CHAPTER III.
LATTICE AND SUSPENSION BRIDGES.
“Theexpense of a tubular bridge would be too great.”
“But if we could get the strength without the expense.”
“What mean you?”
“By iron lattice work we could, I think, gain the stiffness and support needed, without such great cost of labour and material. In other words, I propose a lattice or trellis work girder, instead of a solid sided, or a tubular girder.”
“That is, you would have the sides of lattice or trellis work, instead of solid plates?”
“Exactly. I would use bars of iron placed diagonally. These lattice or trellis bridges are developed from the tubular bridges, also from the loose wooden lattice bridges of America. We make a web of iron insteadof a solid sheet. The same kind of structures are largely used over the wide rivers of India. Sir John MacNeill designed the first in iron, and it was built in 1843 on the Dublin and Drogheda Railway with a span of eighty-four feet. I consider they will be among the most popular bridges of the future for longish spans.”
The engineer’s prediction has come true; for lattice bridges have undoubtedly been very widely adopted. We may suppose that he was advising the directors of a proposed railway, and we doubt not but that he carried the day.
A fine specimen of a lattice bridge is that across the Thames near Charing Cross, for the South-Eastern Railway. It has a total length of more than a quarter of a mile—viz., 1365 feet, and six of its nine spans are 154 feet wide. Two principal girders, fourteen feet deep, are connected transversely by other girders which carry the rails and project on the other side to support a footpath. The two main girders are nearly fifty feet apart and one weighs 190 tons.
The sides have upper and lower booms made of plate iron connected by perpendicular bars, between which are a couple of bars crossing each other diagonally at an angle of forty-five degrees, and fixed to the booms by bolts of five and seven inches in diameter.
The old Hungerford Bridge stood here previously, and its two piers of brickwork were used for the new bridge. Other piers are huge cylinders of cast iron ten feet across, but fourteen feet in diameter in the ground. Thus they are broadly based. These piers are filled with concrete and also brickwork, and are topped with bearing-blocks of granite. They are formed of plates of cast iron bolted together, and they were sunk into the ground many feet below high-water by combined forces; divers scooped out the mud and gravel and clay from within the cylinders; water was pumped out and heavy weights pressed them down. The piers became fixed on the London clay, but when filled were heavily weighted to drive them down again, andfinally they were forced to a depth of over sixty-two feet below high-water mark.
But before lattice girder bridges had become so popular, another class had come into use, and afford some splendid specimens of engineering skill. These are suspension bridges, and, perhaps of all kinds, they are the most picturesque. Their graceful sweeps and curves yield perhaps a more pleasing sight for the eye than the solid, rigid, straight lines of the girder bridges.
It was the genius of Thomas Telford which gave a great impetus to this class of bridge. Like Stephenson after him, he had to bridge the surging Menai Straits, but for a carriage road, not a line of rails; and at length, after various plans had been suggested and abandoned, he proposed the Suspension Bridge.
Now, in its simplest form, a suspension bridge has been known for ages. It is merely a pathway, or even a small movable car, suspended from a rope or ropes across a chasm. Ulloa describes suspension bridges built by the Peruvians in South America. Four stout cables span a river, and on these four is placed the platform of sticks and branches, while two other ropes connected with the platform are useful as hand rails. Such bridges sway with the wind and move with the passenger, but for light loads they appear to be perfectly safe.
In Telford’s Menai Bridge the carriage-way is hung from four huge chains or cables, each chain made up of four others, and passing over high piers. The chains are anchored on the landward side, sixty feet in pits, and grafted by iron frames to the rocks. The chains are so complex and so strong, that parts may be removed for repair without imperilling the safety of the structure. The length of the span thus gained is 560 feet, and it is 150 feet above high-water. The remainder of the bridge is composed of arches of stone, of 52½ feet span.
The piers from which the great span is suspended rise above the carriage-way fifty-two feet, and are toppedby blocks of cast-iron, which can move on rollers to permit the chains passing over them to expand and contract freely with the temperature. There are two carriage-roads, and also a footpath. The roads are separated by iron lattice work, which also gives them stability and decreases vibration.
