Any structural/ concrete engineers out there?

Paul Downes

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Westphalia, Michigan
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?
 
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. ;)
 
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.
 
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.
 
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.
 
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.
 
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.:huh:
Flat-Car-Bridge-2.jpg

Look >HERE<
 
I was thinkng old railroad flat car. Pleanty wide, very strong, clear span, and usually 40' or more in length.:huh:

Those work great Rennie! I've driven over lots of RR flatcar bridges in Alaska.

Make sure you check with your state Fish and Game or DNR.......piling in a stream or bridge building without permits in many states will get you in bigggggggggg trouble.
 
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). :)

Snowmobiling_009.jpg


Jason_and_Joe_Over_Sebasticook2.JPG


Snowmobile_Bridge_Close3.JPG


Shaky_Bridge.JPG
 
I was thinkng old railroad flat car. Pleanty wide, very strong, clear span, and usually 40' or more in length.:huh:
View attachment 25115

Look >HERE<

A railcar is kind of big and hard to transport. But for the span you're talking an old flatbed semi-trailer would work about right. It would have some wood planking that might need replaced and reinforced and you might need to rent a crane to set it in place. But it wouldn't need a support mid stream which would be difficult to build as a do-it-yourself project without diverting the stream. It would need some concrete supports on each end though. As for strength, I drive a cement truck and we deliver regularly to a summer home area that we cross a bridge made from two old semi trailers side by side. It holds an 80,000 lb loaded truck so a single would easily carry a pickup or tractor.
 
Lot's of ideas here. I do have a stash of telephone poles but they got shortened by a tornado and none are long enough. They were 80 ft. before the storm. Alas I milled the long ones a while ago and now am down to 28 to 30 ft. I cut the 2' dia. poles into 8x8's and 10 x 10's They are going up for a barn lean-to. I was planning on putting in concrete/stone footings for the bank ends. And will build a retaining wall for one side to bring it up to height. Lots of re-rod and such to make a permanent structure. I can get more poles, and long enough. I have several neighbors who work for the power company.

I have all winter to stew over the ideas and will have to check regulations on the casual to find out what our dubious government thinks about it.
 
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?

I was going through my vast collection of Fine Woodworking Magazines and there it was...a whole article on how to build a timber trussed 39 foot bridge. I am not making this up.

It was done in Alaska and on the cheap because materials had to be flown in by float plane. The bridge was big enough to support a front end loader going over a stream that often flooded. The article was well written, detailed and showed pictures and drawings though the former was in black and white. If you are interested in trying to find the issue, it is:

Fine Woodworking
March/April 1982, Number 33
Pages 78-81
 
A railcar is kind of big and hard to transport. But for the span you're talking an old flatbed semi-trailer would work about right. It would have some wood planking that might need replaced and reinforced and you might need to rent a crane to set it in place. But it wouldn't need a support mid stream which would be difficult to build as a do-it-yourself project without diverting the stream. It would need some concrete supports on each end though. As for strength, I drive a cement truck and we deliver regularly to a summer home area that we cross a bridge made from two old semi trailers side by side. It holds an 80,000 lb loaded truck so a single would easily carry a pickup or tractor.

What he said. As an ex flat bedder it was nothing to put a 48,000 pound single coil in the middle of a 48 foot trailer.
My brother sold 2 old flatbeds to a neighbor for a bridge so they could get their combine across a creek.

Buy a whole trailer have it delivered. scrap what you don't need to recoup a bit of the investment.
 
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