Need simple, durable bridge ideas

[jeepcj7]

I need a bridge to cross a dry creek that has water in it maybe 2 months out of the year. We had a lot of rain this year and due to the lay of the land I realized a culvert is not an option. Was looking for simple ideas to build a bridge to span a 10 ft gap so I can drive my tractor across in wet season. Was thinking of using an old flat trailer off a semi or something along those lines, any ideas?

 

[BTDT]

Maybe an old tandem axle trailer (like you would haul your tractor on) would work.

 

[RonMar]

How deep does the water get, how much waterflow can be expected? how much vertical distance is involved. If it is low and flat, perhaps a series of smaller culverts buried in gravel...

 

[svcguy]

One thought is to try and find a heavily damaged sea-can. Shipping containers are about $3000 for a good 40 ft unit up in British Columbia, but when heavily damaged, you can get them for about $500 - $1000. If you made arrangements to cut the sides and roof off and take only the deck, you might get it really cheap. (On a damaged unit, you are really in competition with the scrap guys at about .25 a pound). If you could find a 20 foot can and get just the deck, it would be very affordable.

Another possibility is to find a steel scrap yard or recycler and try to pick up a couple of nice, deep used I beams and deck them.

 

[GaryM]

One thought is to try and find a heavily damaged sea-can. Shipping containers are about $3000 for a good 40 ft unit up in British Columbia, but when heavily damaged, you can get them for about $500 - $1000. If you made arrangements to cut the sides and roof off and take only the deck, you might get it really cheap.

I'm not a structural engineer, but those containers are built so the sides and top add most of the rigidity to the unit. I think if you tried using only the deck it would fold like a cheap umbrella in the wind under any kind of a load.

Now if you cut down the sides to maybe 24-30 inches and added a horizintal steel plate about 8" wide welded to the cut down sides, you'd almost have a girder style bridge. I think it would take a much higher load in that configuration.

 

[tuolumne]

Do you have timber on your property? Cheap and simple sounds like two tree trunks dropped over the creek at the wheel spacing of your machine and deck them over. I assumed a 12' span with a 7000 lb concentrated load at the center. Using allowable loads for doug fir, you would need a pair of 12" diameter trees. I am sure you could create some sort of crushed stone area to bear the trees on and minimize rot. I agree with the above poster that a cut down container will have insufficient strength.

 

[kwolfe]

I am no engineer, however a 3 or 4 6x6x16 pieces of pressure treated lumber with 2x8 decking should easily support the weight of your tractor or a truck if you want to take it over as well.

 

[tuolumne]

I am no engineer, however a 3 or 4 6x6x16 pieces of pressure treated lumber with 2x8 decking should easily support the weight of your tractor or a truck if you want to take it over as well.

As a structural engineer, I would be nervous about such a setup...however, ignorance is bliss . Strictly by the numbers, 4 6x6 members would be insufficient for the load I mentioned. 7000 lbs is not an unreasonable axle load for a CUT with a bucket of gravel. Wood is very forgiving, and could perhaps perform OK for this transient loading, so this setup may work even though the calculations do not. Also, with the 6x6 option, there would be little load sharing to the inner members, and the members beneath the load would be severely overstressed. A safer solution would be to put the members in pairs below the wheels. Better yet, use something deeper.

 

[Knotbored]

The beams under modular homes (trailer houses set up permanently) look pretty sturdy, and several of those burn down every year. I would guess they are sold as scrap metal, perhaps an insurance investigator could give a hint where you could bid on one to get the beams.

Remember if you build an access over a stream then some day a curious tourist will sure as heck try to pull a 5th wheel over it and get stuck or swamped then blame you. I would invest as much in warning signs as in the bridge!

 

[SkunkWerX]

I really think we would need to know several things before suggesting anything. As Tuolumne has alluded to, we would need some of the following:

-Horizontal distance (across the creekbed)
-Vertical distance from assumed bridge surface to creek bed.
-Maximum load anticipated (Then apply at least a 2:1 Safety factor)
-Maximum width of load.
-Soil conditions at either side and in the creekbed for possible vertical supports.

Need more specifics.