s of the towers have X-bracing and
the connecting spans have two panels of intermediate vertical sway
bracing between the three pairs of longitudinal girders. In the low
viaducts, where there are no towers, every fourth panel has zigzag
lateral bracing in the two panels between the pairs of longitudinal
girders.

[Illustration: PROFILE OF HARLEM RIVER TUNNEL AND APPROACHES]

[Illustration: SECTION OF HARLEM RIVER TUNNEL DURING CONSTRUCTION]

[Illustration: ASSEMBLING IRON WORK ON PONTOON--HARLEM RIVER TUNNEL]

The towers have columns consisting as a rule of a 16 x 7/16-inch web
plate and four 6 x 4 x 5/8-inch bulb angles. The horizontal struts in
their cross-bracing are made of four 4 x 3-inch angles, latticed to
form an I-shaped cross-section. The X-bracing consists of single 5 x
3-1/2-inch angles. The tops of the columns have horizontal cap angles
on which are riveted the lower flanges of the transverse girders; the
end angles of the girder and the top of the column are also connected
by a riveted splice plate. The six longitudinal girders are
web-riveted to the transverse girders. The outside longitudinal girder
on each side of the viaduct has the same depth across the tower as in
the connecting span, but the four intermediate lines are not so deep
across the towers. In the single trestle bents the columns are the
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 lo