Museum of Science and Industry

Straw Bridges

Tap into your inner engineer as you design, build and test model bridges.

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Lesson at a Glance

Students will plan, design, build and test a model truss bridge. They will identify effective geometric shapes used in bridge design and will identify factors that engineers consider when designing bridges.

Illinois Learning Standards

Late Elementary
11: B; 12: D; 13: B
Middle / Junior High
11: B; 12: D; 13: B

Background Information

There are many types of bridges: beam, truss, arch, suspension and cable-stayed. A truss bridge uses trusses, or a series of triangles, for support. Triangles are structurally the strongest shape because they allow weight to be evenly spread throughout a structure, which allows it to support heavy loads.

Truss bridge construction developed rapidly during the Industrial Revolution. They were first made of wood, then iron and finally steel. During this time, different truss patterns were designed. The Howe Truss, one of the more popular designs, was patented by William Howe in 1840. His innovation used vertical supports in addition to diagonal supports. The combination of vertical and diagonal supports created impressive strength over long spans; this made the truss design ideal for railroad bridges.

Engineers must consider loads when building structures. Loads are weights and forces that a structure must withstand. The dead load of a structure is the weight of the structure itself, which includes beams, cables and the deck. The live load of a structure is the weight that is added to the structure, including people, cars and wind.

As a warm up, discuss with students which shape they think is more stable: a triangle or a square. To demonstrate, build the three shapes pictured below. Stand the shapes up on a table and push down on the top of them. With very little force applied, the open square shape twists, which the the square made of inner triangles withstands much more force.