What Is a Box Girder Bridge, Anyway?

Dumbarton Bridge

The Dumbarton Bridge over the San Francisco Bay is a large box girder bridge.

In The Pickwicks’ Picnic, the family is on the road, heading for a box girder bridge. What’s a box girder bridge, anyway? And why is it so fun to say?

Box girder bridges are very common. You might cross one all the time and not even know it. The name comes from the fact that the girders–the steel and/or concrete parts that support the bridge deck–are shaped like a hollow box.

“A box girder is formed when two web plates are joined by a common flange at both the top and the bottom.” —theconstructor.org

Does that clear things up? No? Personally, I feel that any definition of a thing that mentions something called a “flange” is not going to help me to picture it at all. But the point is, box girders are often used in bridge building. When seen in cross-section, they look like this:

Box girder cross sections

Cross sections of box girders

Sam Houston Waterway Bridge

This is a big box girder toll bridge over the Sam Houston waterway in Texas. It is also described, apparently, as a cantilevered concrete trapezoidal haunched hollow box girder bridge.

Got it now? No? Well, don’t worry about it. Anyway, just trust me, box girders are a popular option for constructing bridges, both large and small. The boxes, or cells as they are sometimes called, make the bridge super strong.

Why is it so fun to say? Box girder bridge, box girder bridge. I think it’s because it has a great rhythm to it, like a waltz, one-two-and-three, one-two-and-three. And it also uses alliteration, with the words that start with B. No matter what, you’d have to agree that “box girder bridge” fits better into the words of the book than “cantilevered concrete trapezoidal haunched hollow box girder bridge” (which is a real thing).