Geo-Joint: The Golden Gate Bridge

Though it’s hard to imagine when looking at it now, San Francisco was once empty hills, undeveloped beaches, and open wetlands. Its protective bay made the place a natural for the safe mooring of explorers’ ships, and eventually a small town developed. As we know, the discovery of gold in the mountains east of the city led to explosive growth and the establishment of a true city. By the early part of the 20th century, it became clear that the coastal edge of California needed a continuous road running north and south. Those who held land in Marin and parts north, above San Francisco’s peninsula, knew that the value of their real estate would increase greatly if there was a convenient connection to The City. But the very thing that made San Francisco Bay possible was the stumbling block to such a road. The mighty Golden Gate Strait, the entrance to the bay, presented a saltwatery gap of a little over a mile along that most direct path. There were ferry services across the mouth of the bay, and had been for many decades, but a bridge would be much more efficient. However, considering the powerful tides that ebbed or flowed through the deep passage four times a day, not to mention the aggravation of high winds, thick fog, and heavy marine traffic, it was thought that a bridge could never be safely or affordably built across it. For a long time that was true, but advances in engineering began to make such a project seem possible.

By 1922, a number of plans had been considered, and one finally got the nod from city officials. It came from an engineer named Joseph Strauss, whose design for a suspension bridge was far larger than the drawbridges with which he had some expertise elsewhere in the country. HIs proposed bridge would be three times longer than any other suspension bridge in the world, but his bid was only a fourth of what others thought it would cost. The very idea of a bridge was anathema to the operators of the ferry services owned by the Southern Pacific Railroad. With their considerable resources they fought the plan with lawsuits but were eventually defeated. That traffic jams at the ferry ports were becoming completely out of hand certainly didn’t help their case.

The next obstacle was financing. While money for huge infrastucture projects is rarely easy to obtain, the timing for the Golden Gate Bridge was disastrous. By the time actual funds were being sought, the Great Depression was underway. Requests for bond measures went out and were approved by voters, but given the abysmal economy, there was no one to sell them to. Eventually the proponents of the project were able to convince financier A.P. Giannini of the Bank of America to front the $35 million for the bonds. One of the things in the bridge project’s favor was the number of jobs it would create, and there were thousands.

A map of the Golden Gate from 1899, well before the Golden Gate was spanned.

Preliminary construction began in 1932, with the building of access roads, and actual bridge work began in early 1933. Trestles were built out to the sites where the towers were to be sunk into the bay floor, one at the Marin end, and the other, called the South Tower. Hardhat divers, who could only work an hour at a time between the rushing tides, laid dynamite to blast holes on the uneven, rocky bottom, creating a workable base for the piers. A structure called a fender was then emplaced. Like a large tube, it created a column of calm, protected water in which to dive for further operations. It had a thickness of 30 feet, and about 50 feet in height, and the area of a football field inside. Though it was not the original plan, the water was eventually pumped out, and the South Tower was built “dry,” completely protected from the ocean’s influence. The tides pouring through the Golden Gate average a flow of 2.3 million cubic feet per second, equal to three and a half times the flow of the Mississippi River at its mouth. But that’s only an average. Peak flow is more like seven times the Mississippi rate, and during times of unusual tidal variations, it can go to fourteen times! A sixth or more of the bay’s water commonly moves back and forth through the narrow Golden Gate during a change of tides. It is a high-energy environment. Creating a safe zone in which to work was essential, but much of the rest of the construction was done on and under open water. Ship collisions, wind, waves and tides battered access piers and delayed work repeatedly.

 

Map of the Golden Gate in 1947 when the bridge was ten years old.

 

Without the Golden Gate Bridge, it’s a long way around the bay.

The whole story of construction is far too long and complicated to try to retell here, but once the 746-foot towers were in place, great cables were spun between them, and then the bridge deck itself was suspended from vertical cables. Photos and movies of the construction process defy belief. The methods of bridge building had been well established, of course, but the Golden Gate Bridge was bigger than any other bridge ever built at the time: a center span 700 feet longer than that of the George Washington Bridge in New York. The two cables you can see swooping from tower to tower today look huge and solid. But they were spun together from individual smaller strands—over 27,500 of them—pressed into a single cable. The combined length of the wires in one cable would encircle the earth three times. The strands had to be individually pulled across the span over the tops of the towers on a wire trolley that continuously ran back and forth. During construction, engineers developed new technologies to spin more wires per trolley transit. Improvements like this meant the bridge was completed and opened by May 28, 1937, more quickly than expected. It remained the longest suspension bridge until 1964, when New York’s Verrazano Narrows Bridge surpassed its length by 60 feet. As other suspension bridges have been built worldwide, the Golden Gate is now the 9th longest.

Early cable work—a high wire act.

Suspension bridges are meant to move laterally in wind, vertically under load, and in San Francisco’s case, when the land itself moves. Powerful ocean wind gusts up to and over 70 mph have been one of the few things to cause officials to close the bridge to traffic, though no permanent damage has ever been done. At mid-section, the bridge is designed to swing 27 feet from side to side. The cables supporting the bridge deck are made to stretch as weight varies. The largest load ever put upon the bridge was during the 50th anniversary celebration in 1987, when 300,000 people walked onto the bridge and the deck’s upward bending curve was deflected 7 feet downward, causing the towers to lean inward. The span is actually designed to flatten down 10 vertical feet, so there was no danger of failure, but people were amazed to see pictures showing that the stress was greater than that from solid automobile traffic. As for earthquakes, the Golden Gate has weathered many, including the magnitude 6.9 Loma Prieta quake of 1989, which caused major damage to the San Francisco-Oakland Bay Bridge, just a few miles across San Francisco Bay.

The deck is suspended from vertical cables.

There are more fun and mind-boggling facts about the Golden Gate Bridge than I can hope to list here, from the 600,000 rivets in each tower, to the 2-billion-plus vehicles that have crossed the span. But one thing to take away from this brief history is the fact that the bridge is named for the span of water it crosses, not the color of the paint that covers it. Steel from Pennsylvania, sent on ships through the Panama Canal (some of which this author’s own father helped sail) arrived in San Francisco covered in an anti-corrosive reddish-orange paint called International Orange. The color caught the imagination of bridge planners, and it was felt its audacious boldness would not only be a distinctive statement in contrast to the blues, greens, and grays of the ocean, hills, and fog surrounding its location, but that it would also serve as a high-visibility safety factor for passing ships. To this day, a permanent crew of painters applies 5,000 to 10,000 gallons of that paint every year to maintain the Golden Gate Bridge, now 80 years old and one of the world’s most famous and beloved bridges.


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PHOTO CREDITS:

1899 map image and 1947 map image
Source: USGS : USGS (Public domain)
Bay Area Map
Source: Wikimedia : USGS  (Public domain)
Cable
Source: Flickr: OFF/AFP/Getty Images (CC by 2.0)
Construction
Source: Flickr – GOGA Park Archives (CC by ND 2.0)

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