ice a
week during the progress of the work. The columns were usually kept about
1/2 in. high so as to allow for compression in the timber bents.

As a rule, no jacking of the elevated railway structure was done while
trains were passing over, and trains were flagged during the operation.
There was generally very little delay, as all jacking was done between
10.30 A.M. and 2.30 P.M., when the traffic was lightest, and frequently the
jacking was done between trains, causing no delay whatever. Steel clamps
were placed, three on the top and three on the bottom of each set of the
girders "C," to bind them together and cause them to act as a unit.

All structures then being supported on girders "C," which were carried on
four concrete piers resting on the central rock core, the excavation on the
sides of the avenue was continued down to sub-grade and the east and west
portions of the concrete north abutment were constructed. The central rock
core was about 36 ft. wide on the top and 45 ft. wide on the bottom, and at
the center of 32d Street it was about 42 ft. high.

It was the original intention to excavate a sufficient width of the sides
of the avenue to erect six rows of the permanent steel viaduct, 5 ft. from
center to center, and this was done on the south portion of the work. On
the north portion, however, the rock was of poor quality, and it was
thought best to excavate for only five rows at first, to erect the five
rows of permanent steel and put the timber bents in place under the ends of
the girders "C," in order to give them some support while the outside
concrete piers were being removed and the excavation was being widened out
to permit the erection of the sixth row. Additional raker braces were put
in these bents temporarily, and were removed when the sixth row of steel
had been erected. This is shown on Figs. 4 and 5.

[Illustration: PLATE XLVIII, FIG. 1.--TW 33, P.T. & T.R.R. Co. Terminal
Station West. East side of 9th Ave. looking North from 31st St., showing
rock excavation and supports of 9th Ave. structures. Dec. 28, 07.]

[Illustration: PLATE XLVIII, FIG. 2.--TW 39, P.T. & T.R.R. Co. Terminal
Station West. East side of 9th Ave. looking North from 31st Street, showing
rock excavation and permanent steel work. March 24, 08.]

[Illustration: PLATE XLVIII, FIG. 3.--TW 73, P.T. & T.R.R. Co. Terminal
Station West. West side of Ninth Ave. Jacking up girders "C" at Elevated
Railroad Column 491, showing method