The Footbridge Project

I have a creekbed that runs through my yard. Most of the year it’s dry (we live in California), but it has a short span of time in the winter when water flows after the rains. Even when it’s dry, it is inconvenient to cross, because it’s between 3 – 4 feet deep, and the bank is irregular.

The dark line is the creek

So I’m going to build a footbridge over it. I have little experience with construction projects, so this will be a learning experience. I hope it will inspire me to try other projects.

Actually I used to have a makeshift bridge over this creek. I threw down some scrap lumber I had around. But one day I came out into the yard and the whole bridge was gone. It wasn’t secured down, so in theory it could have floated away when the water level swelled. But the creek passes through a wire fence, and there was no evidence the wood had gone through that. So I guess someone passing by needed some lumber? Anyway, I need a new bridge, and this time I’ll build one properly with concrete moorings.

The bridge-that-was

Preparing the site

The first step is to clear the weeds and brambles that have grown in the creek. It had gotten quite overgrown. Here’s the thickest growth, next to my neighbor’s driveway bridge. Some of the weeds can be pulled out by hand, but the raspberry brambles are thorny, and that wannabe tree has grown large enough that I had to use lopping shears and finally an electric saw.

It took several weeks and truckloads hauled to the green waste at our county landfill.

$15 dumping fee per load

I chose a location to build the bridge. This spot is in the shade of a couple of trees, and the bank is nearly the same height on both sides.

I admit working in the shade is not an insignificant factor

I’m leaving the Vinca Major ground cover intact, because it’s nice when it blooms, and also because it’s tougher than it looks, and tearing it out would take a lot of work.

Measuring the bridge

I ran some string to mark out the bridge. I wanted to know if I could use 12-foot joists, but I found that was too short. The bank of the creek is pretty crumbly, and I wanted the moorings to be well back from the edge. So I measured out a 16-foot length.

The width of the bridge is 5 feet, determined by a collection of Trex composite boards I had kept when we rebuilt our deck. I plan to make these the planks of the bridge, so I can avoid buying new material.

Part of the creek bank is shored up with slabs of scrap concrete. I assume these were installed by a previous owner of the property. This is another reason to set the end of the bridge a few feet back from the edge, because I can’t dig through the concrete.

I learned how to make square corners with the 3-4-5 method, and how to tie string to stakes using a Larkshead knot. I enjoyed watching a video Using String Like a Pro, by the Essential Craftsman.

You can see one of the Trex boards I placed on the ground for reference, to make sure I was measuring the right width for the bridge.

Building the moorings

I will have three joists spanning the creek (Trex is not known for its bearing capacity, so I want a center joist as well as one on either side). To fix these joists to the ground securely, I’ll connect them to concrete moorings.

I got steel Strong-Ties to fix the joists to the moorings. I want these to be as straight as I can make them, so I built a temporary frame matching the width of the bridge (i.e. 5 feet, the length of a Trex board). These will allow me to keep the Strong-Ties straight as I put them into the concrete. I used construction screws so I can remove these frame pieces from the Strong-Ties once the concrete sets.

The next task is to dig holes to put the concrete moorings into.