The New York Rapid-transit Subway

Proceedings.] PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. 11 special members tending to delay erection ; and thirdly, columns at long intervals introduced a possible source of danger, in that a derailed train might strike one of the columns and wreck not only the train but also the structure, and possibly the street. Separate longitudinal girders, each supported on two columns, removed the objection to longitudinal girders on the score of special members and variations in temperature, but not the danger resulting from collision. In fact, this danger was rather increased, as the knocking away of a single column would be certain to result in the wrecking of the roof. The placing of the columns between each pair of tracks and at the same interval as the roof-girders, without any longitudinal girders, seemed to be the best solution. It involved the minimum amount of steel, the smallest cost, the lightest individual members, the possibility of cutting the cross beams in lengths spanning either one, two, three, or four tracks, as might be most desirable, the bringing of the columns so close together that a derailed train would not have distance in which to get so far from the track as to strike one of them a direct blow, and the introduction of frequent supports for trackmen to take hold of when passed by trains. For these reasons frequent columns were adopted. In order to make the walls thin and of uniform width, so as to leave as much space outside the subway-structure for sewers and house-vaults, X beams were built in the walls to carry the roof and withstand lateral thrust. The columns and wall-beams were usually each of uniform section. This enabled a great reduction to be made in the number of different kinds of members required, and introduced the principle of interchangeable members. Although the New York Subway is regarded as an extreme type of shallow construction, and such was, as explained above, the basic principle of its design, as a matter of fact, shallow construction covers less than one-half of the route. The actual form used at any point was governed by the local topography and con- ditions. Altliough a standard design was adopted as preferable to others, there was no hesitation in departing from it if necessity required, or compensating advantages were to be gained. In some places the line was placed so deep that tunnelling by various methods was resorted to, while in other places where the depth to rail- level was not quite enough for tunnelling, the structure was made with an arched roof wherever the number of tracks did not exceed three. At the upper end of the island, for the sake of economy, a still more radical departure was made from the adopted type, and an elevated structure instead of a subway was constructed ; and on the West Side line, crossing the Manhattan Valley, in order