The floor of the viaduct is carried by fish-bellied cross-girders, as shown in Fig. 34, spaced 5 feet apart, as in Fig. 33. The section of these girders is shown in Figs. 44, 52, and 54. The connection of the cross-girder with the main girder is shown in Fig. 51, the plate stiffener of the main girder being stopped on the upper flange of the cross-girder and riveted to it as shown. The practical continuity of these plate stiffeners, as between the upper and lower flanges of the main girder, is thus secured on both sides, and the entire combination attains the necessary rigidity. In larger and more important structures the cross-girders and cantilevers for the footway would probably be in one, and supported below (or in some cases above) the bottom flange or tension member of the main girder.

Fig. 51. Scale ¾ inch = 1 foot.

Upon these cross-girders are laid bent floor-plates riveted to the upper flange of the cross-girder as shown, the plates being concave in this case, and not convex as in the older-fashioned buckled plate. The joints of the floor-plates longitudinally are covered by tee-steels, which are continuations of the web stiffeners of the cross-girders as shown.

Fig. 52.

Fig. 53. Scale ¾ inch = 1 foot.

The junction of the floor-plates of two adjacent spans, where the main girders meet over the piers or cylinders, is shown in Fig. 44.

This may be considered as a "fixed" end for expansion purposes, while the "free" end, for expansion due to change of temperature, is shown in section and plan in Figs. 52,53, the space between the plates being covered by a 12" X ⅝" strip, riveted on one side, and bolted on the other, in slotted holes so as to allow a certain amount of freedom of movement. But the arrangements for expansion and contraction in a structure of these moderate dimensions are not of the same importance as in bridge-work of a larger class.

The floor-plates are flushed up to a level surface by concrete filling, as shown in Fig. 54, and upon the surface thus formed wood-block paving is laid in the usual manner, upon a layer of asphalte.

The rails are of steel of flat-footed section, weighing 75 lbs. per yard, and are laid on cross-sleepers embedded in the concrete, the space being laid between them in wood blocks as shown, the nature of the traffic generally not allowing of the rails standing up above the general level of the roadway. Surface water is disposed of by drainage outlets at intervals, discharging beneath the bridge into the sea below.

Fig. 54. Scale ½ inch = 1 foot.

A trench is laid in the centre of the roadway with cast-iron cover-plates, to receive pipe-work of comparatively small diameter, such as hydraulic or small gas or water pipes, or electric mains. Any other pipes of larger dimension, such as sewage or large water-pipes, would require to be specially provided for, and would probably be slung under the cross-girders, or, as in some cases, carried alongside the main girder next the footway, and cased in with timber framework and boarding, with occasional doors for examination or repairs.

In cases such as the present, where the roadway is carried between the main girders (which form a parapet), and mixed wheel traffic has to be provided for, it is necessary to protect the plate stiffeners of the main plate girders, or the tension or compression members of a braced structure, from blows which might be received from the hubs or other projections of a wheeled vehicle. To effect this the wheeled traffic is kept off the girderwork by the cast-iron curbs shown in section in Fig. 51, and in further detail in Figs. 55, 56, 57, and 58. Fig. 55 is an enlarged section of the curb, with its capping running back to the plate-web of the main girder, thus covering in the void between the curb and girder. The joints between this cover and the plate-web and stiffeners may be caulked with oakum and pitch to prevent wet from getting in.

Fig. 55. Scale 1 inch = 1 foot.

Fig. 56. Scale 1 inch = 1 foot.

Fig. 57. Scale 1 inch = 1 foot.

Fig. 58. Scale 1 inch = 1 foot.

The projection of the curb beyond the face of the stiffeners is in this case 7 inches. A recent metropolitan example under somewhat similar conditions gives 12 to 13 inches for a similar dimension.

The curb may also be constructed of timber, with an angle-iron at the edge to take the rub of the wheels.

Fig. 55 is a section on the line H,H, in Fig. 56, and Fig. 57 is a sectional plan on the line GG in Fig. 55, while Fig. 58 is a part plan showing connection with the plate stiffener, and Fig. 56 is a part elevation showing the joint in the curb at every stiffener, and over a cross-girder.

Girderwork for Machine or Boiler Shops, Steel Foundries, etc.: Traveller Girders. - An important application of steel girders, whether of the simple rolled-joist, single-web, double-web, or box type, is found in the longitudinal girders forming the roadway or railroad for overhead travelling cranes.