Unitized Curtain Wall Engineering

Unitized construction, a facade method that sits within Michael Stern’s supertall field, is a curtain wall whose parts are finished in a factory and arrive on site as complete units ready to hang. The material story behind curtain walls, from the early steel and glass walls to the aluminum systems of the postwar decades, is kept by michaelsternjds.com; its page The Curtain Wall holds it. Here the subject is the engineering: how a unit is built, how it is anchored, how the joints between units take movement, and where water goes once it gets in.

On the production line

A unit begins as aluminum extrusions cut to length. Vertical members, called mullions, are joined to horizontal members, called transoms, to make a frame about one story tall; glass or an opaque spandrel infill is set into it, gaskets are fitted, and the brackets that will meet the building are attached. Structure magazine’s fundamentals of structural curtain wall design treats the finished unit as a modular section prefabricated in a plant and shipped to site, with a vision area that lets occupants see out and a spandrel area that covers the slab edge and the ceiling plenum behind it.³ Kora Studio’s comparison of the two systems puts typical panel dimensions at about 1.2 to 2.1 meters wide and 3.5 to 4.3 meters tall, sized to a single story, and records that every panel is air and water tested before it leaves the factory.⁶

The plant is also where the tolerances are reconciled. A structural frame is built to roughly plus or minus 25 millimeters while a unitized system is designed to about plus or minus 12, so the difference must be absorbed by anchors that adjust in three axes, a detail the same comparison says has to be settled in design or installation turns into a problem.⁶ Dextall’s account of unitized walls on tight city sites adds that once a facade moves from stick built to unitized, module size drives the grid rhythm of the elevation, the joint layout shapes what the eye reads, and the performance details, from weather tightness to thermal breaks, are fixed much earlier in the project.⁷

Figure

From extrusion to crane hook

1. 01
   FrameAluminum mullions and transoms are cut and joined into a frame about one story tall, sized to the floor to floor height.
2. 02
   InfillVision glass or an opaque spandrel panel is set into the frame and the gaskets are fitted and tested in the plant.
3. 03
   AnchorsThe brackets that will meet the slab's cast in embeds are fixed to the unit before it ships.
4. 04
   TestEach unit is air and water tested under controlled conditions before it leaves the factory.
5. 05
   LiftUnits arrive in sequence, are hoisted by crane and clipped to the slab anchors one floor at a time.

What arrives at the site

On a crowded block the facade is a matter of logistics before it is a matter of architecture. Dextall describes panels arriving by truck in the order they are needed, hoisted to the active floors and hung directly off the structure, with crews working from the slab rather than from scaffolding and enclosing one band of the elevation at a time.⁷ Deliveries timed to the crane keep sidewalks and loading zones clear, cut idle time between picks and avoid the damage that comes from storing panels on a narrow street.⁷ Crawford Tracey, a Florida glazing contractor, puts the installation rate at up to a third of the time a stick system needs, since the panels only have to be hoisted into position.⁸

Repetition is what makes the factory pay. A Facade Tectonics Institute paper on the Getty Center records a campus that needed more than 7,100 units, about 4,500 of them unique, covering some 500,000 square feet, and observes that an office tower with a comparable wall area typically needs only 100 to 150 unique units.⁴ That paper also describes assembly details that seal the framing members to one another without applied sealant, forming the inner face of a rainscreen and improving how the frame behaves under seismic racking.⁴

Anchors and the stack joint

The units hang from the perimeter of the slab. Structure magazine’s primer says that unitized designers use the units above and below for stability and load transfer, which calls for two anchorage points per unit, and that the mullions mate as the units are installed so adjacent members share wind load according to their relative stiffness.³ Because the unit’s weight lands at the slab edge, it induces an eccentricity in the primary structure that the facade engineer and the designer of the frame have to coordinate, ideally through a sealed document showing the loads imposed and where they act.³ Wind usually governs; the suction at building corners can be nearly double the pressure on the interior of a wall, and that load path, from infill to framing to anchor, is taken up in cladding pressures.³

