There are so many descriptions of roof-principals used in railway stations that it would be impossible here to introduce more than a few examples. [Figs. 387 to 405] illustrate by diagram sketches a series of types taken from actual practice. [Fig. 406] gives more in detail the particulars of the roof-principal of 60 feet span, [Fig. 392]. As will be noted from [Fig. 406], the width of 120 feet between the walls is divided into two spans of 60 feet each, the ends of the principals in the centre of the 120 feet being carried on arched wrought-iron girders of 48 feet span, supported on strong ornamental cast-iron columns placed at 48-foot centres. The rain-water from the large centre gutter is taken down inside the columns and conveyed away to drainage pipes laid down for the purpose. The 60-foot principal above
described forms a very strong roof, and is light in cost and maintenance. The weight of ironwork, both wrought and cast, in the principals, arched wrought-iron girders, cast-iron columns, centre gutters, etc., is only 0·51 of a ton per square (of 100 square feet) of area covered. For comparison, the weight of ironwork in the roof, [Fig. 402], of 198 feet span is 1·42 ton per square of area covered; and of the roof, [Fig. 404], of 210 feet span, is 2·07 tons per square.
This increase in weight per square as the spans go on increasing results, not only in a much larger outlay for original construction, but entails also a proportionally heavier expenditure for maintenance and painting. The item of painting alone is an expensive one in all iron-roof work, and must be attended to regularly for the proper protection and appearance of the ironwork. With the smaller spans, the roof-trusses form very convenient supports for painters’ scaffolding or planking, but with the very large spans the greater height and the form of the roof-principals render specially designed scaffolding and appliances necessary for the painting and repairs.
Doubtless there is something very attractive about a large span roof, its bold outline stretching from side to side of a wide covered area imparts an imposing effect which cannot be claimed for smaller or more moderate spans; but where roofs are constructed for purely utilitarian purposes it becomes a question worthy of grave consideration whether a series of smaller spans would not provide the same practical benefits as would be obtained from one very large span. Upon referring to the typical sketch of a terminal station, [Fig. 373], it will be seen that the total width from inside to inside of main walls is 240 feet. The lines and platforms are so arranged that by placing rows of columns at A, A, B, B, and C, C, the entire width may be divided out into four spans of 60 feet; or, if preferred, a row of columns at B, B may be adopted, resulting in two spans of 120 feet, or the entire width may be included in one large span of 240 feet. Any one of the three arrangements will provide an effectual roof-covering, and the selection must be decided by the cost or expediency.
Another way to avoid the introduction of large span-roof principals, and to preserve the covered area free from intervening columns, is to erect strong truss-girders extending across at right angles from the main walls. These truss-girders are placed at