The Forth Bridge/Expansion Joints in Rails
Expansion Joints in Rails.
These are all arranged on the same principle, though of different lengths. In the approach viaduct span, and at the fixed ends of the central girders, the movements are so insignificant that a very short rail-joint suffices to regulate the lateral displacement of the points due to contraction; but, at the sliding ends of the central girders, where provision is made for a longitudinal movement of two feet, and at the ends of the fixed cantilevers, where half that length is provided, the arrangement is somewhat more complicated. Without going more into detail, it will suffice to say that the long rail-tongue is made an absolute fixture to the rail-trough in which its uncut end rests. Its pointed end, cut at an angle of 1 in 63, projects into the opposite trough and rests on a plate fixed therein, laying up close to the backing rail, which is bent away at an angle of 1 in 63 from the point of the tongue. The outside of the flange of the rail-tongue is cut in long steps at the same angle of 1 in 63, and is held down by draw-washers to the plate, which forms one piece with the backing-rail, but in such manner that it can slide in and out with the expansion and contraction. In doing so, however, by means of the sloping steps it draws the lower plate and with it the backing-rail always close up to the joint, which thus retains its position relative to the centre line of the bridge and thus keeps the gauge correct. The contrivance is very ingenious and unfailing in its action. (See Fig. 127.) The wind fence on each side of the viaduct is 4 ft. 6 in. high, and of close lattice work, and it is crowned by a handsome teak handrail of substantial appearance.