same as those just described, but the diagonal bracing is replaced by
plate knee-braces.

The Manhattan Valley Viaduct on the West Side line, has a total length
of 2,174 feet. Its most important feature is a two-hinged arch of
168-1/2 feet span, which carries platforms shaded by canopies, but no
station buildings. The station is on the ground between the surface
railway tracks. Access to the platforms is obtained by means of
escalators. It has three lattice-girder two-hinge ribs 24-1/2 feet
apart on centers, the center line of each rib being a parabola. Each
half rib supports six spandrel posts carrying the roadway, the posts
being seated directly over vertical web members of the rib. The chords
of the ribs are 6 feet apart and of an H-section, having four 6 x
6-inch angles and six 15-inch flange and web plates for the center rib
and lighter sections for the outside ribs. The arch was erected
without false work.

[Illustration: SHOWING CONCRETE OVER IRON WORK--HARLEM RIVER TUNNEL]

The viaduct spans of either approach to the arch are 46 to 72 feet
long. All transverse girders are 31 feet 4 inches long, and have a 70
x 3/8-inch web plate and four 6 x 4-inch angles. The two outside
longitudinal girders of deck spans are 72 inches deep and the other 36
inches. All are 3/8-inch thick and their four flange angles vary in
size from 5 x 3-1/2 to 6 x 6 inches, and on the longest spans there
are flange plates. At each end of the viaduct there is a through span
with 90-inch web longitudinal girders.

Each track was proportioned for a dead load of 330 pounds per lineal
foot and a live load of 25,000 pounds per axle. The axle spacing in