100. For the roofing of train sheds, two distinct systems are in vogue. The first and more expensive method is by means of a curved steel roof, supported on high arched trusses and covering the entire area, without intermediate supports.

The second method is to use short flat trusses, supported by one or more longitudinal rows of interior columns, as in Fig. 238.*

The first method has the advantage of allowing the tracks and platforms to be arranged in any way, involves no waste floor space, avoids any obstruction from intermediate columns, and presents an imposing interior. On the other hand, the long-span roof trusses are heavier and costlier than shorter ones supported on columns and covering the same area. It is difficult to keep the skylights and upper steelwork clean, and the curved roof sheathing is very expensive. The second method requires less steel for a given length and width, the roof may be more quickly and cheaply erected, and the shed is more easily lighted, and it is easier to keep the skylights and steel work clean. Such roofs also lend themselves about as well to an attractive exterior treatment.

*For Description see Engineering Record of March 19, 1898.

Fig. 238.   Train Shed at Providence, R. I.

Fig. 238. - Train Shed at Providence, R. I.

Notable examples of train sheds roofed by one span, are those at Pittsburg, Jersey City and Philadelphia for the Pennsylvania R. R.; at Philadelphia, for the Philadelphia & Reading R. R.; at Buffalo, for the N. Y. C. & H. R. R. R.; at Cleveland, for the Lake Shore R. R., and in the old depot of the Providence R. R. at Boston.

Examples of very large train sheds roofed on the second method are the Union Depot at St. Louis, the South Terminal Station at Boston (described in the Engineering Record of Jan. 14 and 21, 1899), and the station of the Pennsylvania Railroad at Camden, N. J., described in the issue of Sept. 14, 1901.