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  • Got asked to make a bridge today.

    One of the guys I work with asked me to make a golf cart bridge. I've never built anything this big. 15' long and 5' wide.

    Each side: I was thinking 2x2x1/4". My thought is 36" long with 3 degree ends, weld 5 together and then 37" long with 3 degree ends, 5 welded together. The two could then be welded together using more 2x2x1/4" to tie the two together. Add 2x3 angle on the each inside for a wood deck.

    Tying the two together 6 each 2x2x1/4".

    Decking: 2x6 wood

    The drawings are below. I would do load testing in Fusion 360 but it is beyond my capability. My worry is building it and it not being strong enough. Not worried about my welds.



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  • #2
    Like you, I don't worry about my welds too much. I DO worry about my engineering skills. ( I have none )

    Based on what I've gleaned over the years, the structure will be stronger if you tie the ends together with a stringer that is under tension. That would allow the steel 2x2 to be in compression even if some earth movement causes the supports under the ends to move away from each other.

    You will get a lot better advice from others.

    Dan
    At the end of the project, there is a profound difference between spare parts and extra parts.

    Location: SF East Bay.

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    • #3
      I have been thinking of tying the two ends together. I am also thinking about bolting it to concrete on each end. I would have to build points to be embedded in the concrete.

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      • #4
        When you do your engineering figures, keep in mind that stiffness is much more compatible with human emotion than strength is.

        A deck that bounces and flexes may be sufficiently strong, but it will bring terror to the eyes of anyone who hasn't established trust.

        Girders make efficient use of materials, but suffer from age and weather more quickly than beams.

        Allow for free air movement, don't rest materials that rust or rot directly on concrete or soil.

        Allow for painting.

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        • #5
          A simple shallow arch with a stringers is amazingly strong. While not as long as your bridge I bought some aluminum arched ramps many years ago. They were rated for several hundred pounds for loading riding mowers. I welded some modestly heavy aluminum angle to them on each side and used them for about ten years for loading and unloading my scissor lifts for my contracting business. There was some distortion over the years of the rungs on the ramps, but they never had an real issues. One scissor lift was a little over 3000lbs and the other was almost 4000lbs. I even used them for loading a 6000lb forklift once, and a 6800lb tractor a few times. I would never trust somebody else's life to my ramps, but I rode a scissor lift up those ramps many times over the years. I still have the ramps. Is there any engineering behind that? Nope. Just an anecdote.
          *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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          • #6
            Does the bridge have to be arched like that? A straight piece of 2 x 6 tube trussed on the underside with a length
            of 2 x 3/8 flat and some standoffs would give you plenty of strength and be MUCH simpler to build. Making something
            with that many pieces all welded together is exactly the wrong way to tackle that project...
            Keith
            __________________________
            Just one project too many--that's what finally got him...

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            • #7
              I would worry about the tubes rusting from the inside where you can’t see it until it’s too late. C channel should work too - and you can inspect it all.

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              • #8
                The guy is looking for the arch. All that welding is going to hell with distortion. Two beams with angle on the inside with wood decking sound so much easier. I think I am going to try to re focus him on the 12 degree incline it starts off with and how it is going to be a hell of a speed bump. Might be able to do something out of thin stuff to give it the look of the arch.

                Thank you for all the feedback. New drawing coming up.

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                • #9
                  FYI. My arched aluminum vehicle ramps were bent to shape. Not segmented welds. Probably in a tubing roller with square dies.
                  *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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                  • #10
                    Originally posted by outlawspeeder View Post
                    The guy is looking for the arch. All that welding is going to hell with distortion. Two beams with angle on the inside with wood decking sound so much easier. I think I am going to try to re focus him on the 12 degree incline it starts off with and how it is going to be a hell of a speed bump. Might be able to do something out of thin stuff to give it the look of the arch.

                    Thank you for all the feedback. New drawing coming up.
                    Check with your steel yard. I am able to order material already bent into an arch. Give them specifications of what radius you want for a length and they can deliver. Last bit I got they charged $30 for each 24' length. I would consider that cheap versus what you show with all the welded segments. I would suggest that you have two different radiuses on each side. A lower for the bottom and a tighter one for the upper arch. A few stays between the top and bottom arches and you have it.

                    lg
                    no neat sig line

                    near Salem OR

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                    • #11
                      Check out Beamboy: https://www.softpedia.com/get/Science-CAD/BeamBoy.shtml
                      This program easily calculates maximum deflection and stress on a given element. Good sanity check.

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                      • #12
                        How are the ends of the bridge going to be secured if you're using an arch?

                        I would weld side gussets (maybe in decorative shapes) at each angle.
                        Last edited by Tungsten dipper; 05-14-2020, 07:28 AM.

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                        • #13
                          I think you are going to get some decent deflection in your 2x6 decking if it is going across the 60" span unless you tie it together with some longitudinal runners. I'd consider running 8 footers (or full length 16') along the path of travel. Bending them and bolting down at the cross beams if needed. I remember bending 12' long 2x8's for the deck of my dad's trailer without a lot of drama.

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