Bridge Building

[TomIre]

I'm going to build a small bridge over a creek on our property. The only use for the bridge will be a UTV (maybe 2000 lbs max loaded) and pedestrians - no tractors.
Spam is about 8 feet. Will pressure treated 4x4's be enough for stringers? Decking will be some leftover 2 x 12's with 2x4 curbs and scuppers. I could also screw some of the 2x12's together to make stringers but would then have to buy decking.
What say you?

 

[oosik]

I would go two doubled 2x10's - (four 2x10's - glued and screwed together). Guarantee to hold the weight. Be certain that there is some CONSTRUCTED RESTRICTION that does not allow heavier vehicles to be on or cross the bridge.

You know the limits of the bridge. I'm visiting with my 10,100 pound M6040 Kubota to help you with a project. I do not know the limitations of the bridge.

A sign would be nice. A constructed restriction will insure no future liability issues.

 

[747driver]

I just made a creek bridge. 10' span by 7' wide deck. I made concrete piers on both sides and used 5 4x6 treated post for deck beams. Topped the deck off with 2x8 treated boards. I wouldn't trust 4x4s. We use ours for the atv and JD Gator.

 

[s219]

4x4 not nearly enough, in fact they wouldn't even suffice for pedestrian traffic over an 8' span. You need lumber that is "tall" to span distances without deflecting. If in doubt, go study how decks are built, then go at least double or triple what they used for your beams.

 

[oosik]

View attachment 560602


OK - I've got the ideal example for you. This is my ten ton farm wagon I built. Its got ten ton Horst running gear. The span from wheel to wheel - where the trusses rest - is eight feet.

To carry a load of ten tons - Horst recommended the two trusses be made of knot free 2"x 12" - each truss doubled( two 2 x 12's) glued with "glue lam" type glue and screwed together with three inch trumpet screws. I went WAY OVERBOARD - bought a ten by four sheet of 3/4" plywood and made each truss a "sandwich" with the plywood being in the middle.

That's why I recommended doubled 2 x 10's. Guaranteed to carry the load and if in a dry environment - probably last forever.

 

[KennyG]

Unanimous that you need to maximize height of beams and 4x4 is a bad choice. I used 2x8s when I built a bridge because I didn't want more rise over existing structure but 2x10 or 2x12 would have been better. There are beam tables on line that are easy to use to determine load capacity based on beam size and span.

 

[RedNeckRacin]

I'll throw on the pile. A 4x4 sounds good but it's basically a doubled up 2x4. The width doesn't help you much. You need a higher moment of inertia to resist the loading from the deck and live load. The taller you can make the supports the farther they get away from the neutral axis of bending and the stiffer the element gets.

 

[buckeyefarmer]

Agree with others 2x10 or 2x12 much better that 4x4.

 

[TomIre]

I'll throw on the pile. A 4x4 sounds good but it's basically a doubled up 2x4. The width doesn't help you much. You need a higher moment of inertia to resist the loading from the deck and live load. The taller you can make the supports the farther they get away from the neutral axis of bending and the stiffer the element gets.

Scary thing is, that actually made sense to me! Like somebody else said, 2x12 is going to make the top of the bridge really high. Was going to use bedded in telephone pole pieces as the "piers". They're at least a foot high but I can bed the down. Maybe I'll play around with my backhoe a bit and see what kind of grade I can come up with.

 

[RedNeckRacin]

Glad I could help! Can you sink in your telephone poles a little deeper? I wouldn't use wood buried in dirt though. Concrete is by far better for burial if it's in the budget!

 

[fried1765]

Glad I could help! Can you sink in your telephone poles a little deeper? I wouldn't use wood buried in dirt though. Concrete is by far better for burial if it's in the budget!

"wouldn't use wood buried in dirt"......Why not?
Most electric poles throughout the US are wood, and they typically last 40+ years.

 

[TomIre]

Not in the budget! It's a very small stream. I could use a culvert just thought a little bridge would be nicer. Plus it's something my 12 year old nephew can do when he comes to the farm for his summer visit.
Wonder how long the telephone pole pieces would last if I put them parallel to the water flow and pretty much in the creek bed? They would sometimes have water flowing past them. Then I could put 2x12's on top for stringers.

