Engineering Wonders of the World
Volume I

 THE STORY OF THE FORTH BRIDGE. 331 DIAGRAM SHOWING RECTANGULAR LOWER BED-PLATES ON PIERS, AND KEY-PLATES (A A A A, LLLL, C C C) PROJECTING FROM UPPER BED-PLATES. The shaded areas indicate (on a greatly exaggerated scale) the spaces left between the Key-plates and the depressions in the lower Bed-plates, to allow for movements of the Towers and Cantilevers. Preparing the Giant Tubes. the steelwork, were finished, shops at Queensferry were fully occupied in rolling, planing, and shaping the plates, beams, and ties of the various bridge members. For the enormous 12-foot diameter columns and bottom mem- bers between skewbacks, plates 16 feet long and about 41 feet wide were bent to the correct curve by special rolls, tem- porarily assembled, and drilled with all their rivet holes. These gigantic tubes are made up of ten “ strakes ” (corresponding to the staves of a cask) of plates laid inside and outside alternately round the circumference, and “ breaking joint ” every eight feet—all plates made flat or butt-end joints with their two neighbours in the same “ strake,” these butts being covered inside and out by short plates. Internally the tubes are strengthened by longitudinal beams riveted to the plates, and supported inside by circular beams and diaphragms eight feet apart. The 8-foot diagonal tower struts are flattened on both sides, for convenience in making the joints where they cross one another. From each skewback, based on an upper bed-plate, spring five tubular and four lattice girders, which make it a some- what complicated part of As soon as the skewbacks the horizontal members join- Erection of the Steelwork begun. ing each skewback to its three neighbours were built on a platform resting on the piers. Then came the erection of the columns and their struts, and the building out of the lower booms and struts of the first bay. At this period each skewback some- what resembled a huge expanded hand, each finger the commencement of a huge tube, the thumb one - half of a horizontal member between piers. Up to a height of 60 feet above the main staging the parts of the various vertical members were placed with the aid of derrick cranes. When that point had been reached preparations were made for the construction of platforms which should be raised gradually up the col- umns as the work proceeded. There were two platforms to each tower, running north and south—for 190 feet in the case of the Fife and Queensferry, and 350 feet in that of the Inchgarvie. The platforms rested on four longitudinal girders, themselves supported by two cross beams passing through openings in the top plates of the rising columns. These beams rested on shorter beams inside the columns, under which were hydraulic presses attached to lower inside beams. The two last mentioned sets of beams were supported by removable pins. A “lift” of the platform was 16 feet, Movable Platforms for the Towers.