The demolition of the superstructure of the Mills Building left a cellar hole full of masonry walls, as seen on July 20, 1926:
If you look closely at the photo, you can see that some of the old walls and piers were repurposed to support the steel beams that served as the base for construction derricks. This was a big deal at this stage of the work, because the next step after demo and more or less clearing the lot was the construction of caissons to support the new building. A whole bunch of derricks by August 10:
Nine days later, we have a photo looking north along the Broad Street edge of the lot, labelled “Caisson Construction” but also which shows (I think) the rebar being set in place for the west foundation wall.
This is as good a place as any to discuss reinforced concrete in the 1920s. It was in a weird half-formed state as a structural material. Entire buildings were constructed in r/c and had been for more than twenty years by 1926, but the analysis and design methods used were quite crude. Moment distribution would come along in the 1930s and would simplify the frame analyses, but the design tools for concrete remained primitive into the 1950s. The combination of linear-elastic moment models and relatively low allowable stresses used in both the concrete and the steel made reinforced concrete seem weak. As a result, you don’t see a lot of r/c spread footings for big buildings of that era. Instead, they used concrete-encased steel-beam grillages. But r/c was suitable for a foundation wall, as seen here – an improvement over the former use of unreinforced masonry for foundation walls – and in gross compression in steel-jacketed caissons.
A caisson on the narrow Wall Street wing:
And finally, installing sheet-pile by hand at the edge of the site.
I misspoke in part 1: I said I had worked on two of the building in these photos, 15 Broad Street and 37 Wall Street. The building beyond the work site, with the old joist pockets from the demolition of 51 Exchange Place, is 43 Exchange Place. My first-ever facade investigation and repair project, in 1988, was at 43 Exchange. At that time, before J. P. Morgan had completed and occupied 60 Wall Street, the company had a campus of sorts consisting of 23 Wall, 15 Broad, 37 Wall, and 43 Exchange, making up the west half of the block bounded by Wall, Exchange, Broad, and William Streets, which is, I guess, how Ben Minerva at Weiskopf & Pickworth, the partner I was working for on this kind of project, came to have facade repair at three of the buildings.
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