The Logic of a Hybrid

The Manhattan Life Insurance Building at 66 Broadway is one of those early skyscraper that wasn’t something enough. It was the tallest building in the world for five years, taking the record from Chicago’s Masonic Temple and losing it to the Park Row Building, but somehow it’s barely remembered even among people who have some familiarity with the old record-holders. It’s very ornate but (at least to my eyes) not very good looking. And its structure was middle-of-the road for its 1894 date of construction: some modern elements combined with backward-looking ideas. It was demolished in the 1960s for the annex to One Wall Street.

Thanks to a detailed article in the Engineering Record, we have a pretty good idea of how its structure was put together, and by modern standards it is odd. The floor beams (supporting tile-arch floors) were steel. The columns from the sixth floor up to the main roof were steel; the columns at the base were cast iron. I’d guess that the structure of the mostly ornamental crown was also all steel. The rear and side walls were supported by the frame, while the ornate Broadway facade appears to have been largely or entirely self-supporting. It’s not quite a skeleton frame, and not quite one of the predecessors to skeleton frames (which, based on 1890s practice, I usually call cage frames). Here are some details of the lower-floor cast-iron columns:

Figure 11 (the three elevations and two plan sections on the lower left) shows an interior column at the cellar. Figure 12, the plan section at the top center, is a column in a side exterior wall. Figure 13, the part elevation, plan section, bottom reflected plan, and section on the right, shows an interior column at the base. Except for the extreme weight of these column sections, they’re reasonably normal for cast iron. The beam seats in Figure 11, for example, are standard iron details. Figure 15, on the other hand…

…shows a knee-braced connection between a slender built-up steel column and the adjacent beams, at the building perimeter. Putting aside the old technology (the use of built-up members, the use of rivets), this could almost be a detail from a modern tube frame. Finally, Figure 14 shows the transition from steel to iron:

The presence of knee-braces in the steel portion of the frame shows that the frame designer, Charles O. Brown, understood he was dealing with lateral wind forces. But you can’t make a moment frame using cast-iron columns and he made no effort to use a braced frame at the lower floors. So what was he thinking?

First, there was an ongoing debate in the 1890s about the best material to use for the columns of tall buildings. Even engineers who argued solely for steel acknowledged that cast iron was stronger in compression, and if you’re worried about the ability of columns to carry 18 stories above, that is important. The Broadway facade had noticeably smaller windows at the lower floors, and the side walls had no windows there, so the use of those walls as lateral bracing, in the manner of the older cage-frame building type, was not impossible. I’m probably approaching this backwards because I’m starting with the engineering rather than the architecture: Brown many have looked at the general architectural design and decided that the masonry was sufficient bracing at the base but he needed to introduce steel above. Similarly, supporting the Broadway facade would be very difficult even if the adjacent columns were all steel, because of the out-of-plane eccentricity introduced by the ornament, but supporting the flat(ish) side and rear was much easier.