“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 instead of 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, and finally 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.