A Treatise On The Principles And Practice Of Dock Engineering

438 DOCK ENGINEERING. The kentledge is deposited in a spécial compartment called the ballast box, arranged at the extremity of the tail end of the bridge, commonly below the floor level, tliough the space between the webs of box girders is also available for the purpose. The interior surfaces of the ballast box should be washed over with liquid Portland cement, and the interstices between the blocks run with grout, to prevent corrosion. The counterpoise has occasionally been disposed as an ornamental feature, and a massive balustrade or an entrance arch in cast iron may be cited as illustrations. Such artistic pretensions are, however, in question- able taste in situations where the functions of a bridge are strictly utilitarian. The arnount of ballast required to give the requisite stability depends upon the ratio which the length of the tail bears to the length of the bridge forward of the pivot or point of support. A bridge with the pivot exactly at its centre, as is generally the case where two parallel openings have to be spanned, and also for some single openings, as at Naburn on the Ouse, near York, needs no counterweight. In the majority of cases a shorter tail is the rule for two reasons—first, on the ground of expense, for the structure of a bridge is far costlier than even a much greater dead weight of ballast; and secondly, there is less occupation of valuable quay space by a counter- balanced tail. In fact, at some sites, a short tail is absolutely unavoidable. Tlie Whitehaven swing bridge has a tail only one-fourth of the total length, or one-third of the length of the forward portion. In a number of cases the proportion is one-half of the forward portion, while at Marseiiles it is three-fifths. In an interesting paper on the subject, Mr. C. F. Findlay* demonstrates, by an application of the calculus, that if the cost per ton of the bridge structure be five times the cost of the kentledge, for any bridge not of extremely minute span, the length of tail should be approximately one- third of the length of the other section, if the most economical proportion is to be observed. Bridges which depend for their stability upon the downward reaction of an inverted roller path do not of necessity require ballasting, if the path itself be secure. Ballast being an unremunerative form of weight, an attempt has been made, in one case at least, to balance a bridge by placing the hydraulic rams, which work it, within the ballast box. This method necessitates a hollow pivot for the transmission of the water pressure. In the case of a small bridge, the paving of the short end with stone setts, and the long end with wood blocks, has been found an adéquate solution of the difficulty. Setting Apparatus.—For obvious reasons it is not advisable to allow a bridge to rest upon its pivot longer than is required for the operation of turning. While undergoing the stress due to moving loads, the structure is preferably supported on some independent base. Before the introduction °f hydraulic power, when the usual practice was to carry the bulk of the. * Findlay on “ The Design of Movable Bridges,” Min. Proc. L.E.S., vol. ii.