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!