Movement under a passenger train crawling over the bridge was very appreciable, but not startling. There had been introduced, from time to time, additional timbers and iron ties, with the object of rendering the spans more reliable, but leaving it somewhat difficult to determine the function of the several members. The bridge was, of course, reconstructed.
Fig. 86.
Fig. 87. Fig. 88.
Fig. 87. and Fig. 88.
An instance may here be cited showing how badly distorted a timber structure may become without actually falling. The bridge referred to consisted of three spans of 29 feet, each span having two trusses, between which ran a colliery tramroad, 1-foot 6-inch gauge; the corves running upon this, at 4 feet 6 inch centres, weighed, when full, about 10 cwt. each. The trusses were badly out of shape, the centre span having sagged 51⁄2 inches, with one truss of the same span nearly 10 inches out of line at the centre. This little bridge, of which some details are shown in [Figs. 86], [87, and 88], had been in use about twenty years.