# Monthly Mechanics: Arch Bridges

This is Part 2 of a series about bridges. Read Part 1 here.

Arches are the second of three bridge types found in nature, carved from stone by water or weather on a timescale of thousands of years. As we learned in my earlier column on the subject, arches act in pure compression. Masonry works well in compression and is not easily destroyed; thus, some of the oldest extant bridges are arches. Examples of super old arches include the Roman aqueducts, the Pont d’Avignon in France, and (most spectacular for its preservation) the 1600-year-old Zhaozhou Bridge in northeast China.

Right away you’ll notice a problem with using an arch as the basis for a bridge: its shape. You can’t easily transport people or resources over a curved surface; you must somehow connect the arch to a flat bridge deck. All the bridges I’ve mentioned so far are deck arches, meaning the deck is completely above the arch. This solution takes advantage of the materials at hand: loads are transmitted by compression from the deck to the arch. The disadvantage is that the deck might wind up far above the surrounding land.

A more recent development is the through arch, in which the bridge deck passes through the middle of the arch. At midspan, where the arch is above the deck, loads are transmitted by tension and require more modern materials like steel rods or cables. The new Lake Champlain Bridge between Crown Point, New York and Chimney Point, Vermont is a good example of a through arch.

The new Lake Champlain Bridge, designed and constructed in under two years.

It’s very efficient to carry loads axially (in tension or compression) instead of in bending, which is why an arch bridge can span long distances and look slim and trim while doing so. But there’s a wrinkle: the bridge abutments need to resist a diagonal force instead of a purely vertical one. This can be accomplished by anchoring in a wall of rock or, if there’s none handy, by building your own with a large amount of reinforced concrete. Multi-span arches can also use matching horizontal forces from consecutive spans to balance out the piers, as the Lake Champlain Bridge does.

Tune in next month when I explore the third and final bridge type found in nature.