THE CLIFTON BRIDGE.
THE CLIFTON BRIDGE.
In its day, this stupendous bridge was as great a wonder as its later companion over the same Straits—the Britannia Tubular. Six years were occupied in building, and it was opened in 1825. Why, then, did not Stephenson construct a similar bridge when, twenty years or so later, he had to solve a similar problem?
The answer is, that suspension bridges are not—or were not—considered sufficiently strong and rigid for railway work. In America, however, they have been used for this purpose; witness the famous Niagara Suspension Bridge, 2⅓ miles below the Falls, andwith a superb span of 822 feet; but American engineers appear to stiffen the roadway considerably, so as to distribute the stress of the rushing train over a large portion of the cable. The Niagara Bridge is not supported by plate-link chains, but by four immense wire cables, stretching from cliff to cliff over the roaring rapids. Four thousand distinct wires make up each cable, which pass over lofty piers, and from them hangs the railway by numerous rods.
THE BROOKLYN BRIDGE.
THE BROOKLYN BRIDGE.
Probably the famous Brooklyn Bridge is the largest suspension bridge in the world, even as the Clifton Suspension Bridge, in England, is one of the most interesting. The Brooklyn Bridge has a magnificent central span of 1595½ feet over the East River between Brooklyn and New York; further, there are two land spans of 930 feet, which, together with the approaches, make up a total of about a mile and a furlong. The cables, four in number, are each composed of 5000 steel wires, and measure 15¾ inches in diameter. They are anchored to solid stone structures at either end, measuring119 feet by 132 feet, and weighing 60,000 tons; while the towers from which the main span is suspended rise to the height of 276 feet, and are embedded in the ground 80 feet below high-water. It has been estimated that the weight hung between these towers is nearly 7000 tons.
The roadway of the bridge is divided into five thoroughfares. Those on the outer sides are for vehicles, and are 19 feet wide; the centre is for foot passengers, and is 15½ feet in width; while the two others are for tramway traffic. The bridge was opened in 1883, and affords a great triumph of engineering skill.
Much smaller, but none the less interesting, is the Suspension Bridge at Clifton. As far back as 1753, Alderman William Vick, of Bristol, left a sum of £1000 to build a bridge at Clifton. The sum was to lie at compound interest until £10,000 was reached. However, the money was increased by subscriptions, and in 1830 an Act of Parliament was obtained for its construction.
The work coming into the hands of Mr. I. K. Brunel, he designed a bridge of 702 feet span, and 250 feet above high-water. The piers and abutments were built, but lack of cash, which forms an obstacle to so many brilliant enterprises, stopped the progress of the bridge for nearly fourteen years.
Then it occurred that the Hungerford Suspension Bridge was to be removed to make way for the Charing Cross Railway Bridge, so the chains were purchased at a comparatively small cost, and the work at Clifton proceeded, and was finally completed.
Three chains on either side suspend long wrought-iron girders, which help to stiffen the platform; and cross girders between support the floor. The chains pass over rollers on the piers, and are ultimately anchored to plates bedded in brickwork abutting on rock. The platform is hung by upright rods from the chains, and hand-railing is used with lattice-work, to assist in rendering it rigid. The roadway, twenty feetwide, is made of creosoted wood, five inches thick, while the pathways on either side are made with wood half as thick. Between the piers the weight of the structure, including the chains, amounts to nearly a thousand tons.
In all these suspension bridges, however large, the principles are much the same. The platform, or roadway, is hung from chains or cables, which pass over piers and are anchored fast at the ends. Some are stiffened with girders and bracing to prevent undue undulation. The chains take a graceful and definite curve, that of the Menai Bridge dipping fifty-seven feet. The strain is the greatest at the lower part, and is increased, should the chain be drawn flatter over the same space. These bridges became widely adopted.
But there came a time when none of the bridges in vogue seemed to give what was required. A new principle was wanted. Where was it to be found?