A building's performance under attack is of utmost importance. Beyond retaining its structural integrity, it should also protect people from debris and air blast—a challenge made even greater with the prevalence of curtainwalls on the current building stock. Airborne debris is responsible for 75 percent of the damage and injuries resulting from a blast, according to the National Institute of Building Sciences.
“In the event of a blast, the goal is to protect the interior occupants [and] absorb that shock through the glass, into the [curtainwall] frame, and into the structural [components] of that building,” says Andrew Dunlap, AIA, a principal and the building technology studio leader at SmithGroup’s Detroit office. Along with the following considerations, specifying an impact-resistant curtainwall requires the design team to validate the system’s ability to satisfy the particular parameters and requirements of a project.
Before specifying the requirements for an impact-resistant curtainwall, Jim Larkin, a senior associate at Dallas-based Curtain Wall Design and Consulting, says the architect and engineer should enlist a blast consultant to establish the design loads based on the project’s threat concern. For example, a courthouse at risk of vehicles carrying explosives would have different requirements than a mailroom, where threats might come in via packages. “The blast consultant will determine how much force a blast would put on the wall, map out different zones, and determine different blast walls to engineer for,” Larkin explains. Ultimately, says Jessica Marquardt, AIA, a Seattle-based architect at DLR Group, “designing to a defined threat is key to specifying the right framing system.”
Thicker is not necessarily better for a building that is under attack. “You don’t want the glass to be so strong that the [entire] load transfers to the aluminum frame,” Larkin explains. “The glass should break first.”
Laminating glass with polyvinyl butyral (PVB) will help keep shattered pieces together within the window light. Ionoplast is a stiffer option that “usually results in a reduced overall glass build-up thickness” if desired, says John Jackson, AIA, a senior project manager based in the Washington, D.C., office of Simpson Gumpertz & Heger. Though PVB is more cost effective, Marquardt notes, ionoplast is “becoming more commonly specified for structural and security applications” due to its higher mechanical properties, which consequently strengthen the laminate.
In dry glazing, a curtainwall system uses an extruded or preformed rubber gasket to limit air and water infiltration at the perimeter. Also called framing, dry glazing clamps the glass edges with a frame and then locks in it with mullions. In wet glazing, sealant is applied over a backer rod or glazing tape at the window light’s edges. Dunlap typically specifies wet glazing with structural silicone glazing; in the event of a blast, the silicone will hold the light in its frame, which subsequently holds the broken pieces in the opening even upon impact. Larkin also specifies a bead of structural silicone to hold the light firmly in its frame. “You’ll essentially glue your glass to the frame,” he says.
Blast-resistant “curtainwall frames and mullions … are often constructed with either extruded aluminum framing, hot-rolled steel framing, or a combination of aluminum and steel,” Jackson says. Aluminum, Larkin says, can be extruded into a variety of shapes and thicknesses, and is popular in factory-assembled or unitized curtainwalls. It is also lighter than steel, less susceptible to corrosion, and less expensive. Steel is stronger than aluminum, can resist higher blast loads, offers greater fire resistance, and is often used for site-assembled curtainwalls.
Physically testing curtainwalls to ensure they can mitigate potential debris is vital. “The testing protocol varies based on risk category, building type, and owner preference,” Marquardt says. American Architectural Manufacturers Association’s Publication 510, Voluntary Guide Specification for Blast Hazard Mitigation for Vertical Fenestration Systems classifies performance conditions ranging from 1 (indicating no glass breakage) to 5 (fragments project into a space more than 3 meters).
Other protocols include: ASTM F2912, Standard Specification for Glazing and Glazing Systems Subject to Airblast Loadings; ASTM F2927, Standard Test Method for Door Systems Subject to Airblast Loadings; ASTM F1642, Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loadings; ASTM F2248, Standard Practice for Specifying an Equivalent 3-Second Duration Design Loading for Blast Resistant Glazing Fabricated with Laminated Glass; and ISO 16933, Glass in Building—Explosion-Resistant Security Glazing—Test and Classification for Arena Air-Blast Loading.
Note: This story has been updated since first publication to correct Jessica Marquardt's title. She is an architect at DLR Group.