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Thread: Any structural/ concrete engineers out there?

  1. #1
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    Any structural/ concrete engineers out there?

    I'm in the process of building a bridge over a friends creek. The span will be around 28' and that's not counting extra length on the ends for safety. 28' is about what just gets me from bank- to- bank. Because I don't have poles long enough, ( I figured I would need 36 to 40 ft. to be safe) and not liking the possibility of flex in the middle, I have decided to put in a couple of piers in the middle for support. The idea is to sink in some 3 ft. dia. drain tiles upright in the middle of the creek with 12" dia. galvanized culvert in the middle of those all filed with cement and re-rod. The 3' culvert would be about 3' high with the smaller culvert rising up to around 7 or 8 ft. I have been thinking that the 12" culvert/cement posts will be a bit undersized to take the compression from the loads. We planned to just make a 4x4 quad bridge, but it seems to be not much more work to build something capable of supporting a truck or tractor. Any learned opinions out there?

  2. #2
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    I'm not an engineer by any means, but I've done a lot of concrete and structural inspection. I think your biggest concern would not be compressive strength (a 12" diameter of 4000 psi concrete would support about 225 tons). I'd be more concerned with the flexural strength, where that same 4000 psi mix would be more in the 570 psi range. Side-to-side swaying could be a problem. Properly configured rebar and corrugated pipe would help, but at that point the calculations are over my head. My gut reaction is that what you have in mind would be workable, but I'd get confirmation from someone who knows a lot more than me before I built it.
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  3. #3
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    Quote Originally Posted by Vaughn McMillan View Post
    I'm not an engineer by any means, but I've done a lot of concrete and structural inspection. I think your biggest concern would not be compressive strength (a 12" diameter of 4000 psi concrete would support about 225 tons). I'd be more concerned with the flexural strength, where that same 4000 psi mix would be more in the 570 psi range. Side-to-side swaying could be a problem. Properly configured rebar and corrugated pipe would help, but at that point the calculations are over my head. My gut reaction is that what you have in mind would be workable, but I'd get confirmation from someone who knows a lot more than me before I built it.
    What kind of flow does the creek have during heavy rains? Three feet deep, (unless that puts the columns "INTO" Bedrock), Would wash out around the bases quickly in a fast or heavy flow of water. ALL bridges I ever inspected on during construction, OR helped build took the columns down slightly into Bedrock. I agree that you need some "Qualified" help on this, and probably some Core Sampling too.

  4. #4
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    Norman brings up a good point. I meant to mention that the foundation needs to be sitting on something that you know is not going to wash out.

    The more I think about it, I wonder if you might be money and time ahead going with some type of truss to bridge the span without center support.
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  5. #5
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    look into bar joists a live load of 1k# is doable at 30 feet.......you could place the ends on pads on either bank and pour aproaches into the pads.
    as norman said unless the column footings are secure the strength of the columns is irrevelent.
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  6. #6
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    Unless you're going to try to drive a car or truck over it, how much would 2 or 3 telephone poles cost? Seems like they would span the distance ok. Then build the surface on top of them.
    So is the bridge mainly for foot traffic? Jim.
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  7. #7
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    Quote Originally Posted by Jim O'Dell View Post
    Unless you're going to try to drive a car or truck over it, how much would 2 or 3 telephone poles cost? Seems like they would span the distance ok. Then build the surface on top of them.
    So is the bridge mainly for foot traffic? Jim.
    I was thinkng old railroad flat car. Pleanty wide, very strong, clear span, and usually 40' or more in length.
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  8. #8
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    Quote Originally Posted by Rennie Heuer View Post
    I was thinkng old railroad flat car. Pleanty wide, very strong, clear span, and usually 40' or more in length.
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  9. #9
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    Quote Originally Posted by Rennie Heuer View Post
    I was thinkng old railroad flat car. Pleanty wide, very strong, clear span, and usually 40' or more in length.
    Those work great Rennie! I've driven over lots of RR flatcar bridges in Alaska.

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  10. #10
    You need a simple suspension bridge by the sounds. As a snowmobiler I drive over bridges all the time and see lots of different designs. The suspension bridge is the most popular for clear span, no support crossings and they are quick and easy to make. Here are a few I have seen in my travels and feel free to use any you like.

    PS: Not to sound degrading, but I disagree with your assessment of making the bridge big enough to handle cars and trucks instead of just four wheelers (quads as you call them). Weight transfer, weight itself and axle loads are nothing to sneeze at and I think it would be a BIG difference. The only one I know on here that knows anything about this would be Paul Hubbman, but more then likely as an architect he would tell you to be careful and consult a structural engineer. (He told me that just the other day in regards to a safety related issue I had).







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