The Viaduct Works' Handbook

22 The BLACKFRIARS Bridge, for carrying the London, Chatham, and Dover Railway over the Thames in London, may be selected from the foregoing list, as an example of trellis girder bridge top, a perspective view of a portion of which is given on the opposite page; was designed by Joseph Cubitt, Esq., C.E., and manufactured and erected by Messrs. Kennard; consists of five spans for a quadruple fine of way carried on the bottom; each span contains three main girders, namely, one centre and one each side, these latter forming the parapets ; all three are braced together by light overhead arches. The floor is composed of 7-inch plain sheet iron, which rests upon transverse girders placed about 4 feet apart, and attached to the lower portion of the main girders ; between these transverse girders, under each rail, is placed a light longitudinal girder, the top being even with them; the whole surface is protected from weather by a thin coat of asphalte, upon which the sleepers and rails are laid. The main girders are, for uniformity, all of one depth, namely, 14 feet; the centre girder of middle span is built with plates and angle-iron, forming tension and compression bars of the semi- tubular section 4 feet 6 inches in width, and about 2 feet deep, the top plates varying in number and thickness, but having three feathers for attachment of the struts and ties forming the trellis bars ; these bars are made of trough-iron where they form struts, and their section varied to suit the strain they bear ; they are used in pairs, and are also stiffened by lattice bars and distance pieces attached to them, so as to connect the pairs. The ties are of plain bars, varying in section according to the strain they bear. Each main girder is attached to a fixed bearing at one end, the other being free to move on rollers for expansion and contraction. The piers, from high water downwards, are all of masonry built up inside iron caissons 17 feet diameter, and sunk into the bed of the river ; on top of these are placed clusters of cast-iron ornamental pillars, which support the bed plates and rollers upon which the superstructure rests. The approximate weight of ironwork in the centre span is as follows :—