Engineering Wonders of the World
Volume I

THE STORY OF THE FORTH BRIDGE. 335 r Details about their Ex- tremities. box was filled the Inchgarvie cantilevers from their partners of Fife and Queensferry. Four of the cantilevers terminate in ver- tical end posts. The fixed cantilevers, resting on the ends of the approach viaducts, have, instead of posts, large boxes, eight feet long in the direction of the bridge, through which arch-shaped openings were cut to allow trains to pass. Each with iron scrap set in asphalt, to give 1,000 tons of dead weight in addition to that of the steelwork of the cantilever, and so to counteract any tendency of the Fife or Queensferry towers to tilt, should two trains meet at the extreme end of the free cantilever on the Inchgarvie side. As an additional precaution the fixed ends—fixed in one respect only, as they could move longitudinally on rollers—were held down by steelwork projecting from the masonry of the last piers of the approaches, in such a way as not to interfere with longitudinal expansion and contraction. Since such, a check could not be applied to the Inchgarvie cantilevers, their tower was given a length much greater than that of the other two towers. Turning to a consideration of the four “ free ” cantilevers, we find that their end posts all take the form of a hollow box stand- ing on end, 40 feet high, 3 feet wide, 4J feet deep, and lacking the top end and the lid, as it were, so that the end posts of the central girders might enter. At the Fife and Queensferry ends the central girders rested on bearings at the bottom of their respective cantilever boxes, which permitted a slight lateral but no longitudinal movement. At the Inchgarvie ends the top members of the girders termi- nated in a projection, to the under-side of which was attached a huge steel cup, concave side downwards. On the bottom of the cor- responding cantilever box was a similar cup ; and between the two cups stood a massive steel rocking-post, weighing nearly nine tons, with hemispherical ends to fit the cups. This arrangement caused the bottom of the Inch- garvie cantilever end posts to support the top members of the central girders in a manner that allowed of longitudinal and circular movement within well-defined limits. The cranes that had travelled top members of the cantilevers mantled when they reached replaced by lighter machines suited for the task of building out the central girders from both ends as temporary con- tinuations of the cantilevers. Each girder has eight “ bays,” a straight bottom boom, and a curved top boom. The end posts of the girder were temporarily attached to the top member of its cantilever by strong plate-ties, riveted and bolted on ; while the bottom of the girder and cantilever posts were separated by steel wedges, which could be pushed further in or withdrawn by hydraulic presses. The actual erection of the parts of a girder was a comparatively simple matter ; though to make the junction near the centre proved a somewhat difficult ness, owing to the constant changes of perature. The bottom booms were joined first, time when the thermometer gave a reading of 55 degrees Fahrenheit, the gaps being filled in with cover - plates carefully prepared beforehand. Joining- There remained the V-shaped openings in the upper booms, which widened or decreased as the temperature rose or fell, owing to alterations of the curvature of the bottom boom. It was necessary to close the top booms at as low a temperature as possible, to put them in a state of compression at average temperatures, just as the bottom boom would be in one of tension—that is, the down the were dis- ends, and Building the Central Girders. A Clever Device. busi- tem- at a