A Treatise On The Principles And Practice Of Dock Engineering

410 DOCK ENGINEERING. in the second case, both the effective spån and the length of tail are increased. This considération is not without importance on grounds of economy alone. In traversing bridges the effective length is measured between the bearings, and these may be practically at the edge of the coping. 2. The side recesses of swing bridges occupy a large extent of valuable quay frontage—much more than other kinds of bridge—and in the case of wide passages this leads to the necessity for side walls of considérable length, with a corresponding increase in cost of construction, apart from any question of intrenching upon the area of dock accommodation. On the whole, the balance of technical opinion, as evidenced by practice, inclines toward the employment of swing bridges in preference to other types, in so far as heavy trafiic, at any rate, is concerned. There is one case in which a swing bridge offers signal advantages. When two waterways of about equal width lie side by side with an island between, as in fig. 456, a swing bridge may be arranged symmetrically upon a central pivot, so that each wing acts as a counterbalance to the other. In this way the length of bridge necessary for closing the opening is redueed to a minimum, and the rotation of the wings neutralises the effect of the wind pressure upon the surface of the bridge. This last statement, though theoretically convincing, is only partially true in practice, for, as has already been pointed out, the wind does not exert a uniform pressure over large areas. On the contrary, it is given to surging and eddying; consequently it is quite possible that, however symmetrically disposed the wings of a bridge may be, the pressure on one will exceed that on the other. Such was proved to be the case by an incident at Goole, where a bridge over the River Ouse spanning two openings, each 100 feet wide, broke loose in a severe gale and swung violently back- wards and forwards, describing about one-third of a circle each time. The converse of a single bridge spanning two openings is that of a bridge in two leaves spanning a single opening, and we now enter into the merits of single-leaf and double-leaved bridges respectively. Single-leaf versus Double-leaf Bridges.— The relative advantages and dis- advantages of single and double leaves, in the cases of traversing, bascule, and swing bridges, may be summed up as follows :— 1. The depth of a single-leaf bridge is necessarily greater than that of a double-leaf bridge for the same span. If s be the span, w the weight per foot run, t the maximum permissive tension in the top flange—all fixed values— we have, by taking moments about the bottom of the bridge at the side of span— (a) in the case of a single leaf, , ws2 d' = U (b) in the case of double leaves, d2 = ^■, so that the depth (d) of the bridge, treated while in motion entirely as a cantilever, needs to be four times as great in the first case as in the second.