Glass curtainwalls have become a multi-protection product designed to defend damage
The bombing of Oklahoma City’s Federal Building. Hurricane Katrina. Hurricane Sandy. Hurricane Andrew. Judgment-impaired, geriatric motorists with diminished driving skills.
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| Photo courtesy of YKk AP America Inc. |
Threats against buildings and their inhabitants are ever-present in today’s global society. In an effort to minimize them, glass curtainwalls have performed surprisingly well against low levels of loads, force and pressure. Curtainwalls can accept large deformations without the glass breaking hazardously compared to rigidly supported punched window systems. Many are designed to meet federal requirements outlined by the Department of Defense and Interagency Security Committee/ General Services Administration, as well as testing standards for blast performance per ASTM F 1642 Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loading.
And by using curtainwalls, buildings designed to protect their inhabitants no longer have to look like concrete bunkers.
A Building’s Resilience
“Today’s curtainwall is integral to a building’s resilience,” insists James Casper, SE, PE, LEED BD+C, advanced technology studio manager, Enclos, New York City. “The blast design of façade systems has come a long way since the days of the Oklahoma City bombing. Security specifications were a novelty at that time; now it is rare to see a large commercial building project that does not incorporate some level of blast and/or other security requirement.
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| Photo courtesy of Kawneer Co. Inc. |
Curtainwall design and analysis has progressed to allow the owner to tailor the curtainwall design to a specific risk assessment. The threats range from accidental impacts at the pedestrian level, to missile impact from wind-borne debris during extreme wind events, to the impact of high-powered military rifle rounds, and the effects of accidental or deliberate explosions. Enclos has completed many government and other high-security building façades to continually escalating standards, and we expect that trend to continue into the foreseeable future.”
When designing protective curtainwalls, understand that it is an entire system solution and each performance requirement can affect others. This is especially critical for buildings with multiple protection and performance requirements.
“It’s not uncommon for a building to require blast, hurricane and ballistic protection, not to mention acoustic and energy goals,” says Tom Mifflin, government market manager at Wausau Window and Wall Systems, Wausau, Wis. “A system designed for blast does not automatically qualify it for hurricane. It’s extremely important that all performance requirements of the curtainwall system be clearly identified in the specification and coordinated with other specification sections, i.e. curtainwall with glass/glazing. Early involvement with manufacturers can help clear up potential confusion, maintain the intended aesthetic and avoid cost surprises at bid time.”
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| Photo courtesy of Enclos |
Curtainwall manufacturers have realized that a one-size-fits-all approach doesn’t work with hurricane resistance. A design can be prohibitively expensive for one job and unable to meet the performance needs for another.
“Starting with the individual project requirements for structural load, impact protection and cyclic loading, designs have been refined to combine the components to meet all of the project requirements in the most cost-effective way,” says Greg Galloway, ProTek brand manager, YKK AP America Inc., Austell, Ga. “This can involve low-, medium- and high-pressure mullions, different types of reinforcement, low-, medium- and high-pressure glazings, and both wet (silicone) and dry
(gaskets) glazing. An array of anchoring options can also provide cost benefits and flexibility.”
Glass Plus
Glass can often be one of a building’s weakest components but there are ways to strengthen it for a curtainwall. Casper believes that while mitigating solutions vary as loading severity increases, using laminated glass is a start. “Interlayer materials and laminated glass panel makeup can be customized to specific project security requirements,” he says. “Lower range blast loads may be addressed with generic Polyvinylbutyral (PVB) interlayers, while extreme loads may require special laminates including multi-ply fabrications incorporating polycarbonate plies.”
In both hurricane impacts and bomb blasts, laminated glass can break and take the brunt of the force, while still protecting the building and occupants. Indeed, PVB is the dominant interlayer for both these events. It’s a proven material and the most economical, especially at lower wind loads and impacts. But there are other interlayers on the market that also offer protection against higher loads, but at a higher cost.
“Thicker glass may also be used,” says Galloway. “SentryGlas is an ionoplast developed initially for improved performance in wind-borne debris regions. It is up to 100 times stiffer than PVB and more tear resistant. Other higher performance interlayers sandwich multiple types and thicknesses of interlayer material to improve performance. The majority of large, fast wind-borne debris is generated during major hurricanes. The wind-borne debris generated during tropical storms and smaller hurricanes often does not even break the glass, especially with tougher interlayers such as SentryGlas and Vanceva Storm.”
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| Photo: Anne Gumerson, photo courtesy of Icon Exterior Building Systems and Wausau Window and Wall Systems |
Mullion width is important because increasing the glass bite
(how deep the glass is buried into the glazing pocket) allows the system to withstand higher wind loads and more forceful impacts. Galloway believes increasing the curtainwall face dimension from 2 1/2 inches to 3 inches increases the glass bite by 33 percent. This is probably the single change that yields the greatest improvement in both economics and performance, he says.
Also, “Deeper glazing pockets provide more glass bite to accommodate additional system movement required by higher design pressures,” says Christopher Giovannielli, product manager of curtainwall and sun control, Kawneer Co. Inc., Norcross, Ga. “By considering dry systems (gaskets) versus wet system (silicone to glue the glass to the framing systems) overall installed cost can be reduced.”
“The glass thickness affects the overall system depth, and the glass bite affects the system sightline, so it’s important to communicate these requirements early in the design phase to avoid aesthetic surprises later on,” says Mifflin. “Framing design upgrades include thicker extrusion walls, larger size and quantity of connection fasteners, and designs to easily accommodate internal structural reinforcement and ballistic steel. All of these factors play an important role to minimize the amount of anchorage to the building, which is key to the overall installed cost.”
