There is a small drainage ravine on my property, flowing into a pond, which cuts off a trail that circles the pond and the property. I ride on my ATV on that trail and also want to clean it up and make it more walk-able. So I decided to build a bridge over the ravine so the trail would navigate the full loop around the pond.
I found several videos on YouTube which provided some inspiration. Nothing very technical though. What really gave me confidence that the project would be worth the effort, is the US Forest Service publication of detailed plans and recommendations for bridges in the National Forests. With this in hand, I decided to build my bridge.
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Step 1: Determining the Structural Makeup
The span of the bridge needed to be about 20 feet. I did not want any supports in the middle. I’ve calculated loads for beams and joists before but really did not want to have to spend a lot of money and effort on pressure treated framing. I had one abandoned utility pole on my property and a helpful general contractor made two more poles available. Getting my hands on three utility poles gave me a great underpinning to the bridge and let me actually begin the process of building it.
Just getting these utility poles to the site was a major effort. One pole was already close by and I was able to drag it with my ATV. The second was not close and was newer and heavier. I tried using the winch on my ATV to navigate the pole through the woods, but that was not productive. I ended up dragging it with the ATV as well, but it was a major effort, requiring help from the neighbors. The third pole was the shortest of them, so I was able to ratchet it up and strap it onto an 4x8 trailer. My ATV has a ball hitch, so I was able to simply drive that to the site.
Step 2: Placing Headers
The Forest Service design provides for a header to be placed directly on the ground at each end of the bridge. Then the ends of the stringers (joists) sit directly on the headers and extend across to the opposite header. The decking planks are attached to the top of the joists.
One design from the Forest Service recommends resin curb stops as headers, since they are heavy and obviously water proof. Instead, I was able to use the excess length from two of the utility poles which were over 30 feet long. I cut off 8 feet from the thicker end of two of the poles to use as headers.
Digging out a shallow trough at the top of each side of the ravine, I laid in the two headers and leveled them out individually. In order to assure the headers were parallel and directly opposite each other, I put stakes at each end of both headers and measured both straight across to the opposite side as well as diagonally between stakes. I needed a few adjustments to get them lined up reasonably well.
I manipulated the three poles that I was using as the main load bearing stringers up onto the headers, spanning the ravine. After checking that everything was solid, I cut the stringers off flush and evenly, so each end extended just across the headers.
I had decided on making the deck of the bridge about 8 feet wide, so decided the two outer stringers being spaced 5 feet on center. That spacing would leave 1.5 feet of the deck cantilevered off each side of the outer bridge joists. The third stringer is simply centered between the outer two.
Step 3: Attaching the Joists
I used a chain saw to notch slots in each header to cradle the ends of the three stringers. Since the stringers were each a different diameter, the notch cradling each stringer needed to be slightly different depth. This compensated for the varying thickness of the stringers and made the *tops* of the stringers level to support the deck planks.
I discovered the log jack tool, shown in the photo, at Tractor Supply. The tool was critical to manipulating these poles, which are otherwise much too heavy to manage. That said, this model has a serious weak point in the threaded connector that assembles the handle. The threaded joint bent within a few days of use. Still better than nothing.
To tie it all together, I drove 12" spikes through each end of each stringer, down into the headers. The stringers were nearly as thick as the longest spikes I could find, so each place I needed a spike, I first counter-bored a 1” diameter hole a few inches deep. This let the spikes recess down inside the stringers so they could penetrate deeper into the headers. To seat the spikes the last couple inches down into the counter-bored holes, I used a heavy bolt as an extension rod, which let me continue hammering the spikes as they disappeared into the stringers.
Step 4: Decking and End Caps
For decking, I had been watching Craigslist for some time, hoping to find used lumber or scrap deck boards, but was not finding what I needed. I considered buying Pressure Treated lumber from a big box store, but was hesitating on that. I ended up finding a local saw mill operator who showed me Tamarac, which is naturally weather resistant wood and grows locally. So for $200 (about the same as PT), they rough cut enough 2 inch thick by 8 foot long planks to span the course of the 20 foot bridge. I’m happy with the quality and hopefully it lasts for some time. I bought a box of 4 inch weatherproof construction screws to attach the decking.
I first attached two deck boards as backing planks (end caps) on each end of the bridge stringers. These backing planks are part of the Forest Service design and prevent dirt and gravel leading up to the bridge from pushing under the bridge and directly contacting the stringers. They extend the full width of the bridge and would do well to extend even wider. I extended the top backing plank 2” above the top of the stringers to make them flush with the horizontal decking.
As I began attaching the decking planks, I was torn on the amount of gap to leave between each board. The Tamarac was green, so I was expecting some shrinkage, though it would be out in the elements and shaded, so I was not sure how much to expect. I ended up with about a 3/8” gap between deck boards, using two spikes between adjacent boards to keep things evenly spaced. The last board needed to be ripped down slightly to fit cleanly.
Step 5: Building Up the Trail Approaches
I am in the process of building up the trail leading to the bridge at each end (but am anxious and to get this instructables posted!) Anticipating that ATV usage will wear out dirt ramps too quickly, I built up the sides of the approach with larger pieces of broken up concrete. On one end, I also used a spare piece of utility poles as fill for the ramp. I'll fill the rest of the remainder with a combination of dirt and small rocks. And I have begun to level out the trail for the last 20 feet or so leading up to the bridge so the whole approach is smooth even with a little speed on the ATV.
Step 6: Side Rails
And finally, I plan to install low side rails to protect against slippery conditions for ATVs, such as snowy or wet deck surface.
I will installing 12” blocks of Pressure Treated 4x4” along each side of the bridge. These will be attached with galvanized carriage bolts that pass through the blocks, down through the decking and through an additional 12" length of 5/4" PT decking on the underside of the bridge to make it a bit stronger. The idea is to sandwich the decking between two pieces of wood.
My original plan was to then attach 10 foot lengths of 4x4 across the gaps between the smaller blocks making a continuous bumper while letting rain and debris wash away underneath it. But as I was clearing brush near the bridge I removed two tall narrow trees, which will make for great bumpers and a more natural finishing touch to the bridge. I will bolt these directly to the 4x4" blocks with galvanized lag bolts.
The finished bridge makes for an attractive and functional feature along the trail and has been an enjoyable learning experience.