Curtain Wall Systems: Structural Support for Building Facades
A curtain wall is, by definition, non-load-bearing: it carries no gravity load from the floors above, hangs entirely off the edge of the building frame it faces, and its only structural job is to keep its own weight and the wind pressure acting on its surface off the interior and safely back into the primary structure. That narrow scope does not make curtain wall design simple − it makes it a specialized problem in span, deflection, and movement accommodation that is easy to underestimate precisely because the wall is not carrying the loads a structural engineer usually thinks about first.
Load Path Back to the Frame
A typical stick-built curtain wall consists of vertical mullions spanning floor-to-floor, connected at each floor line back to the building's spandrel beam or slab edge through an anchor designed to transfer wind load (and the mullion's self-weight) into the frame while allowing the small rotations and movements a real connection needs. Horizontal transoms span between mullions and support the glass or panel infill, transferring their own tributary wind load and weight to the mullions rather than directly to the frame. Wind pressure on the glass is delivered to the transoms, then to the mullions, then to the anchors at each floor − a load path with several transfer points, any one of which can become the governing design element depending on span and pressure.
Wind pressure on a facade is not a single design number across the whole building height; corner zones and upper floors typically see higher local pressure coefficients than the field of the wall lower down, following the same pressure distribution logic covered under general wind loads on buildings, and curtain wall mullion spacing or glass thickness is frequently increased at corners and upper floors specifically to handle this higher local pressure rather than using one uniform design pressure for the entire elevation.
Curtain wall deflection limits are tighter than most structural members because glass is brittle and unforgiving of excessive movement: a common serviceability limit for mullion deflection under design wind load is span/175 or a fixed absolute limit (often around 19 mm), whichever is smaller, and this deflection limit, not stress, is what usually sizes the mullion − a mullion with plenty of bending strength left in reserve can still fail the deflection check well before it fails in stress.
Movement Accommodation
Every anchor connecting a curtain wall mullion to the building frame at each floor has to accommodate several kinds of relative movement simultaneously: thermal expansion and contraction of the aluminum or steel mullion itself, live-load deflection of the floor slab or spandrel beam the anchor is fixed to, and, on taller buildings, interstory drift − the lateral displacement of one floor relative to the one below it during wind or seismic events. A curtain wall anchor is typically detailed with a slotted connection allowing vertical slip (to absorb structural frame deflection and mullion thermal movement without loading the glass) while still transferring horizontal wind load rigidly back to the frame.
Interstory drift compatibility is one of the more consequential checks curtain wall design shares directly with the structural frame's own seismic design: a stiff frame with tightly controlled drift makes the facade designer's job easier, while a frame designed to the code's maximum allowable drift limit can force curtain wall anchors and glazing pocket clearances to be detailed for movement well beyond what the wall system's stock hardware handles without custom modification, meaning the structural engineer's drift design decisions directly constrain what curtain wall products are viable on the project.
Self-Weight and Anchor Redundancy
Beyond wind, the curtain wall's own dead weight − glass, framing, insulation, and any stone or metal panel infill − has to be carried down through the mullion system to gravity anchors at intervals, typically one floor at a time so that the accumulated weight of an entire building height of curtain wall is never resting on a single connection at the base. Most curtain wall systems use one anchor per mullion per floor designed to carry gravity load and a separate or combined anchor designed for wind load, with redundancy built in so that a single anchor's failure, whether from installation defect or corrosion, does not immediately release the mullion above it; anchors are also among the few curtain wall components regularly inspected and replaced over a building's service life, since they are more accessible than embedded structural connections and more exposed to the corrosion risk of an exterior, weather-facing connection point.