An age-old problem in architectural design is that not everything fits together perfectly. Building sites may be asymmetrical while interior spaces are meant to be perfect squares or rectangles, the grand vision of interior spaces has to allow for secondary support spaces, or a sequence of main spaces may not match the exterior shape of the building. The result is that there are left-over spaces in plan similar to the left-over spaces in section between hung ceilings and the floor structure above them.
There are various types of left-over, all of which can be conveniently illustrated with plans from A Monograph Of The Work Of McKim Mead & White 1879 – 1915.
The oval rooms are nicely shown on the exterior, but since the neighboring rooms are all rectangles, there are large walls of varying thickness adjacent. At the bottom of the building, these walls are shown as solid masonry, with flues embedded in the walls higher up. This is the traditional solution for such plan peculiarities, going back hundreds of years: solid masonry with maybe some chimney flues.
Next, the Rhode Island Capitol:
As is true of many of the state capitols, its architectural design is loosely based on the U.S. Capitol. The floor plan shows that it’s a bearing-wall structure (long parallel interior walls of substantial thickness), but more importantly shows the massive masonry piers necessary to support that central drum and dome:
I’ve outlined in red on the plan of the Mrs. William Vanderbilt house a more modern variation on the chimney flue: an air vent hidden between a curved stair and a rectangular room instead of a plain mass of brick:
At the Harvard Union, thickened walls (at the living room, disguised by the thickness of the fireplaces) hide multiple air vents:
By the time steel framing is taken into account, there’s no more need for heavy masonry walls on the interior, as Sherry’s Hotel shows:
In other words, when the rationale for thick walls disappeared with the switch from bearing-wall to skeleton structure, buildings were left with fewer hiding places for modern mechanical systems. This is the origin of the modern mechanical closet: those systems have to go somewhere. If there’s no leftover space to put them in, then space has to be created for them.
A lot of our time during investigation is spent figuring out which oddities in plan are columns, which are bearing walls, and which are hidden spaces full of duct or pipe.