Beam Bridge

Station

Overview

Beam Bridge Station Simulation

What Happens when A Load Pushes Down on A Beam Bridge?

Resources needed - at least 3 sponges with notches cut into the top (see resource section).

Beam bridge Photo credit: © Andrea Pelletier/iStockphoto

Instructions

Take a small sponge and slice a shallow notch across the top and bottom. Create a beam bridge by supporting each end of the 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.

Compression and Tension

More Background Information

This is more for the teachers and facilitators to acquire more background knowledge of each type of bridges in order to facilitate this session.

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 piers.

Prestressed concrete is an ideal material for beam bridge construction. The concrete withstands the forces of compression well, and the steel rods embedded within resist the forces of tension. Prestressed concrete also tends to be one of the least expensive materials in construction.

The Lake Pontchartrain Causeway, Louisiana Photo credit: © Gary Fowler/iStockphoto

But even the best materials can't compensate for the beam bridge's biggest limitation: its length. The farther apart its supports, the weaker a beam bridge gets. As a result, individual beam-bridge girders rarely stretch 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 continuous span.

In fact, the world's longest bridge is a continuous-span beam bridge. Almost 24 miles long, the Lake Pontchartrain Causeway consists of a pair of two-lane sections that run parallel to each other. The Southbound Lane, completed in 1956, comprises 2,243 separate spans, while the Northbound Lane, completed in 1969, includes 1,500 longer spans. Seven crossover lanes connect the two main sections and function as pull-over bays in emergencies. Although impressive, the Lake Pontchartrain Causeway bridge underscores the drawback of continuous spans—they are not well suited for locations that require unobstructed clearance below.