Presidio of San Francisco in San Francisco City and County, California — The American West (Pacific Coastal)
Types of Bridges
Arches were often used in ancient times to form bridges. Arches carry load primarily in compression (resisting pushing forces.) They can be made out of masonry (stones or bricks) because those materials are strong in compression. Each stone or brick pushes on its neighbor. Masonry is also durable, and most locations have a natural supply of stone that can be cut into shapes or clay that can be hardened with fire into bricks. The Romans built many arch bridges, such as the multi-level Pont du Gard that can be visited in France. That bridge served as an aqueduct to carry water over a river valley. Water ran in a trough at the top level of the bridge.
Cable-stayed bridges have straight cables extending from towers to hold up the roadway deck. As those cables (stays) pull up on the deck, they also pull toward the mast, and thus the roadway deck structure must be strong enough to resist a large compressive force. Crossing the Millau Viaduct in France, cars drive 1,132 ft (343 m) above the valley below.
The straight members that make up a truss are struts or “sticks” of steel in modern bridges. The struts form triangles, which are inherently rigid. Truss bridges carry weight effi ciently because each of their straight members is either pulled (in tension) or pushed (in compression). The tension or compression flows straight through the length of each member, without causing it to bend or flex. When the Firth of Forth Railroad Bridge was completed in Scotland in 1890, it was the longest-spanning bridge in the world (1,710 ft, 521 m) and one of the first to be made of steel rather than iron.
Beam bridges were invented when people first placed logs over streams and walked across. Most short-span bridges crossing roads or highways are supported by steel or reinforced concrete beams. If the beam only rests on supports at both ends, it tends to sag as it bends or flexes. This stretches the material (makes it feel tension forces) along the length of the beam's lower edge. Concrete is good at resisting compression (pushing) but is weak in tension. A concrete beam resting on supports at both ends, as shown here, needs steel reinforcing bars or steel cables embedded along the beam’s length near its lower edge to resist tension.
Captions: • Akashi Kaikyo Bridge Honshu-Shikoku Bridge Expressway Company Limited
• Pont du Gard Robert Reitherman
• Millau Viaduct Jeffrey Kossef
• Firth of Forth Robert Reitherman
• A common type of bridge composed of concrete beams Robert Reitherman
Topics. This historical marker is listed in this topic list: Bridges & Viaducts. A significant historical year for this entry is 1937.
Location. 37° 48.48′ N, 122° 28.534′ W. Marker is in San Francisco, California, in San Francisco City and County. It is in Presidio of San Francisco. Marker can be reached from Coastal Trail. Marker is at lower-level entrance to Golden Gate Bridge Vista Point South overlook. Touch for map. Marker is in this post office area: San Francisco CA 94129, United States of America. Touch for directions.
Other nearby markers. At least 8 other markers are within walking distance of this marker. Facts & Figures About the Bridge (here, next to this marker); Tall and Strong (here, next to this marker); Steel, Fog, Salt, Rust, and Paint (here, next to this marker); Art Deco on a Grand Scale (here, next to this marker); Joseph Strauss Legacy Circle (a few steps from this marker); Men of Vision (a few steps from this marker); Battery Lancaster (a few steps from this marker); Bridging the Gate — the Beginning (a few steps from this marker). Touch for a list and map of all markers in San Francisco.
Credits. This page was last revised on February 7, 2023. It was originally submitted on July 6, 2021, by Duane and Tracy Marsteller of Murfreesboro, Tennessee. This page has been viewed 128 times since then and 6 times this year. Photos: 1, 2. submitted on July 6, 2021, by Duane and Tracy Marsteller of Murfreesboro, Tennessee.