Monthly Mechanics: Truss Bridges

This is Part 5 of a series about bridges. Read Part 1, Part 2, Part 3, Part 4, and Part 4.5.

Trusses were the most popular bridges for a century and a half. Built from timber, iron, and eventually steel, the lightweight and long-spanning forms also had the advantage of providing a flat deck for railroads and later highways. They have fallen out of favor recently, although long-span arches may still show a secondary truss structure, like the New River Gorge Bridge in West Virginia.

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The arch of the New River Gorge Bridge exhibits a Howe truss.

In a freshman-level structural mechanics class, budding engineers learn about many types of trusses, but they rarely learn the advantages and disadvantages of each type. I’d like to rectify that in my little way.

A Howe truss has diagonals that slope up toward the middle of the bridge. Verticals are in tension; diagonals are in compression. Howe trusses are rare because compression is more expensive than tension – that is, a steel or wood member of a given size is stronger in tension. More common is a Pratt truss, which has diagonals that slope down toward the middle of the bridge. Verticals are in compression; diagonals are in tension. Another variation is a Brown truss which has X-shaped diagonals; one leg of each X is always in tension. Brown trusses are common in older wood and iron bridges, using metal rods for diagonals.

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A Warren truss has diagonals that alternate sloping up and down and no verticals at all. Because of the efficient shape of equilateral triangles, they tend to use a minimum of material. A Vierendeel truss, conversely, has no diagonals. Vierendeels are not true trusses because the verticals work in flexure (bending), not pure tension and compression. As a result the verticals tend to be very beefy.

Your turn! Can you identify the following truss types? Hover over the picture for the answers.

Let’s compare a truss to a beam for a moment. A truss is really just a beam with some bits taken out. When a beam bends, what’s actually happening is it gets shorter on the top and longer on the bottom. The top flange is in compression, the bottom flange is in tension, and the web transfers loads between the top and the bottom. Likewise for a truss… except the top flange becomes the top chord, the bottom flange becomes the bottom chord, and the web becomes the verticals and diagonals.

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Some engineers have designed trusses like the K-truss where the diagonals try to mimic the curved paths that loads take in a beam. The result is lightweight but expensive because of the extra connections.

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Alkesh inspects the General Sullivan Bridge, a historic truss bridge near the New Hampshire Seacoast.

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