how to extreme

Building a Bridge

Construction How-To, Landscaping, Outdoor Living January 28, 2013 Sonia


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Building a bridge for a volunteer project demonstrates DIY takeaways for every job.

By Mark & Theresa Clement

 

 

When Theresa and I were asked if we could help out at Rebuilding Together’s Annual National Project called Building a Better Neighborhood, our assignment was to build a small bridge. You know the kind: one that usually spans a small creek or gulley in a back yard.

Small or large, we’ve always wanted to build a bridge, but having seen some pretty awesome—and pretty grim—backyard crossings, we wanted this one to rock.

And even though this bridge is a one-off project, there are some cool things you can use to both build a bridge of your own and solve other home improvement headaches from footings to flooring projects to leaving your city better than you found it, which we’re calling “DIY takeaways.”

To lay out the project you need dependable straight lines to guide your measurements. When I need to create two control points in space, I try and use a full-size template like this giant square we made from super-straight Western Red Cedar.

 

Design

We built the bridge for a neighborhood community center, kind of an anchor point to an urban neighborhood that was rebuilding itself. Our mighty span of 14 feet is over a shallow “bio-swale” that collects and naturally filters run-off from the parking lot and the playground area above—a micro-wetland in the middle of America’s 5th largest city, Philadelphia.

We needed a bridge that could accommodate everything from wheelbarrows to wheelchairs and look good doing it.

Bridge Deck. One key was to make the crossing be part of the landscape rather than something “applied” over the top of it. The building-side of the bridge span (where the walking path and community center were located) was the highest point, and we used that as our layout control point. Because it’s about 12 inches higher than the parking lot, it became important to terminate the building-side into grade (i.e. the level of the existing ground). This makes a smooth transition and lowers the bridge on the parking lot-side for a more accommodating ramp down to the macadam.

Rail. We made the guardrail look cool, but not too busy—and not exactly like a backyard deck or store-bought bridge. At the same time, we wanted it to do the same job of a backyard deck safety-wise.

 

How-To

The bio-swale proved both easy and challenging to build in.

It was easy because the span wasn’t that long, and we could use solid-sawn, pressure-treated dimensional lumber (donated by Culpepper Wood Preservers.)

It was, however, a layout challenge because the tops of the footings were about 16 inches below grade. In other words, our holes were in holes—weird.

On the parking lot side, we could set the joists on the sidewalk curb.

Footings must be 36” deep where we work. But with the hill and depth of the bridge structure, the tops of the footings would have been almost 16” below the walking path grade, so we moved them inward from the end of the bridge to give us a little more working room.

DIY Take-Away—Footing Layout. Whenever I’m building something where there’s nothing dependable to measure to or from, I know there will be problems later if I’m not extremely careful. The best way to short-circuit this problem is with full-size templates. In this case, we built a giant square from super-straight Western Red Cedar 2×4 so that we could locate and mark our footing posts the same distance away from the curb as one another and be confident they were the correct distance apart (4 feet).

Grade Beam Attachment. I’m not a big fan of J-Bolts and wet concrete no matter how carefully I lay out. Using post-bases and drop-in anchors like the galvanized Red Head sleeve anchors works better. To set them we measured again, marked the center of the hole, then drilled 1/2-in. diameter holes in the cured concrete with a Bosch Bulldog rotary hammer. We inserted the anchors and socked them up tight, dead-money on layout.

Next, we set our 4×4 grade beam and fastened through the bracket flange with hanger nails. This method is fast, solid and rated for ground contact.

We installed a 4×4 grade beam and LedgerLoked our bridge to it. This helped us set the structure to minimize digging.

Forget J-bolts and wet concrete. We let the posts cure, the Bosch Bulldogged some ½” holes for galvanized Red Head sleeve anchors.