WORLD-FAMOUS BRIDGES

New Zealand Herald, Volume LXIX, Issue 21144, 30 March 1932, Page 7

WORLD-FAMOUS BRIDGES

COMPARISON WITH SYDNEY. 6 ENGINEERING ACHIEVEMENTS. STEEL AND SUSPENSION SPANS. The recent opening of the Sydney Harbour Bridge, a structure which represents one of the outstanding feats in bridge engineering in the world, both as regards length of span, difficulties in construction and low loss of life in erection, serves to focus attention on the feats of engineers in other countries. The honour of having the longest single steel span belongs to the recently-com-pleted Kill Van Kull Bridge, which connects Stateri Island, New York, with New Jersey. While the Sydney bridge is 1650 ft. ■ long, the New York structure is 1652 ft. lin., a remarkably small disparity, which might lead one to believe that the Kill Van Kull Bridge had been specially designed to take the honour. However, from the point of engineering, the Sydneybridge must rank as a greater achievement, as the weight and difficulties of construction were greater. The Australian bridge provides a 56ft. roadway, four train tracks and two 10ft. pavements, compared with the 40ft. roadway, provision for two trainlines and two 6^ft. pavements included in tho American bridge. New Hudson River Bridge. New York and New Jersey were joined across the Hudson River last October, when the George Washington Memorial Bridge was opened This structure, which is designed on the suspension plan, a design which has been almost exclusively used in New York, has a span of 3500 ft., or about two-thirds of a mile, and is the largest structure of its typo in the world. In all, the bridge and the approaches extend for nearly two miles, while they took years to complete. The roadway will take eight traffic lanes, while a remarkable feature of the construction is that its capacity may be increased in future by tho addition of another floor ?wung below the present roadway.

While the famous Brooklyn suspension bridge in New York City is well known, it is not generally realised that it is but one of several structures of the same type. Manhattan, the heart of New York, comprises a comparatively small area of land, with the East River on one side separating it from Brooklyn and tho Hudson River dividing it from New Jersey. American engineers have preferred to span the East River, with suspension bridges and to cross the Hudson by tunnels.

Tho Brooklyn Bridge, which has a span of 1595 ft., was completed as early as 1883,, and was followed by the Williamsburg Bridge, 1600 ft. in span, and tho Manhattan Bridge, with a span of 1470 ft. A fourth structure, built on the cantilever principle, the Queensborough, was also found necessary to carry New York's growing traffic. Other American Examples.

Another great suspension bridge is the Ambassador Bridge, with a span of 1850 ft., which joins Canada and the United States across the Detroit River. A bridge across the Delaware River, between Philadelphia and Camden, is 100 ft. shorter than the Ambassador, but possesses unusual width for a suspension bridge, 128 ft., giving room for six lines of traffic and four railway tracks in addition to a walk for pedestrians on each side supported by brackets. A peculiarity of this bridge is that instead of the cables being carried ,on rollers at the tops of the towers, to allow them to slide and provide for changes in length due to different temperatures, the towers are made flexible and can bend several feet in either direction without harm.

Although construction on it was commenced in 1883, the cantilever bridge over the Firth of Forth is still considered to bo the greatest bridge in the world. The estuary of the Forth at Queensferry is divided into two channels by an island. Tho depths of the channels precluded the construction of intermediate piers, and two spans of 1700 ft. each were adopted as the basis of the design. In addition to the two main spans there are two largo sections stretching out from the shores, and including these .the total length of the bridge is 8296 ft., or slightly over a mile and a-half. The headway under the bridge is 152 ft. at high water and tho highest part of the bridge is 361 ft. Quebec Disasters Recalled. Another cantilever construction of note is tho Quebec Bridge, the building of which- was attended by two great disasters, each of which contributed valuable lessons to tha science of engineering. After one 500 ft. anchor arm had been completed shoreward, a cantilever span was thrust out over the river from one side for 200 ft., approaching a similar cantilever structure to be built from tho other side. Suddenly, the huge mass of metal collapsed, and hundreds of tons of twisted steel were thrown into tho river. Only 11 of 86 workmen who were thrown into the water wero saved.

After a modified design had been prepared, work was recommenced in 1909. Tho working season was little more than seven months in the year, but by 1915 tho northern sido had been pushed 580 ft. out over the river. On the southern shore the anchor and cantilever spans had been completed, and all was in readiness for hoisting the central arch into position. As tho huge mass of 5500 tons was being lifted into position it slipped from its supports and disappeared into the river with 90 workmen, of whom nine were lost. Later, a new span similar to tho one which had fallen was built and tho structure was completed. Crossing the Zambesi River.

One of tho world's most wonderful bridges is a steel span across tho chasm below the Victoria Falls on tho Zambesi River, in tho heart of equatorial Africa. Its floor is 406 ft. above tho water. The gorgo is 750 ft. wide at the top, and as a journey of nine miles was necessary to got from one sido to the other, tho most difficult task of the engineers was to establish communication across the chasm. This was done by firing a rocket carrying a light line, by means of which first, a wire and then a cable wero dragged across. Another bridge built under extraordinary difficulties was one spanning the Grand Canyon, in Arizona, United Stales, at a height of 475 ft. above the water. It is said to bo the highest steel arch of any si/.o in tho world. The width _is 616 ft. The nearest railway was 130 mrles distant, and all material had lo be transported over desert country.