The New York Rapid-transit Subway

Proceedings.] PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. 13 front of warehouses, and lighter in streets traversing the residential sections where the only loading likely to occur would be that pro- duced at long intervals of time by piles of building materials. It is obvious that there will be a greater loading at the sides of streets than at the centre, which would have to be kept open for traffic. The following assumptions for live loads were therefore made Table IV. Heavy business district Medium „ » Residential „ Centre of Street. Side of Street. Lbs. per Lbs. per Square Foot. Square Foot. . 1,000 1,135 690 950 500 500 The maximum loading on the roof consisted of these live loads added to the weight of the cover, assumed arbitrarily at 100 lbs. per cubic foot. As the maximum live loading would consist of piles of material and be therefore quiescent, or practically dead, and inas- much as the roof-beams were to be sustained laterally by the jack- arches of the roof for their whole length, the permissible unit-stresses were assumed to be :— Beams and girders . . 20,000 lbs. per square inch. length Columns ..............16,000 lbs. - 60 radius of gyration* In order to diminish the number of sizes of beams required, the possible loadings were divided into four groups with the proper section of beam adopted for each, so that only four sizes of beams had to be kept in stock. For the first three, beams 15 inches deep and weighing 42 lbs., 50 lbs., or 60 lbs. respectively per foot, sufficed, and for the maximum loading a beam 18 inches deep, and weighing 55 lbs. per foot was required. The beams were spaced 5 feet apart between centres. To take the thrust of the walls a 12-inch 40-lb. beam was usually sufficiently strong. For the supporting column it was necessary to adopt a section which would be compact, strong against shock, and of a simple character. Taking a suggestion made by Mr. Theodore Cooper, M. Inst. C.E., the Author developed a column consisting of four bulb-angles with an intermediate plate; all the members being, therefore, merchantable shapes and easily manufactured. The thickness of the angles was 4 inch and * inch, according to loading. The intermediate plate varied from 6 inches by + inch to 61 inches by % inch. The over-all dimensions of the columns were 64 inches by 81 inches to 63 inches by 8% inches (Fig. 3, Plate 5).