A beam or "girder" bridge is the simplest and most inexpensive kind of bridge.
According to Craig Finley of Finley/McNary Engineering, "they're basically the
vanillas of the bridge world."
In its most basic form, a beam bridge consists of a horizontal beam that is
supported at each end by piers. The weight of the beam pushes straight down on
The beam itself must be strong so that it doesn't bend under its own weight and
the added weight of crossing traffic. When a load pushes down on the beam, the
beam's top edge is pushed together (compression) while the bottom edge is
What happens when a load pushes down on a beam bridge?
Take a flat eraser or a small sponge and slice a shallow notch across the top
and bottom. Create a beam bridge by supporting each end of the eraser (or
sponge) with a stack of books. Press down on the center of the bridge. What
happens to the top and bottom notches? Notice how the top notch squeezes
together in compression, while the bottom notch spreads apart under tension.
Pre-stressed concrete is an ideal material for beam bridge construction; the
concrete withstands the forces of compression well and the steel rods imbedded
within resist the forces of tension. Pre-stressed concrete also tends to be
one of the least expensive materials in construction. But even the best
materials can't compensate for the beam bridge's biggest limitation: its
The farther apart its supports, the weaker a beam bridge gets. As a result,
beam bridges rarely span more than 250 feet. This doesn't mean beam bridges
aren't used to cross great distances—it only means that they must be
daisy-chained together, creating what's known in the bridge world as a
In fact, the world's longest bridge is a continuous span beam bridge. Almost
24 miles long, the Lake Ponchartrain Causeway consists of two, two-lane
sections that run parallel to one another. The Southbound Lane, completed in
1956, is made up of 2243 separate spans, while the Northbound Lane, completed
in 1969, is pieced together from 1500 longer spans. Seven cross-over lanes
connect the two main sections and function as pull-over bays in emergencies.
Although impressive, the Lake Ponchartrain Causeway bridge underscores the
drawback of continuous spans: they are not well suited for locations that
require unobstructed clearance below.
Lake Ponchartrain Causeway, Louisiana
(back to intro)
(next bridge description: suspension bridge)
Photos: (1) Rita Serderian-Precast/Prestressed Concrete Institute; (2) Gulf Engineers and Consultants, Inc.
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