The New York Rapid-transit Subway

Proceedings.] PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. 17 ment of small members readily handled in a city street; complete embedding of the steel in concrete, thus avoiding all maintenance- charge for painting ; and the ease with which the design may be modified at any point to give greater strength to withstand a specially heavy concentrated load, such as the thrust from the foundation of adjoining buildings or the weight of a monument overhead. Reinforced concrete was also employed in many of the details of the subway with equal success; for instance, in the over- hanging portions of station-platforms, stairways, the strengthening of arches, vault-lights and station-roofs—in fact, in any detail where concrete members are subjected to tension. In the particular case under discussion exposed steel was used for the columns. This was necessary on account of the desire to econo- mize space. In other work subsequently carried out by the Author, reinforced-concrete columns have been employed and consequently any exposure of the steel has been obviated. The experience thus gained during construction, and study of the constructed sections during operation lead the Author to prefer this type of design, unless special local reasons prohibit it. The method is not so suitable where there is necessity to block up and support any overhead structure during erection before the concrete has had time to set, such as the conduit structure of a surface tramway: for this purpose the steel frames or beams offer special facilities. Table V illustrates the comparative economy of the two methods of construction by giving the quantities of the principal items for the same amount of cover :— Table V. Item. Beam Construction. Reinforced Concrete. 2 Tracks. 3 Tracks. 4 Tracks. 2 Tracks. 3 Tracks. 4 Tracks. Excavation . Cubic yards 22-2 32-0 41-7 22-8 32-5 42-3 Concrete . . ,, „ 5-7 7-7 9-6 6-3 8-3 10-2 Steel . . Tons (2,000 lbs.) 0-38 0-56 0-74 0-31 0'46 0-62 Waterproofing Square yards 9-7 12-5 15-2 9-8 12-6 15-4 The reinforced concrete calls for a trifle more concrete but less metal, and the metal used costs less per pound. With equal depths, the arched cut-and-cover work is the most economical of all, other factors being disregarded, as all steel is omitted, except for minor reinforcements, while the quantities of excavation and concrete remain about the same. [THE Inst. C.E. VOL. CLXXIII.] c