The Basics: Introduction to Curtain Wall Anchorage

Both glass curtain walls and storefront facades can be used to introduce more natural light to the interior of a building. While curtain walls are definitely more complicated to fabricate and install, they offer several design and structural advantages over storefront facades. As a result of this added complexity, curtain walls need to be attached to the structure of the building using specially designed anchors.

A very brief history of curtain walls

The use of large glass display windows dates back to the mid-19th century, when manufacturers started to produce large glass panes at relatively low cost. At the time, most buildings were designed with the exterior walls supporting the majority (or all) of the weight of the structure. The first storefront-style display windows therefore had to be embedded in cast iron structures that were able to support the building load. But the development of structural steel and reinforced concrete made it possible to support building loads with columns (rather than exterior walls), and therefore architects became free to design exterior walls that aren’t load-bearing, using light materials like glass for exterior facades (in other words, curtain walls).

Curtain walls can be used in ways that glass storefront facades never could. For example, they can span multiple floors, handle larger wind loads, permit more natural light into the building, and they can be designed in complex shapes. But even though curtain walls don’t bear any load from the building, they still need to be anchored to the structure to support their own dead load and account for factors like thermal expansion and contraction, water diversion, wind load, and seismic forces. The rest of this article will explain how various curtain wall anchors work.

How glazing systems handle building movement

Buildings are never truly still. They expand when it’s hot outside, contract when it’s cold, sway with the wind, and settle over time. Every glazing system attached to a structure has to absorb that movement, or the glass cracks, the seals fail, and water finds its way in.

Watch the 90-second breakdown:

The four types of building movement

Before getting into anchors and splices, it helps to know what we’re designing against. There are four kinds of movement a curtain wall has to absorb:

• Thermal movement. Metal and glass expand and contract as temperatures swing. A 30-foot aluminum mullion can change length by more than a quarter inch between a hot summer day and a cold winter night.
• Live load movement. Floors deflect as people, furniture, and equipment move around inside. The slab the curtain wall hangs from is never perfectly still.
• Wind load movement. Wind pressure pushes the system in and out. Tall buildings can sway several inches at the top in a strong gust.
• Seismic movement. In earthquake zones, the structure shifts laterally, and the glazing system has to follow without tearing.

A well-designed curtain wall handles all four. The way it does that comes down to three components working together: dead load anchors, wind load anchors, and splices.

Dead load anchors

A dead load anchor holds the weight of the system. It’s the fixed point, the place where the curtain wall is locked to the structure and isn’t allowed to move vertically. In most stick-built curtain wall systems, the dead load anchor sits near the top of each mullion run, so the system effectively hangs from above.

Because it’s carrying the full weight of glass, aluminum, and accessories, the dead load anchor is the most heavily engineered connection in the system. It’s also the reference point everything else is measured from.

Wind load anchors

A wind load anchor is the opposite of a dead load anchor. It resists horizontal forces but allows vertical movement. The mullion can slide up and down through it as the building expands, contracts, or deflects. That sliding action is what keeps the glass from being crushed when the structure moves.

A working wind load anchor is what lets the system breathe with the building. It holds tight against horizontal wind pressure, but never fights vertical thermal travel.

Mullion splices

For curtain walls that span multiple floors, you can’t run a single piece of aluminum the full height. It’s too long to ship, too heavy to handle, and it wouldn’t accommodate thermal movement anyway. Instead, mullions are spliced together inside the system.

A splice is a sleeve hidden inside the mullion that lets two sections move independently while staying structurally connected. As the building expands and contracts, the two mullion halves slide along the splice instead of fighting each other. The splice is invisible from the outside, but it’s the single component that lets a tall curtain wall breathe.

The amount of movement a splice has to accommodate is governed by the system’s deflection criteria, typically L/360 for curtain walls.

Why architects, engineers, and glazing contractors have to coordinate

Architects care about how the building looks and how it’s used. Engineers care about whether the system performs structurally. Both want anchors and splices placed somewhere they won’t compromise the design, but they don’t always know where the constraints are. That’s where the glazing contractor earns their keep.

A good glazing contractor reviews the shop drawings with both teams and figures out where to locate dead load anchors, wind load anchors, and splices so they’re hidden behind floor lines, ceiling cavities, or sill conditions, and still in the right structural position to function. Done right, the building looks clean and the system moves freely. Done wrong, you see anchor plates on the curtain wall face, or worse, the system cracks the first time the seasons change.

This is exactly the kind of coordination we cover inside the Curtain Wall Anchorage 201 course on LearnGlazing.com.

Anchor type #1: F&T Clips

F&T clips are used to anchor the head and sill of a typical curtain wall system. The legs of the clips slide into the open ends of the vertical mullions. The F clips which only have 1 anchoring leg and are used at the jambs, while the T clips have both anchoring legs and are used at intermediate verticals.

Anchor type #2: Windload Clips and Deadload Clips

When curtain walls span multiple floors they need to be anchored at the floor lines. This can happen at the slab edge, or a structural member such as an I-beam. Depending on the direction of the design team, and engineers the clips will either need to be slotted (windload) or through bolted (deadload). If the clips are slotted they allow movement while still resisting wind loading. This also allows the floors to move independently of the glazing system. For deadload or through bolted clips the movement must be allowed in the system through working splices.

Anchor type #3: F Perimeter

F perimeter is an aluminum extrusion that can be used at curtain wall jambs. These perimeter anchors help limit deflection of the jamb and keep the joint sizes minimal.

Anchor type #4: Embed Anchors

Embed anchors are worth mentioning, even though the embeds do not actually attach to the glazing system members. Embeds are cast in place anchors that are set into the concrete while it is being poured. All types of system clips can be attached to them by mechanical and welding means.

 

Need help with your curtain wall design?

No single anchoring product is ideal for all applications. Variables like dead load, wind load, seismic forces, the size/shape of curtain walls, budget, lead time requirements, building codes, and more, all have an impact on what is the best anchoring solution. When in doubt, it’s a good idea to consult glass and glazing experts.

MP Drafting specializes in producing complex drawings for glass and glazing systems with curtain walls of any shape, size, and connection type. Contact us today to talk about how we could help you select the right anchoring solutions and produce accurate shop drawings to streamline the fabrication and construction of your project.