The Forth Bridge/Temporary Work in Connection with the Erection
Temporary Work in Connection with the Erection.
If the proud boast is perfectly justified that in building the two 1710 ft. spans across the Forth, and erecting the 11,600 tons of steel massed therein, without having placed a single stick of timber into the river, it is yet equally certain that this mode of erection entails an immense amount of auxiliary and temporary work, which is both costly and wasteful with regard to time. There were many days and weeks lost through waiting for crossings or junctions to be built up and completed, and although much of this may be said to be due to the novel character of the work, yet it is a point which must not be lost sight of. Elsewhere, it has been stated that owing to the general batter of the sides of the structure, and owing to all of its members being out of the perpendicular, as well as the horizontal, they had a natural tendency to either fall together, or else apart, and in both cases apt to be both out of the right direction. Thus in building out a pair of struts or ties, so soon as a length of 30 to 40 ft. had been built, it became necessary to put up temporary girders to hold them apart or keep them together, and a large amount of dangerous work had to be done before a further section of work could be built. The members themselves, even when only bolted up, were often so stiff that hydraulic rams had to be used to force them apart or draw them together. In other cases wedges of hard wood and union screws were able to deal with them. In looking at the illustrations and plates showing various phases of the erection, this feature will immediately attract attention, and in many cases it must be difficult for any one except those conversant with the structure to distinguish between temporary and permanent work.
For reasons explained above, and easily understood, it was not possible to fix the wind-bracing so close up to the new work that temporary appliances could be dispensed with, and therefore an immense number of lattice-girders, some heavy, some light, had to be used to allow the permanent members to be corrected, and when corrected held in position. In the bottom members, especially near the piers, where they were some 120 ft. apart, centre to centre, the temporary girders naturally had to be very strong and heavy, but after the completion of three bays, timbers were in most cases sufficiently strong to take all resistance, and this much simplified matters, for timber is both light in weight and easy to cut and shape to requirements. Thus in the bottom members alone, at every vertical tie and at every junction, the necessary corrections required the placing of temporary struts and wire rope ties, and in most cases when finally fixing the horizontal wind-bracing girders, similar girders had to be placed to fix the position of the member. A repetition of this kind of work with every tie and strut, in bottom members and top members, at every vertical tie, and at every intersection, made up an amount of work of which no visible trace is left, yet which was as real in its day as any which helped to build up the mighty fabric.