the 10 x 15-foot
rectangular concrete tunnel, through which the underground coal
conveyor is installed between the shore end of the pier and the power
house.

[Sidenote: _Steel Work_]

The steel structure of the power house is independent of the walls,
the latter being self-supporting and used as bearing walls only for a
few of the beams in the first floor. Although structurally a single
building, in arrangement it is essentially two, lying side by side and
separated by a brick division wall.

There are 58 transverse and 9 longitudinal rows of main columns, the
longitudinal spacing being 18 feet and 36 feet for different rows,
with special bracing in the boiler house to accommodate the
arrangement of boilers. The columns are mainly of box section, made up
of rolled or built channels and cover plates. They are supported by
cast-iron bases, resting on the granite capstones of the concrete
foundation piers.

Both the boiler house and the engine house have five tiers of floor
framing below the flat portion of the roof, the three upper tiers of
the engine house forming galleries on each side of the operating room,
which is clear for the full height of the building.

The boiler house floors are, in general, framed with transverse plate
girders and longitudinal rolled beams, arranged to suit the particular
requirements of the imposed loads of the boilers, economizers, coal,
etc., while the engine-room floors and pipe and switchboard galleries
are in general framed with longitudinal plate girders and transverse
beams.

There are seven coal bunkers in the boiler house, of which five are 77
feet and two 41 feet in length by 60 feet in width at the top, the
combined maximum capacity being 18,000 tons. The bunkers are separated
from each other by the six chimneys spaced along the center line of
the boiler house. The bottom of the bunkers are at the fifth floor, at
an elevation of about 66 feet above the basement. The bunkers are
constructed with double, transverse, plate girder frames at each line
of columns, combined with struts and ties, which balance the outward
thrust of the coal against the sides. The frames form the outline of
the bunkers with slides sloping at 45 degrees, and carry longitudinal
I-beams, between which are built concrete arches, reinforced with
expanded metal, the whole surface being filled with concrete over the
tops of the beams and given a two-inch granolithic finish.

[Illustration: 58TH ST. POWE