Page:Cassier's Magazine Volume XV.djvu/25

Rh water, the steel viaducts were built upon them, and were raised by hydraulic power, stage by stage, until they reached their final height of 150 feet above high-water level. As the viaducts were thus raised in stages, the granite piers were built underneath, this operation being so successful that not a hitch occurred during the carrying out of this work. The weight raised in this operation in the case of the longer viaduct was well-nigh 2000 tons of steelwork.

After the main piers had been completed the erection of the superstructure began at three points, namely, North Queensferry, Inchgarvie and South Queensferry. The first portion of the steelwork was one of the most complicated parts in the whole, and is technically known as the skewback. The skewbacks rest immediately over the piers on steel bedplates, and some idea may be formed of their intricacy when it is known that in these skewbacks converge five different tubes and five different sets of wind-bracing in the form of large girders.

The skewback proper, with its connection to these tubes and girders, although neither very long nor very high, weighs somewhere about 500 tons. While in one sense they are complicated, in another sense they are not, because the design is such that in every hole the rivet intended for it could be put, although in some cases special means had to be taken to effect this. A considerable portion of the riveting in these skewbacks was carried out by small riveting machines, capable of being easily lifted by one hand, but strong enough to withstand a water pressure of from two to three tons per square inch. The smallness of the machine was necessary to allow it to go into the small spaces in the skewback, and it says a great deal for the design, as well as for the system adopted, that every rivet hole was accurately filled by a good rivet as originally intended.

After the skewbacks and the steelwork immediately over the main steel piers had been completed up to a height of about 50 feet, temporary platforms were erected, stretching from column to column in connection with hydraulic lifting arrangements within each of the four vertical steel columns. These platforms were raised in stages from the vertical columns which supported them until they reached the final height of 350 feet above high-water level. As the platform was raised the column was built up, and after the platform reached its final height the upper portions of the structure were built upon the platform itself, until they were completed sufficiently far to enable them to sustain their own weight. While the platform was being raised, these main columns were rivetted by special riveting machines attached underneath.

The riveting machines were of a double character, having a cylinder inside the column as well as one outside. These cylinders were secured to girders running longitudinally with the column, and were raised and lowered by hydraulic power. They had also a circular motion round the column, so that every rivet in the full circumference of the column could be put in by the same power. These machines were so effective that as many as 800 rivets could be driven in a shift of nine hours. The machines were raised with the platform as it rose.

One of the main difficulties in connection with the working of these machines was the supply of steel rivets. Hitherto rivets had either been heated in small hand-blown fires or coal furnaces; but small hand-blown fires could not give the supply of rivets required, and as room for large coal furnaces in a convenient position to the riveting machines was not available, experiments were instituted with a view to adopting oil for the purpose of heating. These experiments were entirely satisfactory, and it was found that a small furnace, 2 feet 6 inches long by 18 inches square, was sufficient to heat easily all the rivets required for the machine. Since then heating rivets by oil has been very largely adopted in the various iron industries.

After the main piers had been carried to their full height, the erection of the cantilevers was immediately taken in