 

[RedNeckRacin]

"wouldn't use wood buried in dirt"......Why not?
Most electric poles throughout the US are wood, and they typically last 40+ years.

So how much life does a salvaged pole have left and don't forget about the wet environment next to a stream? I have no idea, but for a couple bucks, you could pour some nice concrete piers in sono tubes and have a much longer lived footing.

 

[EddieWalker]

If it was me, I'd probably just pile up a bunch of concrete sacks for a base and run some rebar through them into the ground below. Then lay treated lumber on the flat on top of the concrete with some rebar going through the board too. Leave a little sticking up so you can bend it over with a hammer once the concrete sets up. For the flat boards, I'd go with treated 2x6's or bigger. It's not that important how big the flat lumber is. Then lay your treated 2x10's on edge on top of the flat boards and attach them with metal brackets. Since you want it wide enough for an ATV, six feet should be about right. Two 2x10's glued and screwed together for your beams under the tires, and another 2x10 in the center. Start with that center 2x10, then go out two feet either way and put your doubled up 2x10s there. That is where the tires will be. Then deck it with treated 2x6's and screws. Never use nails on any exterior project.

A 4x4 will bend just from it's own weight over an 8 foot span.

 

[Jstpssng]

"wouldn't use wood buried in dirt"......Why not?
Most electric poles throughout the US are wood, and they typically last 40+ years.

They also are pressure treated before they are set, and are checked and retreated periodically

 

[Streetcar]

I was told with the new chemicals local power company will not teuse pole. Chemical leach from pole into soil for protection. If moved not enough chemical in pole

 

[John Fitzgerald]

Any footbridge or small vehicle bridge that I have seen around here has eventually washed out by a larger than expected rainfall. Even one the electric co-op built for a public park with big power poles to hold the bridge. Our creek bed is nearly the same size as the OP's, just a little larger, maybe 12 feet wide. With a 500 acre watershed, any bridge would be a major project, like public works would do for a road. I can cross with my walk behind brush mower by putting up a ramp on the steep side and going down a shallow slope on the other. No tractor access unless through the neighbor's property. I only have about a half acre on the other side of the creek, no it doesn't matter much, as it's all low lying.

It depends on the watershed the OP's creek has and the largest rain expected. Our 12 foot wide creek bed spills over and becomes probably 40 feet wide every two or three years.

I would say the OP has a good chance of a washout with that small bridge, unless the bed is very very deep under the bridge. It's not so much the water that destroys things. It's the debris. Our creek can carry 30 foot trees downstream and has recently rolled 300 pound rocks.

 

[KennyG]

I would also suggest to put in concrete foundations. It's not hard and it will eliminate problems down the road. My approach would be to pour an appropriate rectangular foundation under each end, perhaps 8 to 12 inches deep. Below that, dig several footing holes with a posthole digger, another 3 feet or more if possible. That will give an easy surface to build the bridge on and a treated wood structure on top of that will last many, many years.

 

[TomIre]

Washouts can always happen but I'm not too concerned here. I can literally step across the creek. I just don't want to drive the UTV up and down the bank turning it to a muddy mess. Just upstream, three feet maybe, is a woven wire field fence that would catch/slow most debris. The creek also just drains the hillside in my property. Maybe 10 acres or so. Dries up most summers. Really more of a fun project to do with my nephew than anything.
I've got a full size power "telephone" pole standing in my property. About three years ago the local EMC reran the power supply to my house when we renovated. They took all their equipment off the pole and I just left it standing for some future project. This may be it! Now I'm thinking I'll just put in two pole lengths as stringers and deck it over. Yes, the pole ends will be sitting in the dirt. If I get 5-10 years out of it I'll be happy. If it comes apart in two years, no biggie. And all material will be what I have on hand - zero out of pocket

 

[Tractor Seabee]

Standby; some day in the future you will kick your butt and say why did I not make this, higher, longer and heavier. Railroad ties make good stringers and sills. Have built several like that over the years. Could drive my 1 ton loades with gravel over it.

Ron