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    CNGreen's Avatar
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    Opinions needed for bridge design

    I am in the planning stages of a bridge that I intend to build over a creek at my hunting property, actually 1 of 2. This bridge will span 20' and be 10' wide. The use for this bridge will be ATV/UTV and tractor use. My tractor will be the heaviest item to use the bridge with a weight including tractor and implements approximately 11k pounds. I have placed a pipe in the creek and it has held up fairly well but the limbs, leaves and other items coming down the creek are clogging up the inlet side and I am looking for a better long term solution as I do not live on the property and do not get to visit as often as I would like.

    Here is what I am thinking...I plan to use concrete on both sides where the beams will rest. The foundation for the beams will be poured into the banks on either side measuring approx. 18"x18"x12'. The beams will be anchored to these and will span the 20' across the creek. I plan to use 3 pieces of plate steel in between the beams to anchor them all together to prevent rolling or shifting. I plan to use approx. 35 4"x6"x12' pressure treated timbers laid horizontally spaced approx. 2" across the I beams. I will then place 3 2"x8"x20' PT boards across the bridge at the location of the tractor tires. All fasteners will be hot dipped galvanized.

    The only item that I am unsure of is what size I beam do I use. I am considering 4" or 6". While I am no engineer, I would think that a bridge designed and built like this would hold more weight than what I need. It would also open up the creek under the bridge to prevent obstructions from stopping the flow of water. Any input or suggestions would be welcomed. I have attached a drawing of what this bridge would look like.Click image for larger version. 

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    This looks like a very interesting project.
    You have certainly thought this through well so far.

    Timber bridges are an incredibly fun engineering exercise.

    Well I am unable to lend an opinion on your question of beam size, if you have somewhat of an engineering and mathematical mind, I can point you to a great document

    http://www.woodcenter.org/docs/em770...ublication.pdf

    It is very technical but, if you understand it... you can figure out your beam requirements from the formulas and data in there.

    If you aren't so inclined the best piece of advice I can give is that overbuilding is often cheaper for the layman, and keep in mind that a moving point load (tires on a tractor) requires substantially more strength than a uniform static load.

    Good luck with this project and I hope to see pictures of your progress
    Last edited by mn1025rfilb; 06-04-2015 at 01:54 PM.

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    CNGreen's Avatar
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    Quote Originally Posted by mn1025rfilb View Post
    This looks like a very interesting project.
    You have certainly thought this through well so far.

    Timber bridges are an incredibly fun engineering exercise.

    Well I am unable to lend an opinion on your question of beam size, if you have somewhat of an engineering and mathematical mind, I can point you to a great document

    http://www.woodcenter.org/docs/em770...ublication.pdf

    It is very technical but, if you understand it... you can figure out your beam requirements from the formulas and data in there.

    If you aren't so inclined the best piece of advice I can give is that overbuilding is often cheaper for the layman, and keep in mind that a moving point load (tires on a tractor) requires substantially more strength than a uniform static load.

    Good luck with this project and I hope to see pictures of your progress

    Thanks for the link. I would not consider myself a math wiz but I can usually fumble around and get the answer I need. I intend to take several pictures of this project as it goes along and will share with everyone here.
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    Good project

    Quote Originally Posted by mn1025rfilb View Post
    This looks like a very interesting project.
    You have certainly thought this through well so far.

    Timber bridges are an incredibly fun engineering exercise.

    Well I am unable to lend an opinion on your question of beam size, if you have somewhat of an engineering and mathematical mind, I can point you to a great document

    http://www.woodcenter.org/docs/em770...ublication.pdf

    It is very technical but, if you understand it... you can figure out your beam requirements from the formulas and data in there.

    If you aren't so inclined the best piece of advice I can give is that overbuilding is often cheaper for the layman, and keep in mind that a moving point load (tires on a tractor) requires substantially more strength than a uniform static load.

    Good luck with this project and I hope to see pictures of your progress
    You will also have substantial weight from the decking above the beams. That needs to be included in the loading. I would use "wet" weight for the lumber as the bridge is out in the open and wood absorbs water.

    The point about a dynamic weight vs. static weight is important, particularly with implements that could be bouncing on the back of the three point hitch.

    Treefarmer
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    Opinions needed for bridge design

    Does the creek flood? Hard to tell with out knowing the creek banks but I think you may want more depth in you cement footings. Even a small flood can wash out a lot of material real quick and may take your footings with it.

    We have a wood over steel bridge that our neighbors use. Tonight or tomorrow I'll go and take some measurements of the beams and some photos for you. They did use to move a house trailer across so that is probably over kill for you.

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    Last edited by felixm22; 06-04-2015 at 03:46 PM.
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    Quote Originally Posted by Treefarmer View Post
    You will also have substantial weight from the decking above the beams. That needs to be included in the loading. I would use "wet" weight for the lumber as the bridge is out in the open and wood absorbs water.

    The point about a dynamic weight vs. static weight is important, particularly with implements that could be bouncing on the back of the three point hitch.

    Treefarmer
    Thanks for the input TF. I will take that into account
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    CNGreen's Avatar
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    Quote Originally Posted by felixm22 View Post
    Does the creek flood? Hard to tell with out knowing the creek banks but I think you may want more depth in you cement footings. Even a small flood can wash out a lot of material real quick and may take your footings with it.

    We have a wood over steel bridge that our neighbors use. Tonight or tomorrow I'll go and take some measurements of the beams and some photos for you. They did use to move a house trailer across so that is probably over kill for you.

    Sent from my iPhone using Tapatalk

    Thanks Felix, that would be great. The creek has never flooded that I am aware of. This creek is actually a feeder creek for a larger creek that is as deep as 10' or more in places. I have seen the water reach to within a foot or so of the top but never come out of the banks, at least not within the last 25 years. I will be at my place this Saturday and will be able to get a better idea of where the footings will go and how much water contact the banks will get.
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    Box culvert

    You might also consider a precast culvert. Precast Concrete Box Culverts by American Concrete of Auburn Bangor Maine I'm sure there is a supplier closer to you. This just happened to be the first web page I saw.

    Treefarmer
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    Your beams need to be 10". Rule of thumb for beams based on deflection, Span in feet/2= beam depth in inches.

    The number of beams you need will be determined by the load and how the load is distributed. I'm guessing you will need 4.
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