What is a curtain wall? Visually, we see an almost uninterrupted wall of glass — a modern engineering feat.

Technically, a curtain wall is defined as any external wall that carries no building loads. The wall is hung, like a curtain. In the case of 900–910, and all the towers that followed its example, the larger I-beam mullions stiffen the curtain wall, and the light glass sections prevent air and moisture from entering the building.

The engineering pioneered in 900–910 was a major progression from the 860–880 buildings. 860–880 were a first step along Mies’ way to creating his often imitated, but never equalled, glass and steel high-rise architecture. Unlike the later 900–910 buildings, the exterior walls of 860–880 are not true curtain walls because the windows are placed within the structural frame of the building so they do not form a continuous, separate curtain wall structure. By contrast, at 900–910 the curtain wall stands apart from the buildings’ structural skeleton as a separate continuous element.

Unlike facades in more traditional architecture, the facades of these buildings expose the structure rather than hide it. Finding the beauty in exposed structural elements was a hallmark of Mies’ modernist philosophy.

Photo: Alec McAusland.
Diagram: © Alvin Ho/Canadian Centre for Architecture, Montréal