Building a Bridge?

[vinese]

Hi Everyone,
First post so I'll try to keep it brief.

I have a 100 acre parcel of land that is bisected by a swampy little stream bottom. Years back during a dry summer we had gone through the property with a dozer. I'm guessing the operator just pushed some debris into the stream bed and crossed it that way. Several years later all the debris has washed through so now I have no way of accessing approximately 80 Acres of the property.

The stream is probably 5-6ft deep and 8' wide at the point I'm wanting to cross. It's a really slow bayou like stream with fairly level banks on either side. When it floods it just comes of of the banks and spreads..

My original Idea was to lay 3 16' treated 6x6's across the stream and use concrete footer pads on each side to rest the beams on. I was going to make it 8' wide and use treated 2x6's as the deck material. To Anchor it I planned on setting 4x4 post in concrete at each corner and attaching it to the bridge to keep the whole thing from floating away during floods.

The primary use of the bride is to cross with ATV's and a 6 seater Polaris Ranger. Secondary use would be to support our 40 hp? New Holland tractor and 6' bush hog so we can occasionally clip the trails.

My Questions is.. After reviewing other peoples bride designs online I see many people using 2x10's or 2x12's 12" OC in lieu of the 6'x6' beams.

I'm curious what load rating would be for the 3 6x6 configuration over the 8' free span would be vs 8 2x10's 12" OC for the same span?

Would either support the weight of the tractor?

 

[radioman]

using this fact - Over 12 feet, which is the maximum allowable span between supports, a 6-by-6 beam will hold up to 102 pounds per lineal foot. That said, your design is too simple and will fail. if you built it today, you can cross the creek, but it wil not last and your efforts will be gone. you need to add some triangles to your design so it will become strong. Look at the pic I added. With the angle supports- you effectively cut the span length . Also I would use 12" OC beams. This will ensure it will be strong enough for your polaris - but for your tractor? I am not sure how much it weighs.

 

[Paddy]

Vinese,

A 40 hp tractor will weigh more than a car. I had a similer creek to cross on my previous property but it was dry 75% of the time. First bridge was made of rail road ties, spaced real close. After 5 years it rotted. The problem with using square timber, 6x6 or 8x8, it's very inefficient use of material. All beams have a ratio where the "H" is much larger than "W". This is because the formula for spans at certain deflections "H" is cubed, as in HxHxHxw=L

The second bridge I poured out of concrete with lots of steel. When I formed it up, I formed box beams where the tires would hit. These were 14" deep with 5/8" Rebar.

 

[crazyal]

Call around and see if you can get your self some i-beams. Often they will have short pieces they will sell for cheap. Just figure out how much your tractor will weigh and the span and they can figure out what size and how many you'll need. I've done several bridges this way. For decking I've used pressure treated 2x4 on their side bolted together.

 

[bcp]

Do you have a place where the "valley" is wider and shallower? If so, how about some loads of rock for a solid-bottom ford instead of a bridge? ATV's and tractors should have no trouble.

Bruce

 

[Chris2]

I need to build a bridge myself. My creek is smaller than yours at 3' deep and 4' across. I'm planning on using a 20' gooseneck trailer; it’s already designed to handle all the weight of your tractor. Simply back it in, winch it across, anchor it down, cut off the hitch add a dirt ramp and your done. I found an old one with a steel deck and very bad axles for $900 just trying to talk the guy down some. You may be able to find something similar in your area

 

[KennyG]

I was thinking about starting a thread on a bridge I'm in the process of building. I need to span a concrete spillway from my pond with a 12 foot bridge. It's very similar situation to yours. In your situation, a culvert and fill would probably suffice and be the cheapest way to go.

I-Beams are good but I haven't been able to find a deal on them and the attachment of the deck to the beams will be more complicated. Mine will have concrete footings and 12 2x10's for beams. I will also use 2x6's for the deck. I've done the calculations and the bridge will safely support about 8 tons as a distributed load or 4 tons as a point load. It could be a good bit more depending on the actual strength of the wood but I'm being conservative.

As others have mentioned, the vertical dimension is most important. Taller beams work better as long as they don't buckle which is why good blocking is required. 2x12's would add a good bit of capacity to my bridge but I don't need it.

 

[senecak19]

opcorn: Following this one. I have a small creek to cross as well. Haven't decided how to go about crossing it just yet. My parents use three large timbers to access their barn loft. I think there some 8 x 12 or something close to that and we've driven an F450 dump truck over them without issue. I thought for sure I could get a couple of them and mount them up on concrete peers, but now you guys have me second guessing. I'm on a smaller tractor though... 32hp

 

[KennyG]

senecak, I did a quick calc, assuming a true 8 inch by 12 inch beam with a fiber stress capacity of 1000 psi (which is pretty conservative) and came up with a capacity of 128,000 lbs for a one foot long beam. To get the distributed load capacity, you just divide that value by the actual length. For instance, if the beams are 10 feet long, they would have a capacity for an evenly distributed load of 12,800 lbs each. The point load capacity is half the distributed load.

You didn't say how far you have to span, but those beams will have a lot of capacity. I usually use 1000 psi for the wood strength because treated wood isn't much better than that these days, but good wood can be 1500 psi or better.

 

[Tractor Seabee]

Somewhere in the archives is a thread I responded to on this subject. I do not know how to search the forums. I built mine using 3 16' Railroad ties setting on a 10" tie in each bank. Turned them so the wide dimension is vertical for max strength. Dug down 4' and filled under the sill ties with crushed rock and set the ties the height I wanted then packed gravel around them. Deck is rough sawn fir planks 2" X 12" anchored with 6" nails into the ties. I tied all the ties together with 24" long steel form stakes. I routinely take my 1 ton dually Dodge across with 2 tons of gravel in the bed. I doesn't deflect or creak. If the ground is marshy on the approaches, excavate a couple feet and fill with 4" minus crushed rock topped with finer material. I have about $300 in mine in material. Had to have the planks milled.

The post above on using a trailer frame and bed is also good if you get one big enough. I would look for at least a 16,000# trailer. Remember if it flexes during use it will gradually work loose any fastenings to the sill logs/piers and the deck planks. Pretty soon everything will be floating if the water rises higher than the bottom of the bridge.

Ron