LARGEST BRIDGE IN WORLD

Press, Volume LXXII, Issue 21966, 15 December 1936, Page 16

LARGEST BRIDGE IN WORLD

♦ San Francisco Bay Spanned COLOSSAL UNDERTAKING COMPLETE The opening of the San FranciscoOakland Bay bridge, by far the largest bridge structure in the world, passed almost unnoticed here when it took place last month. This bridge dwarfs the Sydney harbour bridge, and. indeed, every other notable bridge in the world. The bridge took three years and five months to build, and in the construction many “bridge-building” records were broken. Some of the piers are larger, and have foundations at greater depths than any built before. Yerba Buena Island, dividing the two main sections of the structure, was pierced with the largest bore vehicular tunnel in the world; and in the whole of the superstructure there is more steel than in any other bridge. Completed, the total length of the bridge works is 43,500 feet, or eight and a quarter miles. The mam structure of the Sydney harbour bridge is less than a mile long. The longer spans of the new bridge are considerably longer than the great sing!le arch span at Sydney, being 2319 feet, against 1650 feet. Some of the piers are 240 feet below water, and the heights of the towers above water are 474 and 519 feet. The bridge cost 77,200.000 dollars. Five Sections “Two graceful suspension bridges, built in tandem, span the waters between San Francisco and Yerba Buena Island,” states one account of the project. “Each a complete bridge in itself, each nearly half a mile long, they clear high water at an elevation of more than 200 feet. High above the tallest masts of ships will sweep the traffic of Oakland and San Francisco, eastward through the 76 feet, doubledeck tunnel on the island, curving on to the East Bay cantilever crossing that connects with the Oakland shore. Six lanes of motor-cars will speed along the upper deck. Three lanes of heavy trucks and two rows of trains will run underneath. The crossing of the bay, formerly a matter of hours, will be possible in 10 minutes.” The bridge may be divided roughly into five sections —the San Francisco approaches, the West Bay crossing, the Yerba Buena Island tunnel and crossing, the East Bay crossing, and the East Bay approaches. The East Bay section is the longest, although constructionally it involved less adventurous engineering than the West Bay section with its remarkable deepwater piers. With the decision to plan the course of the bridge along the highest existjrig ridge of bed rock between San Francisco and Yerba Buena Island, began eighteen months of work under water. And during these months of dangerous work the lives of 12 men were lost. Of ; the 51 piers in the actual bay crossing 44 are under water. The smallest of these is a solid block of concrete 44 feet wide, 108 feet long, and 45 feet high, resting on wooden piles 100 feet tall. The undertaking is said to be the greatest under-water project ever carried out. For the founding of the deepest pier bed rock was reached at a depth of 235 feet. Huge Towers The piers finished, the erection of the towers, from which the cables hang, was begun. These four towers n the West Bay are all 19 feet by ■"2 feet at the base, two of them standig more than 500 feet above water. It is strange to thing of such towers moving; yet they will move as much .' s three and a quarter feet. The maximum bending can be caused by unequal stress on the decks below, ' hough even the action of hot sun and cold winds will bend the towers as much as seven inches. Two cables between the towers, of ■ ■ire spun to a thickness of 28J inches, ' 6 feet apart, support the tremendous weight of each span. Four cables in all were needed, for the two suspen--ion bridges joined at an anchorage between San Francisco and the island, with a total of more than 70,000 miles of wire. For the East Bay section a “fill” 3900 feet long extends from the Oakland shore, made largely by pumping mud through pipes from the bottom of the bay. On this are placed most of the piers which support a mass of steel, which in turn supports the bridge. On this section stands the third largest cantilever span in the world. Value of Work On the social and economic side the bridge is estimated to be of incalculable value to the two metropolitan areas, formerly separated by eight miles of bay water. It is expected that there will be in a few years an increase of 60,000 to 90,000 in the population of the East Bay side; and this increase not merely by the shifting of population from one side to the other. A population of 500,000 on the east side has been joined to one of 750,000 on the west, creating a single metropolitan area with 1,250,000 people. In this way the significance of the bridge is considered to exceed greatly its importance as a mere convenience for transport. Early financial obstacles were overcome by what one writer calls the nation-wide and tragic need for largescale employment.” When a . re P°,“ was first made on the scheme in 1930 the difficulties raised were not so n} uc *i of construction as of finance: without a huge taxation burden on the cities the sum needed could not* be raised. But in 1932 the loan was made, on the sole security of the future earnings of the bridge, and to provide employment for thousands of hands in some of the principal industries of the country.