Greg Header, president of Solar Innovations Inc., Pine Grove, Pa., suggests using structural silicone on an aluminum receiver clip system or field glazing the curtainwall with a silicone back seal, including unitized and conventional applications. “You will generally receive a higher performance if you back seal it with silicone,” he says. “Factory sealing the clips or a factory-glazed unitized system typically yields higher results, and reduces field labor and field issues. This leads to a much higher success rate for passing impact, air and water testing at higher design pressures. By using the silicone you’re typically able to achieve a higher rating than when using structural glazing tapes, which yield mixed results depending on application variables. Silicone back bead designs also typically yield a higher performance for blast in equivalent situations.”
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| Photo courtesy of C.R. Laurence Co. Inc. |
When designing protective curtainwalls, Galloway stresses the importance of selecting glazing with the least glazing resistance load to meet the blast specifications. “Framing and anchoring loads are multiples of the glazing resistance load, so if you use a
‘stronger’ piece of glass, it drives the entire system cost up dramatically,” he says. “For this reason, if your project has both hurricane-impact and blast-resistance requirements, select the hurricane impact glazing first and then check to make sure it meets the blast specifications.”
Aluminum Protection
Curtainwall framing systems are being strengthened with additional aluminum in the mullion extrusions. Aluminum’s strength-to-weight ratio, finishing capabilities and shape customization make it an ideal material for this.
“Blast-resistant curtainwall systems are comprised of laminated glass infill within standard or ultra-thermal aluminum frames secured to the surrounding building with special anchoring,” Giovannielli says. “Because air blast pressures, impulses, and/or durations often vary project-to-project, the framing systems are designed based on a combination of blast load and standoff distance requirements. In addition to these resistant considerations, fabrication methods such as screw spline or shear block joinery designed into the overall system need to be evaluated for hurricane resistance.”
Casper believes heavier extrusions, high-strength ductile alloys and steel-reinforcing are enough to address the load requirements of most situations, but that ballistic threats may call for special continuous inserts to provide protection along the mullion seams. Also,”The analysis and simulation of façade system response to extreme loads has improved dramatically due to software advances and the use of cloud-based cluster computing,” he says. “Developing know-how and computational power is facilitating the design of less bulky systems to provide the required level of protection.”
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| Photo courtesy of LEO A DALY |
Resistance Test
Full-scale curtainwall mock-up testing is required for hurricane impact qualification. Galloway says in addition to air, water and structural standard testing, impact systems must pass missile impact testing per ASTM E 1886 and 1996, followed by cyclic pressure testing (9,000 positive and negative cycles to full design pressure). “This simulates debris impacts and the pulsing wind pressure that occur during a hurricane,” Galloway adds. “The key to successful testing is an elegant test plan established with an independent professional engineer that leverages the least amount of testing to qualify the most options and configurations.”
To simulate the wind pressures on a building, an airplane engine can create higher wind load speeds on a curtainwall mock-up. “During the test, water is sprayed on the face of the curtainwall together with a negative pressure inside the curtainwall trying to suck water through the system,” Giovannielli says. “This type of test simulates a wind force on the outside of a building along with a vacuum force on the inside, trying to pull water into the interior of the building.”
A curtainwall mock-up is a representative sample of what will occur in the field. When tested, it allows identification of weak areas and even areas that are overdone. “We can maximize the efficiency of the design and improve those weak spots to yield a much higher performing system,” says Header.
“This eliminates surprises in the field and basically provides a second chance in any situation. Although it adds to the initial upfront cost, what is learned during this initial testing will pay for it through the education that is gained; especially in newly designed systems. Typically, if you’re system performs well in hurricane testing, it will perform well in blast situations, at lower-level blast ratings. When testing is involved, make sure there is always an accredited, third-party testing facility involved to ensure the testing you receive is valid. Give yourself plenty of time for the entire process. Extra time typically yields better results and fewer issues throughout the project.”
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Photo Courtesy of Solar Innovations Inc. |
Sidebar: Metal Architectureasked three architects about protective curtainwalls.
As an architect, it is our responsibility to provide a safe enclosed building that protects its users and occupants from exterior adversary forces. These forces include rain, temperature, light, air, wind, fire, smoke and anything that may be projected at the system. The level of which a curtainwall is to be designed to handle these forces varies based on project type and location. A curtainwall in coastal Florida would obviously have a more severe requirement for impact and hurricane resistance than a curtainwall in Chicago. A curtainwall in Chicago may have a unique design based on dealing with the weather extremes at both ends of the spectrum.
Nathan Casteel, AIA, LEED AP BD+C, architect at GREC Architects, Chicago
Each curtainwall system has to be carefully considered for the project that it will be installed on. Some projects have blast, safety or hurricane criteria, and the type of mullion system and glazing is preferably already tested to industry standards and proven to perform as anticipated. There are varying levels of blast so the blast level criterion is also a consideration on reinforcing required. Building scale and applicable codes may also be a factor in which system is chosen. Most of the systems are primarily shop assembled to provide the best quality of product for a project.
Tami Merrick, AIA, senior associate/senior project architect, Page Southerland Page Inc., Houston
This is an exciting time for curtainwall technology, thanks to the work of glassmakers and curtainwall manufacturers. We’re now able to offer our high-security clients the safety they need, while giving them the benefits of daylighting and views. Before, it was an either/or scenario. We are able to achieve blast-resistance by specifying tested curtainwall assemblies, insulated glazing units that meet the performance criteria and adequate structural support at the mullions. The glazing units have become more sophisticated, and offer coatings and laminations that help us achieve the desired solar protection, thermal insulation and blast resistance.
Bill Baxley, AIA, vice president, director of design, LEO A DALY, Omaha, Neb.