Bridge Strength
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    Bridge Strength

    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
    Attached Thumbnails Attached Thumbnails Photo Aug 12, 2 35 42 PM.jpg   Photo Aug 12, 2 45 30 PM.jpg   Photo Aug 12, 2 35 10 PM.jpg  
    2012 Deere 1026R w/
    FEL and 54D MMM

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    BridgeMan's Avatar
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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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    Scotty370's Avatar
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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.
    Andy B.

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    2012 2520 - DELIVERED 27APR2012!!!

    Tractor that I used to own - 1954 40 Utility
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    Quote Originally Posted by andy b. View Post
    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.
    ---

    2011 JD 2520 with 200cx loader, 61" materials bucket, and Artillian JDQA Pallet Forks (42" forks). 62D MMM, ballast box, turfs, and loaded rears.

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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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    Quote Originally Posted by meburdick View Post
    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.

    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.
    Andy B.

    1966 110 Lawn Tractor
    2012 2520 - DELIVERED 27APR2012!!!

    Tractor that I used to own - 1954 40 Utility
    KB3WPN

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    BridgeMan's Avatar
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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.

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    Quote Originally Posted by MattF View Post
    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.

    Kudos!

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

    2011 JD 2520 with 200cx loader, 61" materials bucket, and Artillian JDQA Pallet Forks (42" forks). 62D MMM, ballast box, turfs, and loaded rears.

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