Starting a Bridge

[Kapnfriday]

I've enjoyed reading all the posts about bridge building on TBN so thought I would share my own. We have a small creek on our property and being able to cross it will allow us access to 3/4 acre that we own but have never used. We'll also be able to walk to a city park. Initially this was just going to be a foot bridge, but then I thought it would be nice to be able to get the tractor over there to clean up/build trails.

The plan is for an 18' span that will be 6-7 feet above the creek bed.



This shot is of the creek at normal water level - not much more than a trickle.



Here's a shot from the opposite bank after we had about 2" of rain over a couple hours.

I have five utility poles that I'm planning to use as the main "beams" across the creek. Initially I was only going to use 3 but I checked with an electrical engineer friend who checked with a structural engineer coworker and they said that would be pushing up against the maximum capacity for my estimated 3,000lb load of the tractor (Kubota B7610) and FEL and any implement I might have on.

The poles are between 10 and 14" in diameter. They will rest on railroad ties pinned to the banks with rebar. Hard to tell in the pictures, but the rail road ties are about 3-4 feet back from the top of the banks. I plan to marry two poles together on both the left and right side of the bridge and strap them together with some kind of cable or other metal strapping. These will be set apart about the distance between my left and right side tractor wheels. The fifth pole will run down the center. I initially thought I'd deck it with pt 2x6's, but now I'm thinking of looking into full dimension 2x6 white oak from a local sawmill.

A couple still undecided things: how to anchor the poles to the railroad ties and the best way to attach the decking to the poles.

 

[Kapnfriday]

First challenge with the bridge was how to get the RR tie for the far side over to the far side. I have no way to access that side with the tractor and there was no way I could drag it over by hand. Then I thought of laying my extension ladder across the creek and pushing the tie across that.



Keeping the tie balanced on the ladder was a bit tricky, but not bad.



The ladder surface was slick enough that I could inch the tie the last few feet by hand.

 

[JoeL4330]

You might find a "come-along" useful.

Aside from any wetlands issue, did you simply consider sloping an approach, and laying down some concrete to make an all-weather ford for the tractor, and a stepping stone or two--between the wheel width--for people crossing

 

[Tractor Seabee]

I decked a 16' bridge with 2" rough sawn fir 12" wide planks. Nailed them down with 8" galvanized nails to 3 RR tie stringers. I drive my 3500 Dodge dually with 5000# of aggregate in the bed over trhat bridge and it doesn't deflect or creak. If you can get that oak in 12" you would be better off if nailing it. I held the first nails back 2" from each edge with 3 in-between and never had a split. You should consider on that oak, especially the 6", to pre-drill the nails holes just a little smaller than the dia. I wish we had access to oak up here.

Ron

 

[Kapnfriday]

I had the come-alongs and chains ready to inch the RR tie across, but it was a lot faster and easier to push one end of the tie and then the other.

I think you mean something like this: http://www.americantrails.org/photoGalleries/cool/25images/12.jpg ? There would be wetlands issues. My friend and neighbor used to work for our state wetlands division and I checked with her about doing a culvert instead of a bridge. Again, there would be issues.

Also, the creak banks are about 4-5 feet high so doing a sloping approach would've required moving a lot of dirt. At least what I consider to be a lot to move with my tractor.

Free utility poles also swayed me toward a bridge. The decking will be the most expensive part of this project.

 

[EddieWalker]

How far are you into using the railroad ties? Here in East Texas, where we have quite a bit of humidity and close to 4 ft of rain per year, they rot out really fast. Railroads put them on a bed of gravel so water will drain under them because when left on the ground, they ROT!!!!!!!

Eddie

 

[rd_macgregor]

I've mulled over wha kind of support would work for my (future bridge). I think the best (but not easiest) bet would be steel piles with a steel beam across in a square, inverted U configuration. A number of years ago, I supported a hefty footbridge this way and it has worked great. However, driving piles for a tractor bridge is a bigger undertaking!
I think (mostly) buried Jersey barriers would work really well for abutments, if they could be had cheaply enough.
Final is something just sitting on the ground. This is least desirable, even with a not-deteriorating substance, since it is more subject to movement (up, down, sideways and unevenly). Personally, I wouldn't worry too much about using a solid, well-treated RR tie, piling or piece of power pole as an abutment, knowing that it will eventually degrade (say in 25 to 50 years).

I'm still looking to track down an old flat deck container or flat bed truck frame for my bridge itself; the abutments will be customized to fit whatever I end up with.

Bob

 

[MacLawn]

Three words - OVERBUILD!!

Hehehehehee, yep, it'll rain about ten times harder than you ever thought, bugs, crits, vines, everything will try to destroy it. Overbuild.

 

[radioman]

First off - I see that you have sandy soil- and the creek appears to be undermining the bend and its going to get worse esp with sand. I would be putting the supports for bridge farther back as possible. I would also start putting some stones in that bend to slow down the erosion.

Next - how long are the poles you have? Since you mentioned a 18 ft span - if you want, you could build a file folder or commonly called A truss bridge. If you can use poles for across, and building A frame above bridge with a center pole attached to make equal right angle triangles. You could literally split the weight of the tractor in half when its in the middle of the bridge. Lets say the tractor is 4000 pounds and you drive it to the middle, the weight is distributed 2000 lbs to the each END of the bridge. Keep in mind the center pole of A bridge has BOTH 4000lbs of compression on bottom joint and tension on top so your connection needs to be super strong to handle that much weight you plan to have on your bridge.

 

[Tractor Seabee]

Kapn, When I did my RR tie frame bridge I used 10' ties as sill logs each side parallel to the creek. One side was real humus laden material 2 feet deep the other side sandy gravel. I dug down a trench each side 3' wide X 12' long and 3-4' deep. Filled the trenches with 1 1/4 minus crushed gravel, well compacted, and packed gravel all around the sills up to the top. I knew from experience here in the PNW that RR ties do not like laying on dirt, hence the gravel beds down to solid substrate. In the winter I get water flow within 6" of the underside of the stringers. Your pictures indicate you are just setting the sill ties on top of the ground. Those will be easily undermined and/or rotted quickly. You indicated that there is no access to the area on the other side, that complicates things. You will need a temp solution to get the tractor (I assume you have or can get a BH) and gravel over there to work that side. I placed my stringers by getting one end under the front brush guard and strapping about 6' out to the FEL. Picked up that end and curled up the bucket to get the other end off the ground, just eased it across. You could drag across with a line if you can get a block tied off the something on the other side also.

Another thought; I tied all the framing members together by drilling holes and driving in concrete form steel stakes. On those poles you may want to consider cutting a flat spot on each end where they rest on the ties.

Ron