Why do so many bridges use isosceles and equilateral triangles in their structures? Triangles look nice, but they also help the bridges bear heavy loads. Without triangles, bridges wouldn't be able to handle the weight of traffic.

#### Why It Matters

A bridge must distribute forces evenly, or it will collapse. A **truss bridge** uses triangles to spread out the weight of the vehicles crossing the bridge. Truss bridges are lightweight and fairly inexpensive to construct and maintain. In the United States, they became popular in the 1800s. Today, most truss bridges, like the one below, support railroad tracks. However, their designs are also popular for walking and biking bridges.

Truss bridges often use equilateral and isosceles triangles to distribute weight because the equal angles allow forces to spread evenly across the bridge. Triangles are one of the best shapes for distributing weight because they take force from a single point and distribute it across a wide base.

See for yourself: http://www.mathsinthecity.com/sites/strength-triangles-bloomsbury-tour

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Explore More

Try building your own bridge out of triangles. The link below explains how to build a bridge out of ordinary straws.

http://www.msichicago.org/fileadmin/Education/learninglabs/lab_downloads/CS_straw_bridges.pdf

Experiment with bridges made from different kinds of triangles. Which bridge held the most weight?