The New York Rapid-transit Subway

14 PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. [Minutes of As the subway for the greater part of New York rested on a foundation of rock or sand, it was not necessary to use an invert other than a flat concrete floor. There were places, however, where the bottom of the subway was beneath standing water sufficient to produce a decided upward hydrostatic pressure. To resist this upward pressure, beams were embedded in the floor, extending transversely from wall to wall. It will be seen, therefore, that in the steel framed structure all stresses, both vertically downward from the overhead loading, vertically upward from hydrostatic pressure, or laterally from the thrust of the earth at the sides, were taken by steel beams and columns. The floor, walls, and roof were always made of concrete, rather than brick, as being both better and cheaper. To have filled in all the spaces with brick and to have ensured a complete embedding of the beams, would have been a difficult matter in the contracted space for working. By using the fluid concrete it was made certain that the beams were completely embedded and that all work was tight. Concrete was found to be more economical than brick, as it could be put in place by less skilled, and consequently less expensive, labour ; and further, as both stone and sand were encountered in the construction, much of the material used for concrete was taken from the excavation. It was found in practice not to be necessary to place foundation- stones beneath the columns either in the walls or in intermediate rows. A foundation of concrete was brought up to the approximate level of the base of the columns, and the steel framework was erected and riveted, and then wedged up to tlie true gradient. Afterwards the space between the foot-plates and the concrete foundation was filled with cement. The details of a two-track structure are shown in Fig. 4, Plate 5, and the general arrangement of a four-track structure is indicated by Fig. 5. Wherever the topography of the ground gave sufficient cover, and the number of tracks did not exceed three, the subway was given an arched roof. The arch varied from the ordinary semi- circular arch with a minimum span of about 13 feet for a single track, to a three-centred arch with a clear width of 38 feet for three tracks. The thickness of the walls, of the invert, and of the roof varied according to local conditions and possible pressures. The three-track arch, with a rise above the springing of 14 feet 4 inches, was given a thickness of 2 feet 3 inches at the crown, with about 20 feet cover. The largest arch constructed in cut-and-cover was that of the Mott Avenue station, with a clear span of 50 feet and a maximum width, between the outer faces of the side walls, of