A great photo, in lamentable condition, from the New York Public Library. The B. Altman department store moved to 35th Street and Fifth Avenue in 1906, and soon after was extended to fill the whole avenue block front to 34th Street. The original building plus that southern extension covered most of the block bounded by 34th, 35th, Fifth and Madison, and in 1913 an eastern extension was built to complete the block. The picture above shows that extension underway: the foundations are complete and the new frame is out of the ground.
The AEC team was a bunch of heavy hitters: Trowbridge & Livingston were the architects, Samuel Weiskopf was the structural engineer, and Marc Eidlitz & Son was the general contractor. That was as good a team as you could assemble at that time in New York. That was probably a direct result of the owners of the store wanting to do things right, but it may have been related to the building’s context. Madison Avenue in this area was still almost entirely a high-end residential street at that time, and the people who lived there were potential customers. Even if they were not, making enemies of them would be a mistake. The picture shows a tidy construction site, with a clean and solid fence around the entire perimeter, and unobstructed temporary walkways on both 34th Street and Madison Avenue. Note that the site runs full out to the curb lines, since the building has sidewalk vaults.
The steel is going up much the same as it does today: two-story sections of columns followed by the beams. The floors between the beams are most likely terra cotta tile arches, and we can see (badly, because of the low resolution and deterioration of the photo) the wood forms used to build the masonry vaults. There are two big derricks visible – one in the foreground where the steel is only up to the first floor, and one in the background – each with a vertical mast and a boom held out on a diagonal. These light derricks would be disassembled and reassembled at higher floors as the frame went up.
Finally, there’s a funny logistics issue with this kind of extension: when do you connect to the existing building? Doing so means removing at least part of the existing curtain wall – brick here, unlike the stone facades facing the streets. Either you have to build a temporary enclosure inside the existing building, which takes floor space out of commission, or you are opening up the interior to the weather. A common solution is to build as much of the extension as you can, including its skin, before you make the connection. If you do it right, you can, mostly, use the extension to keep the weather out as you remove the old wall. This photo was taken too early in the process to see if that’s what they did here, but I think it’s likely.