the most celebrated of iron bridges is the HighLevel Bridge across the Tyne at Newcastle. It is a double bridge, resting on six massive stone piers which support a carriage road, with footpaths, and a railway 25 feet above. It was designed by Robert Stephenson and Mr. T. E. Harrison, and was opened in 1849. The loftiest railway bridge in England is the Crumlin Viaduct, in Monmouthshire, constructed in 1853-7. It crosses a mountain gorge at a height of about 210 feet, and consists of 10 spans of 150 feet. The cast-iron piers are strengthened by diagonal braces. Robert Stephenson's Britannia Tubular Bridge, across the Menai Strait, opened in 1850, was one of the greatest triumphs of engineering science in its day. It is situated about a mile from Telford's suspension bridge. The two tubes through which the trains pass are laid side by side, are rectangular, and are constructed of riveted plates of wrought iron. Each tube is divided into four parts, joined end to end at the piers. The bridge consists of four spans, two of 460 feet each, over the water, and two smaller ones of 230 feet each, over the land. The Conway Bridge, over the river Conway, is identical in principle with the Britannia Bridge, but on a smaller scale. The tubular railway bridge across the Damietta branch of the Nile has this peculiarity, that the roadway is carried above instead of through the tubes. The Victoria Bridge over the St. Lawrence at Montreal, completed in 1860 as a tubular bridge, was even more remarkable than the Britannia Bridge, being nearly two miles in length. The upper portion has recently been reconstructed, and the tubes removed in favor of open railway tracks. Wrought-iron girder bridges followed the development of the tubular system, and latterlyespecially those on the lattice-girder principle have become exceedingly common. The girder bridge across the Firth of Tay at Dundee was opened in 1887. It is 2 miles 73 yards long, has 85 spans, is 77 feet high in four of the midspans, and carries two lines of rails. It was built to take the place of a similar bridge that had not stood long when a great part of it was blown down in a storm in 1879. American engineers have been very successful as iron-bridge builders, adopting various forms of girder, and constructing also some splendid bridges with arches of great span built up of wrought-iron and steel. Some important bridges in the eastern hemisphere have been constructed by Americans with materials sent from their own works in America, competing directly and successfully against British bridgebuilders on their own territory. Particulars of some of these bridges are given under the title BRIDGE-CONSTRUCTION and BRIDGE-CONSTRUCTION, MODERN METHODS OF. The iron lattice bridge, so called from having sides constructed with cross-bars, like latticework, is the natural outcome of the tubular bridge for long spans, developing equal strength with considerable economy of material and labor. Lattice girders are now almost universally adopted for iron bridges for long spans. Cantilever Bridges. The principle of the cantilever has recently been developed to a considerable extent, especially in the United States, where enormous spans are daily in process of construction. The cantilever is a bracket resting on a pier in the river or ravine to be crossed, and so balanced and secured that the extended arm will support both its own weight and its movable load. It is connected at its mid-river extremity by a girder span, with a similar bracket reaching from the opposite shore. There are several of these bridges in the United States, the first of any size being the Niagara cantilever, built in 1883. Its total length is 910 feet, and it is 295 feet above the surface of the river, with steel towers 130 feet high. The Hudson River Bridge at Poughkeepsie, built in 1889, has a length of 6,767 feet, and is built in five spans; the first, third, and fifth being true cantilever spans with fixed continuous spans connecting them. The new Blackwell's Island Bridge, New York, is an example of the cantilever principle. There are four channel piers, 85 by 45 feet at the base, and 135 feet above high tide. These piers contain 810,000 cubic feet of granite. The bridge is two miles in length, with two channel spans of 846 feet each, and a span across Blackwell's Island of 613 feet. The distance from the floor of the bridge to the top of the girders forming the span is 100 feet, making the top of the structure 235 feet above high tide. Other notable cantilever bridges are those across the Colorado River at Red Rock, Cal., and across the Mississippi River at Memphis, Tenn. A fine example of this method of construction is found in the great railway bridge over the Firth of Forth at Queensferry, Scotland, completed in 1889. It has two chief spans of 1,710 feet, two others of 680 feet, 15 of 168 feet, and seven small arches. The total length of the viaduct, including piers, is 8,296 feet, or a little over 11⁄2 miles, of which almost exactly one mile is covered by the great cantilevers. The clear headway under the centre of the bridge is 152 feet at high water, and the highest part of the bridge is 361 feet above the same level. The metal columns above each pier, forming the bases of the cantilever, are 12 feet in diameter. The members under compression are tubular, those in tension are of open braced forms. The wind pressure is assumed from calculation at a maximum of 56 pounds per square foot. The maximum possible stress on any member of the bridge is calculated to be at the rate of 71⁄2 tons per square inch of sectional area, leaving a plentiful margin of strength, since the steel of which the bridge is constructed is capable of resisting a tensile stress of from 30 to 33 tons per square inch, and compression to the extent of from 34 to 37 tons per square inch. Between the two main girders a double line of railway is carried on an internal viaduct supported by trestles and cross girders. The way consists of heavy bridge rails laid on longitudinal sleepers bedded in four steel troughs, into which the wheels will drop in case of derailment, when they will run on the sleep ers. In the piers there are about 120,000 cubic yards of masonry, and in the superstructure 44,500 tons of steel. Suspension Bridges.