ENGINEERING. 



315 



entire structure, and the two arms of the can- 

 tilever are different in length and in details of 

 construction. The cantilever type of high- 

 level bridges is a development of the use of 

 cast-steel, which combines with rigidity a ten- 

 sile elasticity that enables it to resist lateral 

 strains to a certain degree. Like the suspen- 

 sion-bridge, the cantilever span can be carried 

 over places where, as in the Niagara chasm, it 

 is impossible to erect temporary supports. The 

 two gigantic steel towers which bear up the 

 cantilevers of the Niagara bridge are 132 feet 

 high, and rest on stone piers 39 feet high. 

 They are composed of four columns of plates 

 and angles riveted together, braced with hori- 

 zontal struts and ties. They converge upward 

 with a batter of 1 in 24 in the direction of the 

 length of the bridge and 1 in 8 at right angles 

 to the middle line of the bridge. The canti- 

 levers are each 393 feet in length. A space of 

 120 feet between the river ends of the canti- 

 levers is spanned by a girder resting on the 

 extremities of the arms. The total length of 

 the bridge is 910 feet between the centers of 

 the anchorage-piers. The clear span between 

 the towers is 470 feet. The height of the 

 bridge is 239 feet from the surface of the river 

 to the rail. The cantilevers are composed of 

 two trusses, 28 feet apart, having a depth of 

 56 feet at the towers, 26 feet at the extremi- 

 ties of the river arms, and 21 feet at the shore 

 ends. The materials used in the bridge are 

 steel and wrought-iron, the former for the tow- 

 ers and the lower chords, center posts, and all 

 the pins, and the latter for all the tension mem- 

 bers. The steel pins connecting the members 

 fit into the bored holes with the utmost ac- 

 curacy. The lower chords and center posts 

 are latticed channel-plates. The upper chords 

 are heavy eye-bars. A compression member 

 is packed between the chords of the shore 

 arms. The shore ends of the beams are an- 

 chored to masonry abutments by short links, 

 which serve also as expansion-joints. Joints 

 are provided also at the connection of the in- 

 termediate span with the river ends, to allow 

 for contraction and expansion due to changes 

 of temperature. The floor-beams are wrought- 

 iron plates and angles, 4 feet deep, riveted be- 

 tween the vertical pos v ,s. On these rest four 

 lines of stringers, consisting of plate-girders 2 

 feet deep. The width of the floor is 32 feet, a 

 plank walk and iron railing at the side of the 

 tracks being supported by the white-oak ties, 

 one half of which project beyond the tracks 

 for the purpose. Each column of the towers 

 stands on a limestone pier, 12 feet square at 

 the top and battering 1 in 24. The piers are 

 connected by walls 3| feet wide at top. The 

 courses of the piers are 2 feet deep. The 

 foundations are a solidified mass of bowlders, 

 beton, and cement, 20 by 45 feet and 8 feet 

 deep under each pair of piers. The anchorage- 

 piers are 11 by 37 feet under the coping, and 

 consist of blocks of masonry, each measuring 

 460 cubic yards and weighing 1,000 tons, raised 



upon 12 iron plate-girders 2^ feet deep and 36 

 feet long, resting in turn on 18 15-inch I-beams 

 through which the anchorage-rods pass in such 

 a way that the pressure is distributed evenly 

 over the entire mass of masonry. The maxi- 

 mum uplifting force of the cantilevers is 678,- 

 000 pounds, or only about one third of the 

 weight of the piers. 



After the towers were built, the shore arms 

 were constructed by the aid of temporary 

 structures, in the usual way. After they were 

 completed and attached to the anchorages, the 

 river arms were built out over the river, one 

 panel at a time, by means of huge traveling 

 steam-derricks. When each panel was con- 

 structed and braced, the traveler was moved 

 forward and the next panel erected. The in- 

 termediate 120-foot span was specially designed 

 with bottom compi'cssion members so that it 

 also could be built out from the end of each 

 arm by the aid of temporary stays, which 

 were removed when the two halves of the 

 girder were fitted together in the middle. 



The bridge is designed to bear a running 

 load of a ton per lineal foot, that being one 

 fifth of the calculated ultimate resistance, and 

 for a wind pressure of thirty pounds per square 

 foot on twice the exposed face of the truss, 

 floor, and train. 



Forth Railway-Bridge. The completion of the 

 Niagara cantilever bridge lends interest to a 

 description of the one over the river Forth at 

 Queens Ferry, in Scotland, which was begun 

 in 1883. The engineers of Great Britain, to 

 whom the development Of the cantilever prin- 

 ciple is due, have never taken kindly to the 

 suspension principle, just as they are in gen- 

 eral skeptical of the stability of the lighter 

 structures which American engineers design 

 for equal stresses. Yet, after condemning 

 the principle for a whole generation, while the 

 Niagara suspension-bridge stood as a practical 

 demonstration of its soundness, at last Sir 

 Thomas Bouch adopted the American idea in 

 his design for the projected bridge over the 

 Forth. The river was to be bridged by two 

 suspension spans, with towers nearly 600 feet 

 high, one on each bank and two on the island 

 of Inchgarvie in the middle of the estuary. 

 The foundations were already dug, when the 

 Tay bridge disaster first brought to the knowl- 

 edge of engineers facts relating to the intensity 

 of wind-strains which meteorologists had al- 

 ready published to the world. The designs 

 for the suspension-bridge, whose author was 

 the designer also of the collapsed Tay struct- 

 ure, were discarded. Fowler and Baker, the 

 new engineers, drew plans for a cantilever 

 truss double-span bridge, all of steel, which 

 will be the most stupendous structure of its 

 kind. The material is tested for an ultimate 

 resistance of thirty tons per square inch in ten- 

 sion, and thirty-four tons in compression, and 

 the structure is planned to sustain four times 

 the combined strain of a wind pressure of 

 fifty-six pounds to the square foot and a maxi- 



