Timber bents were erected on girders "C" to carry the I-beams under the surface railway structure, as shown on [Fig. 3], and all temporary posts under these I-beams were removed. The bents were framed with a jack, as tightly as possible, and very little settlement of the track occurred.
A cradle was then built under the 24-in. water main and placed on girders "C," and, as a temporary footwalk had been constructed on the west side of the avenue, it will be seen that all structures were thus carried on girders "C."
All structures were put on the girders "C" before continuing the excavation on the sides of the avenue because, in case of a slide of rock, there would be less danger than to individual structures. The outside piers, on which the girders "C" rested, might even be lost, without affecting the stability of the structure, and posting could readily be done beneath these girders in case of necessity.
A very careful record of levels, taken on the elevated railway columns, was kept, observations being made during each jacking up and at least twice a week during the progress of the work. The columns were usually kept about ½ 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].
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.
