# Monthly Mechanics: Beam Bridges

I designed bridges for the first five years of my career, and to me they remain some of the purest feats of civil engineering. A bridge has no unnecessary parts – every bit contributes to the structure, stability, and safety. (Most architects will argue that a well-designed building also has no unnecessary parts. But engineers do most of the design work on a bridge whereas architects do on a building, so everybody is biased.)

Three types of bridges are particularly interesting because they mimic structures that occur in nature: beam bridges, arch bridges, and suspension bridges. I’ll address these types one article at a time.

The world’s oldest bridge? (flickr – creative commons)

Beams are possibly the oldest bridges ever used by people (think a log over a stream), and by far the most numerous today. They comprise most of the bridges that you drive/walk/ride over and under every day, the ones you barely notice. Look closer, though, and you’ll see no two bridges are quite the same.

For starters, there are an astounding number of different beams. They might be made of steel, concrete, or (not so often nowadays) wood. Steel beams range from smallish I-beams to huge girders built up from steel plates. Concrete beams might be I-shaped or T-shaped, or they might be box girders so wide that a single piece holds the entire roadway. (Fun fact: box girders are hollow, and bridge inspectors go inside with flashlights to check the condition a couple times a year.) Timber beams are sometimes just that – single timbers – and sometimes glued laminated lumber built up from many thin pieces of wood to form something larger and stronger.

Or maybe it’s the configuration of the beams that varies – how frequently they are spaced, whether they are continuous over several spans or terminate at each pier, whether smaller beams support the bridge deck by running perpendicular over the main ones. When an engineering firm begins a new bridge project, it will often submit a type study, presenting the client with many bridge options. The options may differ in their ease of design, ease of construction, ease of repair, aesthetics, and cost.

One of my favorite technologies is the self-propelled modular transporter, or SPMT, which enables a beam bridge superstructure to be replaced in a weekend. A new bridge is constructed next to the old one and placed on an SPMT; then, when the old bridge is demolished, the new bridge can simply roll into place. Watch this timelapse of an SPMT bridge replacement I worked on in Boston.

Next time you travel through a highway underpass, look up – you’ll surely see something you never noticed before.