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Thread: Beam Calculations for Load

  1. #1
    Associate Engineer
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    Beam Calculations for Load

    Hi,

    First post. I've spent a lot of time reading everything I can find about this topic on here. So I am aware of all the risks regarding advice on this forum, I will contact my local gov't and I will seek professional engineering services. However, I still want to know how to do this so I ask for help with the process of learning only.

    I want to design a bridge that will hold my tractor. I estimate the weight, fully loaded, to be a max of 6000 lbs.

    The bridge beams will be separated by the same width as the tractor tires.

    To start, I intend to use this calculator to determine stress and bending on the beams:
    "Structural Beam Bending Equations / Calculation Supported on Both Ends Loaded Two equal Loads"

    Is it necessary to do calculations for load at center as well?

    Thanks,

    Bob
    Last edited by Kelly_Bramble; 06-22-2013 at 09:56 PM. Reason: Fix link

  2. #2
    Technical Fellow
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    Quote Originally Posted by boballeg View Post
    and I will seek professional engineering services.
    An old adage pops to mind here, "why have a dog and bark?" Save yourself the time and effort and contact that "professional engineering services," as you say you are going to.

    For a variety of Engineering (failure) reasons, the bridge the same width as the "tractor tires," is a swift recipe for disaster, probably the first time out, if not, then a trip over shortly thereafter. The load "load at center" would be the least of your concerns at this point.

    We are really not that silly here that we could not see through the ruse of asking for assistance and the assurance of getting Professional help. Pass "Go," collect $200, save your life as a reward and get that Professional help.

    Just one old fart's opinion who has seen a lot of failed structures that had been hastily contrived by novices.
    Last edited by PinkertonD; 06-23-2013 at 01:00 PM. Reason: Damned Dyslexia!

  3. #3
    Lead Engineer RWOLFEJR's Avatar
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    I'll add these couple of tidbits...
    A bridge will only be as good as its foundation. Foundation fails and it won't matter how sturdy the bridge was.
    Odds are you're bridging a crick or creek, however you like to say it... And DEP and local ordinances can be very expensive folks to deal with should you go for it without their approval and disturb any waterways in any way. We had a short bridge that was being rebuilt that was held up for a couple years in part due to a small hole in the crick that fish liked to hang out in near the bridge. Somebody decided that a few trout needed a voice in the matter and that they might not find another equally pleasing hole somewhere else in the waterway... Took a long time for some folks to find and agree on a very difficult solution of digging another new hole... add some rocks and Voila... happy fishies.

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    Quote Originally Posted by boballeg View Post

    To start, I intend to use this calculator to determine stress and bending on the beams:
    "Structural Beam Bending Equations / Calculation Supported on Both Ends Loaded Two equal Loads"
    Bob
    This calculator will not determine the uneven loading that your tractor will impart unto the bridge. You could assume "W" at both locations to be equal to the total weight of the tractor and this will allow for FOS.

    Quote Originally Posted by boballeg View Post
    Is it necessary to do calculations for load at center as well?
    For a simply supported on both ends bridge - yes as this is where the most deflection and stress will occur.

  5. #5
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    Quote Originally Posted by RWOLFEJR View Post
    I'll add these couple of tidbits...
    A bridge will only be as good as its foundation. Foundation fails and it won't matter how sturdy the bridge was.
    Yes, I was planning on building a foundation and I realize the importance. I simply wanted to ask one simple question at the time as I explore the process.

    Quote Originally Posted by RWOLFEJR View Post
    Odds are you're bridging a crick or creek, however you like to say it... And DEP and local ordinances can be very expensive folks to deal with should you go for it without their approval and disturb any waterways in any way. We had a short bridge that was being rebuilt that was held up for a couple years in part due to a small hole in the crick that fish liked to hang out in near the bridge. Somebody decided that a few trout needed a voice in the matter and that they might not find another equally pleasing hole somewhere else in the waterway... Took a long time for some folks to find and agree on a very difficult solution of digging another new hole... add some rocks and Voila... happy fishies.
    Around here, a small county in NC, people are generally permitted to do what they want on their own property. That said, the creek is small about 2.5 ft deep. It's a run-off from a culvert that flows underneath an elevated road about 30 feet from where I would like the bridge. This flows into a larger creek that borders my property which feeds into a water reservoir. The bridge would be easily visible from the road and there is water supply involved so would be foolish to not call and ask about permits. Very little water in the creek so no fishies problems. Just a muddy mess.

  6. #6
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    Quote Originally Posted by Kelly Bramble View Post
    This calculator will not determine the uneven loading that your tractor will impart unto the bridge. You could assume "W" at both locations to be equal to the total weight of the tractor and this will allow for FOS.
    I see. In fact, the front end loader with a payload would be about 2000 lbs and the the cutter on the rear about 1000 (all of these are high estimates). So there's one disparity.

    Quote Originally Posted by Kelly Bramble View Post
    For a simply supported on both ends bridge - yes as this is where the most deflection and stress will occur.
    OK thanks. I spoke with a local metal fabricator who sells i-beams and he gave me a reference for a good engineer. This project is very large and for something I don't really need so I'll have to see what he says about the cost and scope. These tractors are heavy beasts!

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