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28′ bridge footings

sonofmany | Posted in Construction Techniques on February 13, 2022 09:15am

Hello,

We are building a 28′ bridge to cross a creek on some family property, and I am a bit stuck on figuring out how to attach the bridge to some piers/footings. I have attached a CAD mock-up of the bridge, which uses three 4″x8″ – 28′ foot beams salvaged from an old barn that are rock steady. The bridge is 5’4″ wide, with 2″x6″ decking. 

The reason we are keeping it so long is the geography of where it will be placed. The east side of the creek has a drop-off that sits about roughly 3′ high then where the bridge will end on the west side. This will lead to a 4-8 degree grade in the bridge. The 28′ length easily spans the few foot creek and avoids any of my concerns towards future erosion.

I was planning on casting 6″-8″ concrete footings for each beam on both sides, but I am looking for ideas on how to fasten them with this angle in mind. I was hoping to cast fasteners into the concrete. My first thought is we could use temporary supports to the beams and cast the fasteners at the angle they will sit at. Or I could cast the fasteners level and add shims below the beam in the fastener to help them have level contact with the footing. I would like to avoid making piers for the west side to make up the difference in height. Any advice on this would be greatly appreciated!

Note that the attached photo has 11″ cube piers that I was just using as a place holder.

Edit: added sketches of my thoughts to hopefully clarify. Seeing the open space in the first between the base of the fastener and the footing is worrisome.

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Replies

  1. User avater
    2sheps | Feb 13, 2022 10:55am | #1

    I'd like something a bit more substantial. Maybe like this.

    1. sonofmany | Feb 13, 2022 11:14am | #2

      Great idea! I have never built a form that complicated though. Getting the angles right is my biggest concern. I like this a lot though, and I will have to ponder it more. For what it was worth, I was planning on using thicker guage fasteners and 3/8"-1/2" carriage bolts to attach the beams, trying to be overkill. Thank you for the idea!

  2. User avater
    2sheps | Feb 13, 2022 11:37am | #3

    My concern would be more with wood preservation than anchoring. You'd need to consider waterproofing those beams and allow water to escape the seat area.

    1. sonofmany | Feb 13, 2022 03:16pm | #6

      Good thoughts, I have been pondering waterproofing as well. Some sort of sealant I was thinking. And yes, stand-offs for the seats will be a must. Appreciate your time.

  3. florida | Feb 13, 2022 12:05pm | #4

    28' is a huge span, and you'd certainly need more than 3 even for a foorbridge. Do you have any idea what species of wood they are because that is a big factor in determining strength? Even for a footbridge that size you really need an engineer and you probably need steel beams.

    1. sonofmany | Feb 13, 2022 03:13pm | #5

      These are tie/cross beams that are oak I think. I did some rough FEM work on it and it looks solid, but it was with FreeCAD and I am not a trained engineer so I don't trust it necessarily. I will have to find an engineer to take a look, and also consider adding some mid-length support. Good thoughts, thank you!

    2. mikemahan3 | Feb 14, 2022 01:08pm | #7

      A foot bridge could tolerate a whole lot of deflection. It's not a floor.

      1. florida | Feb 16, 2022 03:04pm | #10

        Yes, I agree but I'll bet it wouldn't be a comfortable walk.

  4. User avater
    stewyyy | Feb 14, 2022 02:24pm | #8

    simple footing's like the one above. maybe with a turnbuckle cast into them for a simple upside down cable truss bridge. 5/16 wire winch rope, I've driven atv's, snow mobile's, and such over quite a few. Footings not even needed. With a steel end plate and a couple large fence post driven into the ground on the low end will keep the bridge from moving.
    Treat everything with copper-coat....seeing as we can't use creosote any more,

  5. Tim_William | Feb 15, 2022 02:54pm | #9

    Instead of trying to hit the perfect angle, could you use a through-bolt or roller on the beam and support it with a box or brace? I'm thinking of a tidal dock that can rise and fall with the water.

  6. plumb_bob | Feb 18, 2022 11:50am | #11

    Casting your fasteners in place is great if you hit it perfect, but it might be easier to drill in anchors after the fact. Hilti makes great big wedge anchors and such with huge load ratings.
    I would build a substantial abutment like in reply #1, more concrete but more substantial.
    Your stringers should be on some sort of bearing, even a rubberized matting or similar to keep capillary action from rotting your wood.
    I agree that a 28' span is asking alot, but for a simple foot bridge it may be acceptable. If a center support is possible it would help. or maybe a suspension system if you really want to have some fun!
    Cool project

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