Fort Point Arch

Fort Point Arch
The steel arch spans 318 ft (97 m) over the historic Fort Point
Photo credit: Robert Reitherman

The curve of an arch experiences pushing forces, or compression.

At the south (San Francisco) end of the suspension bridge, a large steel arch carries the roadway deck over Fort Point, the brick fort below.

Fort Point was completed in 1861, long before the Golden Gate Bridge was constructed in the 1930s. The chief engineer planning the new bridge, Joseph Strauss, preserved the historic fort by spanning over it with an arch bridge.

A suspension bridge is the structural opposite of an arch.

The curving cables of a suspension bridge are in tension, experiencing pulling forces. The opposite or upside down picture of those curves looks like an arch. The forces in the arch, compression forces, are the opposite of the tension forces that the suspension bridge cables experience.

Fort Point Arch - Compression and Tension

A tension force is a force that pulls. The curving cables pull on anchorages at the two ends of a suspension bridge. Those anchorages embedded in the earth must supply reaction forces, exerted outward, to withstand the tension in the cables that tries to pull the anchorages inward.

Exhibit designed by Robert Reitherman, CUREE