Movement is handled in two places. Movements that occur before the facade goes on, such as slab deflection under its own weight and construction tolerances, are taken up by the adjustability of the anchors; movements after installation, including thermal change, live load deflection and sway, are taken up in the horizontal joint between one unit and the next, known as the stack joint.³ Curtain wall extrusions are often custom, so a late change to the design movements can mean new dies and wider joints, which is why the entry on joint movement treats early coordination as the economical path.³

Figure

Two ways to build the same wall

	Unitized	Stick built
Where assembled	In a factory, as complete panels	On the building, piece by piece
Typical height	Standard above about 20 floors	Suited to 3 to 12 floors
Geometry	Regular, repeating modules	Complex transitions and odd angles
Quality control	Tested before shipment	Tested after installation on the open site
Lead time	6 to 12 months of fabrication	No production line to set up
Field joints	Fewer, at the unit edges	Many, at every member and infill

Where the water goes

Curtain walls sit just in front of the structure they hang from, like actual curtains, and one of their core jobs is to establish the control layers that manage air, vapor and water. Building Design + Construction’s guide to leak investigation notes that such systems provide for movement at or near each anchor and are engineered for the specific project.¹ Inside a unit, water that passes the outer gaskets is meant to be redirected by dams in the mullion glazing pockets into the horizontal transoms, through drainage holes in the pressure bars and out through weep holes in the snap on covers.

The guide lists missing dams, excess sealant that blocks drainage, deteriorated outer gaskets and clogged weeps among the common reasons the path fails.¹

The spandrel zone covering each slab edge is its own small assembly. The Construction Specifier’s guide to spandrels and shadow boxes lays out the layers: an exterior panel, an air cavity typically at least 25 millimeters deep and either vented or sealed, a metal panel behind the glass that gives it something to show, semi rigid mineral wool insulation, and an interior membrane or metal back pan sealed to the framing.² Solar heat builds up behind transparent glass in such a box, so the glass is usually heat treated and the panel attachments allow for expansion.² A Perkins and Will research paper found no industry consensus on whether to vent the cavity at all, reviewed four strategies, and concluded that venting indirectly through the mullion cavities addressed most of the problems but is feasible only with certain unitized systems.⁵ Whether a given design works is settled on a performance mockup long before production.

Figure

Where a drop goes inside a unit

1. 01
   Outer gasketThe weather gasket at the glass edge turns most water away; when it hardens or tears, more water enters the glazing pocket.
2. 02
   Zone damDams in the vertical mullion pocket redirect water sideways into the horizontal transom pocket; a missing dam lets it run down inside.
3. 03
   Pressure barDrainage holes in the horizontal pressure bar carry water outward; excess sealant can block them.
4. 04
   Weep holeWater exits through weeps in the snap on cover; dirt, pollen and construction dust clog them over time.
5. 05
   Heel beadA poorly applied inboard sealant bead lets water reach interior covers and operable sash corners.

Unitized against stick built

The alternative is to build the wall on the building. Crawford Tracey describes stick systems as lengths of aluminum set vertically and horizontally between floors to form the mullion grid that later receives the enclosing panels, a method that works for many projects but demands a number of separate operations at height.⁸ Kora Studio places the practical threshold at about 12 to 15 floors: below it, scaffolding for stick installation remains manageable, above it the scaffolding becomes a dominant cost, and at 20 floors or more unitized is the standard choice because the crane is already on site.⁶ Stick keeps its place for complex geometry, small volumes and markets without factory capacity, and one building can use both, with unitized panels on the repetitive floors and stick assemblies at odd corners and lower levels, provided the boundary between the two is defined in design development.⁶

Seen that way, the choice is less about the finished wall than about where the labor happens: in a plant under a roof, or on the slab edge in the weather.