Thursday, October 2, 2008

Sydney Harbour Bridge

The Sydney Harbour Bridge, irreverently known as “the coat hanger” to Sydneysiders, is the largest, although not the longest, one-bow bridge in the world. It crosses from Dawe’s Point on the downtown side to Milson’s Point on the North Shore, and its realization was a remarkable economic accomplishment in the years of the Great Depression—using laborintensive technology, the project employed 1,400 men—as well as being one of the twentieth century’s major engineering feats.

Before 1932, the only connections between the city center and the residential suburbs on the North Shore were ferries or a circuitous 12-mile (20-kilometer) road route that crossed five bridges over narrow inlets of the extensive harbor. The notion of a single bridge surfaced from time to time during the nineteenth century, but serious thought was not given to the project until the 1890s. In January 1900, on the eve of the Australian Federation, the New South Wales government invited tenders for the design of a North Shore bridge across the harbor. Twenty-four proposals were received but all were rejected; only one had involved a single-arch bridge.

In 1912 the civil engineer Dr. John Job Carew Bradfield was appointed chief engineer of Sydney Harbour Bridge and Metropolitan Railway Construction, and he developed the basic design with Department of Public Works engineers. Bradfield recommended an arch bridge with granite-faced pylons. In 1922 the government held an international design competition, for which six companies entered twenty schemes. The winning tender was one of six submitted by Dorman Long and Co. of Middlesborough, England. It proposed a single arch that would be built from both shores and supported by cables until it joined at midspan. The complex structural calculations for fabrication and erection were made by consulting engineers Ralph Freeman of the London firm Sir Douglas Fox and Partners and G. C. Imbault. The British firm of Sir John Burnet and Partners was named consulting architect.

The acquisition of land for the approaches and the construction yards called for the demolition of 800 houses, and their occupants were displaced without compensation. Construction began in 1923. The bridge (including the railroad line) took eight years to build. The contractors, with Lawrence Ennis as site manager, established workshops at Milson’s Point where the steel components were fabricated; almost 80 percent of the steel was imported from England. The 39-foot (12-meter) reinforced high-grade concrete foundations were set in the rock upon which the whole of Sydney stands, and 118-foot-long (36-meter), U-shaped tunnels were excavated to anchor the 128 steel cable restraints that would temporarily support each side of the bridge. Once the approach spans were built, in November 1929 work started on the 1,650-foot (503-meter) single-span hinged arch. It supports the bridge deck, the hinges at either end transferring the massive load to the foundations at either end. The two halves of the arch were erected to meet in the middle. The components were transported from the workshops on barges, positioned, hoisted by two 640-ton (580-tonne) electric creeper cranes, and assembled as the cranes traveled toward the center of the bridge. For about ten months the halves gradually reached out for each other across the harbor, finally meeting in August 1930. The 160-foot-wide (49-meter) steel deck, carried 194 feet (59 meters) above sea level, was then commenced, working outward in both directions from the center of the bridge to save time moving the cranes. The decking was fixed within nine months. The last of the 6 million rivets was in place late in January 1932. In February ninety-six railroad locomotives were used to test the bridge. The four 278-foot-high (85-meter), Moruya granite-faced concrete pylons, each standing on the gigantic pads that support the pins of the arch, have no structural role; they are there for esthetic reasons, to define the ends of the main span.

The official opening on 19 March 1932 attracted large crowds. The state premier John T. Lang was about to declare the bridge open when ex-Army Captain Francis De Groot spurred his horse forward and slashed the ribbon with his saber. The total cost of the bridge, in today’s terms, was about $A12 billion—some sources put it considerably higher. Recovered by tolls on automobiles, it was amortized by 1988. The Sydney Harbour Bridge is an essential rail and road artery in and out of the downtown area. When it was first opened, average daily traffic (in both directions) was about 10,900 vehicles. Toward the end of the century, that figure had approached 152,000. The bridge carries eight vehicle lanes, two train lines, a footway, and a cycleway. The growing pressure of traffic volumes was somewhat relieved after August 1992, with the opening of the four-lane, 1.44-mile (2.3-kilometer) Sydney Harbour Tunnel, close to the bridge. Built at a cost of $A738 million, it offered a ten-minute reduction of crossing time at peak hour. Providing an eastern bypass of the city, it is reputed to save 3 million gallons (13 million liters) of fuel a year.

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