. Railroad structures and estimates . span — full width of street. B. Two spans — supports in center of street. C. Three spans — supports at sidewalk curb lines. D. Four spans — supports at sidewalk curb lines and centerof street. The usual clearance of subways is 12 ft. to 13 ft. for streetswithout street cars and 13 6 to 14 6 for those with street cars. In all types the aim is to keep the floor as thin as possible soas to limit the height and thereby reduce the amount for fillin embankment, consistent with construction that will producewater tightness, noiselessness, good drainage, and easy
. Railroad structures and estimates . span — full width of street. B. Two spans — supports in center of street. C. Three spans — supports at sidewalk curb lines. D. Four spans — supports at sidewalk curb lines and centerof street. The usual clearance of subways is 12 ft. to 13 ft. for streetswithout street cars and 13 6 to 14 6 for those with street cars. In all types the aim is to keep the floor as thin as possible soas to limit the height and thereby reduce the amount for fillin embankment, consistent with construction that will producewater tightness, noiselessness, good drainage, and easy mainte-nance, avoiding projections extending above the rail unlessproper clearance is provided to make it safe for trainmen. Atype of floor construction that is very common is shown. Fig. 15;the depth of floor is only 2 | and the girders project about 1 the base of rail. The floor is composed of 9 X 10 steeleye beams 15 apart, concrete filled, over which is placed awaterproof membrane and the ballast. -11 8 c. to Fig. 15. Shallow Floor. ADVANTAGES Or BALLASTED FLOORS. 77 In Chicago the street subways are in general 00 ft. wide be-tween abutments with curb lines 10 ft. from the walls. At thecenter of the subway a space about 3 ft. wide is taken for a lineof columns and the wheel guards. This leaves about 21 6 forroadway, which permits street cars and fast vehicles to passslower vehicles moving in the same direction. For wider streets the 66 ft. subway is usually maintained, ex-cept at boulevards. The sidewalks are narrowed to 10 ft. to make up for the spaceoccupied by the central row of columns and their clear width isfurther reduced to about 8 ft. by a line of columns just inside the10 ft. width. The spacing of tracks is generally 13 feet centers and if longspans have to be adopted it means that the girders must neces-sarily project above the base of rail, resulting in greater hazardto railway employees. The four span subway permits the use of forms of con
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