Building a Forest Bridge-Steel Superstructure

With the footings in place and the stream bed re-finished, the 5 - 20 foot long, 1 foot tall steel I-beams could be set. You can see that the rebuilt stream bed has filled in and looks nice.

When the footing blocks were poured, 14" long steel anchor bolts were set into the blocks at locations that would secure 4 of the 5 beams. My intention was not that these bolts would rigidly secure the bridge to the footing blocks but hold the beams "in place" allowing some deck flexibility. Though I provided detailed drawings of bolt locations, the bolts ended up in different places than I'd anticipated. I had to cut the existing bolts off and drill about 12" into the blocks to epoxy half-inch threaded rod where I needed it to secure the beams.

Things don't always (don't ever?) go as expected and I like to have at least one alternative plan brewing. Initially, the beams were going to be set in place and the 3 sets of cross braces welded as the beams were placed in position. After rehashing this idea, I decided to move on to the more laborious and costly "plan B".

I rented a generator/welder (as opposed to hiring someone with a portable welder). Renting a portable stick welder was less expensive and, based on my limited welding experience from two welding classes, I figured I could do what I needed to. Plan B called for welding angle iron onto the beams (these would be pre-drilled to accept three bolts); then the cross braces would then be bolted to the angle iron as the bridge superstructure was assembled. This allowed me to weld the braces on to the beams while they were flat on the ground; a much easier task for one person to tackle by themselves.

Making the angle iron brackets took a while as I don't have any real metal working equipment in my workshop. 24 pieces of angle iron were cut with a sawzall. In each of these pieces then had to be drilled 3 - 5/8" holes.... yup, thats 72 holes in all (whew!). I have a small table top drill press for woodworking projects and I had to coax this little machine to drill through 1/4" steel. I wasn't finished drilling once these holes were done, I still had to drill (coax) an additional 72 matching holes through the cross braces. This approach added a bunch of time onto this project but in the long run it did help ease things on the assembly end. Keep in mind, I'm doing all this work by myself. The beams weigh in at over 500 lbs each and each of the cross-braces weigh about 20 lbs. So the strategy was to make the assembly job as easy for one person as I could and bolting was easier than vertical welding for me.

I decided to paint the I-beams instead of leaving them to rust (which they were already doing). A gallon of Rust-Oleum Rusty was about $30. at a local retailer. I put our generator in the back of my truck and used an orbital disk sander to clean the surfaces prior to painting on two coats of primer. One coat of brown topcoat applied with a small (4") disposable paint roller kit finished the beams. I don't know if the time I invested was worth it and this paint job will last or if it will inevitably make the steel last any longer but it satisfied my perfection-ist side and the bridge superstructure sure looks nice!

The I-beams were set, one by one, on top of the cement blocks, starting with the middle one and moving out to the sides. Sandwiched between the bottom of the beams and footing blocks were strips of tire tread cut from used radial tires (cut with a Sawzall). I figured this small bit of rubber would help absorb miscellaneous bridge flexing from traffic.

The top of each I-beam needed to be drilled with 26 bolt holes (130 holes in total). The bottom of each beam also needed to be drilled with two holes on each end to bolt to the footing block. The easiest way to do this was with a mag drill. A mag (magnetic) drill is a small (but heavy!) portable drill press that electro-magnetically secures itself to metal. I used a regular metal cutting 5/8" drill bit to bore through the beams. These holes will secure the bolts that hold the hemlock beams which create the bridge deck.

Steel is not as forgiving to work with as wood. All of my holes were precise to 1/16" of an inch and even then, I had to finesse the beams to get the bolts through the cross-ties and angle iron. First, the center three, drilled and painted beams were placed in position using the cross-ties as guides to precise placement. A shot of spray paint was squirted through the bottom holes to identify placement of the anchor bolts on the concrete blocks. The outside two beams were moved, holes drilled, threaded rod cut to fit and epoxyed in place. The center beam was not attached to the blocks because it sits over the joint between the two and the holed would have been too close to the edge of the block. The rubber tire pieces were similarly marked, holes drilled and placed over the rods. The beams were then placed and loosely bolted. The cross ties were also placed and bolted loosely.

The last two, outside beams were similarly placed and attached. When the entire bridge superstructure was assembled, all bolts were tightened. Scratches and hardware were touched up/painted to complete the superstructure. Though is was more costly (by about $500.00) and consumed more time than I had planned on spending, I am very pleased with the results. The first load of hemlock beams was delivered yesterday and it's on to finishing the bridge deck!