Curtain Wall

Once the frame carried the building, the wall was free to become something else. It became glass.

What it is

A curtain wall is an exterior skin that hangs from the floor slabs and carries no load but its own weight, the wind pressure, and the weather. Aluminum mullions and transoms make a grid; insulated glass units, spandrel panels and gaskets fill it; sealant closes it. Modern curtain walls are unitized: assembled as floor-height units in a factory, trucked to site, and hung from embeds in the slab edge one crane pick at a time. A crew can close a floor in a day.

The history

Willis Polk's Hallidie Building in San Francisco (1918) hung a glass wall in front of a concrete frame. Lever House (Skidmore, Owings & Merrill, 1952) and the Seagram Building (Mies van der Rohe, 1958) on Park Avenue made the glass curtain wall the uniform of the corporate city, and the world copied them.

What it does

Keeps water out (a pressure-equalized rain screen lets the outer face leak into a drained cavity rather than the office), keeps conditioned air in, lets light through, and moves — thermally by a centimetre a floor between winter and summer, and laterally when the building sways — without breaking. Thermal bridging through the aluminum, condensation on cold mullions, and solar gain through the glass are the physics the designer fights.

The trades

Glaziers hang the units. Ironworkers set the embeds before the slab is poured. Sealant crews close the joints from swing stages. The crane operator makes every pick. The drafting/BIM technician models every unit and every embed so the factory and the site agree. The inspector water-tests the wall with a calibrated spray rack.

Why it matters

When the wall stopped carrying load, the floor plate could be all window. Rent per square foot rises with daylight and the view; the curtain wall is where the vertical push meets the tenant's eye.