The Forth Bridge/The Preliminary Tests

by Wilhelm Westhofen

The Preliminary Tests.

On January 21, 1890, two trains entered upon the bridge, side by side from the south end, and composed as follows:—Each train had two locomotives of 72 tons each at the head, followed by 50 waggons weighing fully 13 tons 10 cwt. each, and one engine at the rear of 72 tons. Total weight of each train, 900 tons; total weight on bridge, 1800 tons; total length of train when close-buffered, 998 ft. 6 in.; the same, when open or loose buffered, 1040 ft.

The train was moved slowly until the two engines in front were three-quarters through the central girder connecting Inchgarvie with Queensferry, the rear engine being about the centre of the Queensferry Tower. This is considered the most unfavourable load for the Queensferry north cantilever. In that position the following observations were made: The vertical columns of the central tower in Queensferry were drawn north to the extent of 1+³⁄₈ in. The end of the Queensferry north cantilever deflected 5 in., the end of the Inchgarvie south cantilever deflected 1+⁵⁄₈ in., the end of the Queensferry south cantilever (at that time not fully loaded by nearly 200 tons) rose to the full amount of play existing, namely, ³⁄₁₆ of an inch. The Queensferry south cantilever in itself took an upward deflection of about 1+¹⁄₈ in.

The train then moved forward until the two front engines of the trains were three-quarters through the north central girder, the most unfavourable condition of loading for the Inchgarvie north cantilever. In this position the vertical columns of the Inchgarvie central tower were drawn north to the extent of 1+¹⁄₄ in., while the Fife central tower was drawn ¹⁄₂ in. to the south. The vertical downward deflection of the end of the Inchgarvie north cantilever was 6+⁷⁄₈ in., and the same for the end of the Fife south cantilever was 2+¹⁄₂ in., while the end of the Inchgarvie south cantilever rose to the extent of 3+¹⁄₂ in.

It is not necessary in the face of further trials by the Board of Trade, to say more than that all these movements were well within the calculated amounts.