Golden Gate Bridge
The Golden Gate Bridge is a suspension bridge that spans the bay between San Francisco and Marin County. A well-known landmark, the bridge was named after the "Golden Gate Strait", the entrance to the San Francisco Bay from the Pacific Ocean.
The bridge is considered one of the very few human structures that enhance the physical beauty of nature, and was ranked as the fifth most popular American design in a poll of Americans.
The Golden Gate Bridge's 75th anniversary occurred on Memorial Day weekend in 2012.
Stretching 1.7 miles high above the mist-enshrouded waters of San Francisco Bay, this peaked, vermilion-painted suspension bridge (the color is officially known as International Orange) is as striking today as it was when it was completed in 1937. The natural surroundings—including the coves and forested bluffs of Marin County; the island of Alcatraz; and numerous sailboats, barges, kite-surfers, and even frolicking seals—can all be seen from the bridge on a clear day. 
The bridge is built of fabricated steel, and is painted with an orange color that protects the Bridge's steel components from rust and corrosion caused by the high salt content in the air from the Pacific Ocean.  The construction of the bridge took four years to complete (Jan 1933 - May 1937) at a cost of $35 million dollars. 
The 6 traffic lanes of the bridge are located on one level, and have movable barriers which are changed at set times during the day to accommodate peak flows of traffic in both directions. On weekday mornings, when commute traffic is heaviest going into San Francisco, four southbound lanes are allocated for vehicles, and two for northbound traffic. In the evenings the configuration is reversed to allow for heavier traffic leaving the city. On weekends there are generally 3 lanes open for traffic in each direction. 
The bridge district charges a $5 toll for vehicles approaching from the North (southbound, entering San Francisco from Marin) and is free for vehicles travelling the opposite direction (leaving San Francisco). Pedestrians and bicyclists may travel across the bridge using the Pedestrian walkway free of charge. 
- December 4, 1928 - the Golden Gate Bridge and Highway District was formed as the entity to design, construct and finance the Golden Gate Bridge. The District consists of San Francisco, Marin, Sonoma, Del Norte and parts of Mendocino and Napa counties. Representatives from each of the six counties sit on the District's Board of Directors.
- November 4, 1930 - The Voters within the 6 counties of the District approve a $35 million bond issue to finance construction of the Golden Gate Bridge.
- January 5, 1933 - Construction of the Golden Gate Bridge begins.
- May 27, 1937 - The day before the grand opening, the bridge opens to pedestrian traffic.
- May 28, 1937 - The Golden Gate Bridge officially opens to vehicle traffic when President Franklin D. Roosevelt announces it by pressing a telegraph key in the White House.
The engineering team was headed up by Chief Engineer Joseph Strauss who initially designed the bridge. Irving Morrow was the architect responsible for the Art Deco designs, and engineer Charles Alton Ellis and bridge designer Leon Moisseiff collaborated on the mathematics involved in building the bridge. In 1955 a statue of Joseph Strauss was placed in a landing near the bridge to recognize his contributions. 
The Golden Gate Bridge, at 4,200 feet, (1,280 meters) long was the longest suspension bridge in the world from the time it opened in 1937 until New York City's Verrazano Narrows Bridge was opened on November 21, 1964. 
There are currently 6 other suspension bridges that are longer than the Golden Gate:
- Akashi-Kaikyo Bridge, Japan 6,532 feet (1,991 meters), built in 1998
- Great Belt East Bridge, Denmark 5,328 feet (1,624 meters), built in 1997
- Humber Bridge, England 4,626 feet (1,410 meters), built in 1981
- Jiangyin Yangtze River Bridge, China 4,544 feet (1,385 meters), built in 1999
- Tsing Ma Bridge, China 4,518 feet (1,377 meters), built in 1997.
- Verrazano Narrows Bridge, New York 4,260 feet (1,299 meters), built in 1964 
The suspension design of the bridge was chosen to help it withstand an earthquake. A suspension bridge suspends the roadway from the main cables, which extend from one end of the bridge to the other. These cables rest on top of high towers and are secured at each end by anchorages. The towers enable the main cables to be draped over long distances. Most of the weight of the bridge is carried by the cables to the anchorages, which are imbedded in either solid rock or massive concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.  
In October of 1989 when a 7.1 richter earthquake hit the Bay Area, the nearby Oakland-San Francisco Bay Bridge, (built from 1933 - 1936), was unable to withstand the tremors, causing the upper deck of the bridge to fall unto the lower level. 
Seismic engineers evaluated the Golden Gate bridge after the earthquake and determined that although not damaged, a retrofit was required to strengthen the bridge. To design the retrofit supercomputers were used to simulate an earthquake's effect on each part of the bridge. The construction is planned in 4 phases, each addressing separate areas of the bridge. The areas to be worked on include the structural steel, the approach viaducts, the concrete piers, pylons and anchorage housings. The main cable saddles that run to the tops of the towers will be reinforced, and the steel tower shafts and the struts that connect them will be overhauled. 
Construction began in August 1997 and is tentatively scheduled to be completed by 2012. The total cost of the retrofit is estimated to be around $405 million dollars. Once completed, the bridge should be able to withstand an 8.3 magnitude earthquake.