I bought a steel trailer from a farmer that had altered it to move round bales. I don't think it was actually licensed for the road any longer, other than short distances between a couple different properties that he owned, I don't remember for sure but with the length extensions he added I think it would have been difficult to get through inspection or licensed.(crude but effective, for him, I cut it off because I didn't need a bridge that long.) I only paid like IIRC $500, with the agreement that we would get the axles and the extension pieces after he installed it over a small stream.
Granted this was someone that I knew, not close friends really, more like friendly neighbors, not complete strangers. We left the extensions attached not knowing exactly how long of length,was needed or where exactly it was going for sure. The stream snakes through my property pretty well, I had a general idea where I wanted it but didn't really consider specific aspects of its installation. Like for instance, the approaches on either side. with limited width it's probably unwise to have anything other than a straight ahead approach, having the trailer deck as level to the ground on either side of the deck is pretty nice. (Note: in a vehicle like a car it is very difficult to even see a flatbed trailer set up as a bridge once you get close enough to it. Ive taken cars across mine, but it's precarious to say the least. There is probably just enough room to have the car land upside down in a stream just about deep enough for the cabin to fill with water. Needless to say, the "bridge" gets very limited (NONE with my permission) vehicle traffic, it is intended for a sub-compact tractor with a mower, to maintain a tree farm use only, on private property.
We scoped out a location where the banks on either side of the stream were fairly level to each other and that were high enough above the water level to accommodate the trailer's structural elements plus some "breathing room" for if/when the water level rose. My land is in flood-plain where this stream meets the Mississippi River and does get flooded. This "breathing room" is important depending on your stream, more in a little bit.
When the day came to move this behemoth to my timber, we snuck it down back roads between our places and pulled it to its final destination with a dozer from the county road, used a torch and cordless grinders to remove all "non-bridge components from the trailer. next we made notches in the earth at the top of the banks on both sides of the stream to allow for minimum rise when approaching the bridge, we made a "road" through the stream for the dozer and pulled the trailer until both ends were basically equally resting in out notches. I had set railroad ties into the earth on one side, to have something more than just dirt for it to rest on. After the trailer/bridge was roughly in place I again used railroad ties to level it side to side and end to end. I used my tractor's loader and 2" rock and the earth that was removed from notches to build up and contour the approaches on both sides. The install was complete and worked perfectly for years. Until...
Every stream and every area with a stream has a watershed associated with it they all, or most anyway, have flood-plain surveys and flooding frequency data associated with them also, available at your local Natural Resources Conservation Service (NRCS) center. you should at least be aware of this information. Here's why...
About ten years or so of my trailer/bridge being in service, working great and only needing a couple decking boards replaced or attached better in the later spring when the ground was pretty saturated with snow melt it started to rain, and rain, and rain some more. Flashflood warnings were issued for areas across all of eastern Iowa. I wasn't too worried; the bridge had survived many such storms previously. This one was a little different.
My bridge is in the timber, a little way from and unable to be seen from my house, everything I'm offering is speculation based on physical evidence witnessed by myself and offered by others. After the rain had significantly let up and imminent danger had passed, I decided to go take a look, to my amazement the bridge was sitting right-side up downsteam 45-50 feet turned 90 degrees, parallel to the stream, in perfect shape, not even any boards missing. The notches and railroad ties that I spoke of earlier still exactly where they were in perfect shape, almost no work needed to be done to them after I got a guy with a giant excavator to swing the bridge was the stream does one of its goosenecks, turning almost 180 degrees back onto itself from this turn, up on the land was a "river' of perfectly collated limbs, sticks, lumber pieces and basically anything that would float. The watershed area probably isn't much over a hundred square miles, but near the source there are some deeper ravines and lots of limestone bluff and solid rock just below the service. I found out later that two neighbors had their ponds overflow washing out the earth around their rim, dumping their contents into the already swollen stream, "The Perfect Storm", that I wish I could have seen live.
My theory is that enough floating debris had built up under and upstream against
the trailer's under carriage that when the contents of the ponds, a wall of water met all that debris provided the buoyancy to lift that trailer almost completely vertically out of the notches, with very little damage done to them, carried it downstream above the height of the banks and when it couldn't make the turn, the water and debris kept going and leaving the traileron top of the bank on the inside corner of the turn. Which was amazing because it could have just as easily flipped it over and buried it under tons of mud and not so buoyant debris. I was spared disaster somewhat.
So what's the moral of the story? Spanning a stream with a make-shift bridge, depending on your particular stream and it's watershed area up stream and other places that are holding water that could become compromised ALL contribute to what could or could not make it such a good idea. Other things to consider are the "attractive nuisance" liability aspects of installing it, this is like when someone puts in a pool without fencing or some way to prevent someone falling in and drowning. Even though it may be on your private property with "No Trespassing" signs everywhere, If there wasn't some visual, physical attempt to keep it secured you will be liable for someone damages. Someone falling or driving off the bridge that has no railing. someone jumping from the bridge into water below can be fun until there is something partially submerged in the water or protruding from the bottom that could be impacted while jumping. Where the bridge could be carried to or what damages it could create if it were to wash out is something else to consider.
I'm not trying to be Danny downer, scare you or dissuading you from doing it, just trying to provide some insight of what you could be getting yourself into. Awareness is rarely a hinderance. It seems like most people have better luck than I. I wish you MUCH BETTER!
