a bridge very similar to what you did. Additions I did:

1) Bolted on a 6" piece of 'C' channel to each main beam.

2) Bolted on a 2x4 on the outside edge to increase the strength of the 2x6 cross pieces you drive on. If you're off by a bit, you've got support at the outside of the cross pieces

these are easy things to do post- construction.

I assume you but the beams where the tires are....

No pix, it's at the old house...

Pete
 

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Margin of error, unseen faults and weakness. Species of wood makes a huge difference. For the price of a few extra pieces now versus later, I always go for over kill. I would suggest at least two and maybe three 6x6's bolted through the sides and never look back as far as underpinings and 2x for the deck. A couple of years from now when you start jonesing for that 2720 this bridge will bother you.
 
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