Raise the Drawbridge — Or Swing It

If you build your bridge high enough, you don’t have to worry about letting navigation through.

The Golden Gate Bridge in San Francisco has 220 feet of clearance between the water and the bridge at high tide: ample for most ships.

Topography, infrastructure and budget require that any number of bridges are something less than 200 feet above a navigation channel. That means you need a drawbridge.

Portland, Oregon is located at the confluence of the Willamette and Columbia Rivers, both of them important transportation corridors. There are also a large number of bridges for every type of traffic, and they represent a wide variety of drawbridges.

One of them, Steel Bridge, was built in 1912.

You can probably figure out how this one works. That center section lifts straight up, pulled by the elevator towers on either side.

That is the most straightforward form of drawbridge. Stop traffic, lift the center section, let marine traffic pass.

Technically, it’s a “vertical truss lift drawbridge.” It was built by the Union Pacific Railroad. The lower level carries rail, bicycle and pedestrian traffic. The upper level carries road traffic as well as a line of Portland’s MAX light rail system. Considered one of the world’s most versatile bridges, although a more contemporary term is “multimodal.”

At the top of the elevator towers, you can see the massive counterweights. They counterbalance the weight of the truss, so the elevator has less work to do. The structure on top is the operation center, with a clear view up- and downriver.

Another common form of drawbridge is more familiar than its name suggests: the bascule bridge. Here’s one, the Siuslaw (sigh-OOSE-lah) River Bridge in Florence, Oregon.

Built in distinctive Art Deco style by the U.S. Public Works Administration in 1936, the Siuslaw is one of a number of historic engineering feats on Highway 101 along Oregon’s coast, crossing the numerous rivers that flow into the Pacific.

I don’t have a photo of a bascule drawbridge in operation, but you can picture the halves of that center section being raised as ramps. Some bascule bridges have one “leaf.” The Siuslaw Bridge is a “double leaf:” Two opposing ramps raise to allow a tall center space for marine traffic.

This blog post was inspired by a bridge just a few miles upstream on the Siuslaw: The Cushman Swing Span Bridge.

When I encountered it, I was absolutely unaware of how it worked, or why there was that odd-looking shack on top of it.

Built in 1914 to carry a railroad line, the Cushman is one of a large number of “swing span” bridges, although I was unfamiliar with them before encountering this one.

A closer look at the “swing span” helps explain how it works.

The center span pivots 90 degrees on its central axis, directly beneath the control house, “swinging” aside to leave the channel clear for ships.

I was particularly struck by the picturesque operator’s control station atop the center of the bridge.

Bridges are iconic structures, dating back to earliest civilization. Almost all ancient cities were located along rivers or lakes, and bridges were critically important. In earliest times, they were merely logs or stone slabs, flopped over streams. River crossings figure in stories from from the dawn of civilization, and figure in tales of errant knights and wayward travelers.

And so today.

Is there a drawbridge in your town? Do you sometimes have to wait for it to allow traffic through? Please leave a comment.

© Brad Nixon 2019