-The principle of suspension-bridge construction is very old, and races of limited civilization have used some form of suspension for their bridge-ways. South America and the East Indies are the principal home of these bridges. Ropes of fibre, hide, or tough vines are attached to trees at either side of a river or ravine, and from these a rude platform is suspended. But they are available only for foot passengers, and accidents from failure of the supports are frequent. The use of iron con cables, however, for purposes of suspension, is of comparatively recent origin, and massive stone or steel piers take the places of trees as supports. The earliest chain bridge is said to have been one thrown across the river Tees, England, in 1741. It was about 60 feet wide, and had chains at either end to help to steady it. Various iron suspension bridges were structed early in the 19th century; but the first really great structure of the kind was the suspension bridge constructed by Telford over the Menai Strait, between the island of Anglesey and Carnarvonshire in Wales. It was finished in 1825. The roadway is 100 feet above the surface of the water at high tide. between the points of suspension is 580 feet. The opening The suspending chains are formed of straight iron bars united by coupling-bolts. The main chains are 16 in number, disposed 4 chains one under the other on each side of the central footpath, and 4 at each side of the platform of the bridge. The ends of the chains are fixed in tunnels in the rock the strait. On the top of the piers the chains on either side of lie loosely upon cast-iron saddles laid upon horizontal rollers that lie in grooves formed in a cast-iron platform on the summit. The saddles can move backward and forward a few inches in the direction of the length of the bridge, and thus expansion and contraction of the chains produce no other effect than to move the saddles as they lengthen or shorten, raising or depressing the roadway a little without producing an injurious strain on the materials. The chains were formed link by link, working from the fastenings in the tunnels at the extreme ends. There are many other suspension bridges in Great Britain. The Union Suspension Bridge near Berwick is 449 feet long. The suspension bridge over the Avon at Clifton is 702 feet long, and 245 feet above high-water mark. The Freiburg Suspension Bridge in Switzerland is 880 feet long; that over the Danube at Budapest is 666 feet. In the United States such bridges are also common. The railway suspension bridge over the Niagara, supported by wire cables, is 822 feet long; the platform, which carries three lines of rails, being 245 feet above the river. Another bridge, seven miles below the falls, has a span of 1,040 feet. A suspension bridge of great magnitude, connecting the city of New York with Brooklyn, was opened in 1883. The central or main span is 1,595 feet from tower to tower, and the land spans between the towers and the anchorages 930 feet each; the approach on the New York side is 2,492 feet long, and that on the Brooklyn side 1,901 feet, making the total length 5,989 feet. The height of the platform at the centre is 135 feet above high water, and at the ends 119 feet. The roadway is 85 feet broad and is divided into five sections, the two outside for vehicles and trolley cars, the two inner for electric and cable trains, and the middle one, 12 feet above the rest, for foot passengers. The anchorage at each end is a solid cubical structure of stone, measuring 119 feet by 132 feet, rising to a height of 90 feet above high-water mark, and weighing 60,000 tons each. The towers are 278 feet high. The weight of the whole structure suspended between the towers is nearly 7,000 tons. The stress of suspension is borne by four cables of 5,296 steel wires each, 154 inches in diameter. The foundations of the towers were laid by means of caissons and compressed air, at a level of about 80 feet below high-water mark. About two miles above the Brooklyn Bridge anburg Bridge (q.v.), was completed in 1904. other suspension bridge, known as the WilliamsIts clear span is 1.600 feet; width, 118 feet; height, at highest point, 135 feet; total length, 7,200 feet; height of towers (which are of maand of steel above that point), 235 feet. It is sonry under water and for 20 feet above, supported by four cables formed of parallel steel wires, measuring 53 inches in circumference. driveway, and two footpaths. tracks, four electric passenger-car tracks, a Its roadways comprise two elevated railroad forms of movable bridges is found in the Movable Bridges.- The prototype of some which spanned the moats surrounding fortresses mediæval pivot or trunnion bascule bridges, or castles and which, when closed, effectually shut off communication. vertical direction, or were counterbalanced on revolved upon hinge pivots or trunnions in a These bridges either the principle of the see-saw. During the first half of the century which has just closed a number of pivot bascule bridges were built, the spans ranging from 20 to 50 feet. 1869 saw the completion at Copenhagen, Denmark, of the largest bascule bridge which had The year later the honor of ranking as the largest bridge up to that time been constructed. Nine years of this type passed to a structure at Rotterdam, Holland, which gave a clear channel of over 75 feet. led directly up to the invention of the rolling lift bridge, the latter type having been devised The development of the pivot bascule bridge completed. This bridge consists of a suspension just as the Tower Bridge at London was nearly and a bascule bridge combined, and has three spans. pension chains and contain machinery for workTwo massive towers support the susspan of 200 feet. ing the bascule portion, which forms the central which are drawn up flush with the towers when a vessel passes. This is in separate halves, height of 140 feet, which passengers reach by Above is a footway at the is not available. It was commenced in 1885 and means of elevators and stairs when the bascule completed in 1894, and cost more than $4,000,000. bridges of late years can be illustrated by comThe advance which has been made in movable paring the Tower structure with a rolling lift bridge of greater span in Chicago. The weight of the iron and steel in the London bridge is 14,000 tons, while that in the Chicago bridge is but 2,250 tons, and the entire cost of the latter ing machinery alone of the Tower Bridge. was $126.000, less than the cost of the operat bridge is extremely simple. On the approach The mode of operation of the rolling lift of a boat the bridge seemingly splits across the middle and each half rears upright on the bank power is used to operate these bridges and the on which its shore end is resting. Electric power required is surprisingly light, the movable Less than 20 seconds are required for the comspans being perfectly counterbalanced and roll or rock with a minimum amount of friction. plete operation of opening and closing the spans of one of the largest bridges, and the task is accomplished by one man. By a system of counter-weights the movable leaves comprising a BRIDGES Copyright by the Scientific American. BRIDGE OVER THE RHINE AT BONN, PRUSSIA (Upper) BRIDGE OVER THE AARE AT BERN, SWITZERLAND (Lower) |