Designing Façades to Withstand Natural Disasters
By Joshua Huff
Façades are critical to a building’s design, protecting the structure and occupants from wind, rain, extreme temperatures and environmental hazards. The vast breadth of considerations forces architects and glazing contractors to examine different strategies for designing and installing façades to meet various threats.
Design strategies include analyzing loads imposed on the façade and choosing the proper materials, such as tempered and laminated glass, sealants and glass mounting systems. They also include aerodynamic shaping and reinforcing connections between the façade and main structure and designing façades to resist horizontal inertial forces.
The utilization of these strategies differs geographically. Professionals must meet design requirements imposed by regional building codes that are tailored to a specific location. Some regional codes focus on hurricanes, others on tornadoes, and others on earthquakes.
Seismic Considerations
Designing glass façades to withstand earthquakes requires consideration of strength, connections, structural support and resiliency. Studies have shown that earthquake-related damage to a façade is usually a result of inter-story drifts in the building frame, which are absorbed by the clearance between glass panes and framing members.
In an article titled Performance of Structural Glass Facades Under Extreme Loads–Design Methods, Existing Research, Current Issues and Trends, an international team of researchers wrote that seismic-related damages include various serviceability failures.
The researchers explain that the failures include “glazing gasket dislodging, sealant damage, glass edge damage and glass cracking, which often require expensive, disruptive, building envelope repairs, but can also lead to more serious failures such as falling glass and falling wall system components.”
Julia Schimmelpenningh, architectural industry technical manager for the Advanced Material Interlayers business of Kingsport, Tenn.-based Eastman Chemical Company, says falling glass is the greatest concern during an earthquake.
“Glass will break if there is contact between the glass and the frame due to this racking or shifting of the glass off the setting blocks,” she explains. “If the glass breaks, it can tend to fall out of the opening, causing falling and flying glass dangers. Glass retention in these instances is key, ensuring that if the glass breaks, the shards of glass are held together, and panes of glass remain in the opening.”
Lothar Erkens, an engineer at Saint Louis-based Winco Window Company, adds that proper sealants are vital to increasing façade resiliency. Though glass might remain intact following an earthquake, he explains that sealant failures will rear their head eventually, causing air and water to infiltrate the structure.
Schimmelpenningh concurs. She says there can be tremendous unseen damage to glazing units during an earthquake that does not emerge until later. This includes broken seals and small cracks.
The size of the glass also plays a role in a façade’s ability to survive an earthquake unscathed. Retired architect Chris Arnold writes in his Seismic Safety of The Building Envelope analysis that severe glass damage from an earthquake has often occurred in storefronts, where “glass panes are often large, the lateral movement of the building extreme and the quality of glass installation may be deficient, particularly in older buildings.”
However, Arnold adds that glass has shown considerable in-plane strength and out-of-plane flexibility during earthquakes, depending on the installation. He says glaziers must allow proper clearance to enable the glass to move independently of the building’s structure.
Resilient design also needs to be considered. The process is vital to mitigate costly repairs due to operational failures. Mark Nelson, associate principal of Advanced Technology and Research at Arup, a sustainable development consultancy with locations worldwide, explains that current building codes are in place to protect occupants. They aren’t designed to enhance building resilience.
As such, Arup developed the Resilience-based Engineering Design Initiative (REDi) Rating System to improve building resilience following earthquakes and storms. The system is a framework to ensure owners, architects and engineers design buildings that can quickly resume operations and provide livable conditions soon after a disaster.
“There’s a growing recognition that we need to do something more to protect property from damage and to improve the ability to respond and recover from events rather than having your building be uninhabitable for two months,” says Nelson. “… REDi is basically a stretch code, much like Leadership in Energy and Environmental Design (LEED). If you want to be more resilient than the code, follow REDi. It basically starts by saying, ‘What are your objectives?’ So, how well do you want to perform regarding financial damage, downtime? You would then follow REDi down a pathway that would make you do certain things to your building design-wise to make it more resilient.”
Tim Fuller, senior façade engineer in Arup’s Façade Engineering group, adds that when it comes to resiliency following a seismic event, the baseline is the avoidance of glass breakage and, beyond that, the movement of combination within the sealing lines of the envelope.
“I always like to look at it in terms of air, water and thermal,” says Fuller. “The avoidance of water is most critical for reducing damage to interior components that aren’t going to be made of water-resistant materials. So, if it’s a simple system, it’s just joint sealants that aren’t appropriately sized according to their movement capabilities, and making sure those are installed properly.”
Despite the various considerations for designing and installing glass façades in earthquake-prone regions, the international team of researchers who examined glass façades under extreme loads explains in their paper that damage due to earthquakes “is expected and accepted.”
Considering Water and Wind
Florida knows the danger of hurricanes. The fallout from Hurricane Andrew in 1992 forced the Sunshine State to re-think its patchwork set of codes and adopt a universal code in 2002. The Florida Building Code requires new construction to be able to withstand hurricane-force winds and feature shutters or impact-resistant glass in all openings.
Schimmelpenningh says the first thing to consider when designing a glass façade in a hurricane-prone area is understanding the wind zone and the loads that might be placed on the building. She says this process will help the designer understand the rated products needed.
“For instance, a building that is 20 miles inland from the coastline and the same building right on the coast with beachfront access in northern Florida, are going to have different requirements from each other, and both of those buildings would be different if a third exact building is built in the High-Velocity Wind Zone of Dade County, Fla.,” she says. “Once this is determined, the system needs to be selected based on the capability of the frame and glass or glazing construction to meet the missile impact and cyclical load pressures required for the location of installation geographically and on the height of the building.”
Mark Jacobson, North American market manager for Houston-based Kuraray America Inc., adds that glass choice is crucial. He says laminated glass is the most common product for hurricane-resistant glazing. Typically, polyvinyl butyral is used for smaller glass or small missile impact, while ionoplast is more desirable for glass larger than 30 feet, he says.
Aside from protecting occupants from windborne debris, engineers are also concerned about preventing a breached façade. Schimmelpenningh states that ensuring the envelope is not infiltrated keeps the internal pressure down and reduces wind and rain damage, mitigating the spread of bacterial and fungal growth.
She says no window system will “save a structure in the direct path” of a tornado. However, structures fitted with impact-resistant windows outside the full brunt of a tornado have a much better chance of surviving. Jacobson adds that past experiences show that double-laminated insulating glass units provide the best protection.
Schimmelpenningh explains that the difference in considerations between fortifying a façade in a hurricane zone and a tornado zone is minimal. The same steps must be taken, including considering wind zone, ratings, systems, selection and installation.
Glazing Systems to be Tested as 2023 Hurricane Season Nears Peak
Hurricane season is about to peak. Though the season officially started in June, nearly 85% of hurricane formation occurs during August, September and October. Fueled by record-high ocean temperatures, the strength of hurricanes is expected to increase, claim meteorologists at Colorado State University.
The enhanced forecast will test the effectiveness of hurricane-resistant systems. Before glazing products can be distributed, they must undergo testing. Julia Schimmelpenningh, architectural industry technical manager for the Advanced Material Interlayers business of Eastman Chemical Company, says the systems are anchored into mock-ups using the same materials that will be used during installation. The tests cover air, water and structural load capacity to determine air and water infiltration rates.
During the tests, the glazing systems are impacted with lumber. Schimmelpenningh states if the missile doesn’t penetrate or cause the system to fail, it is then moved to the cyclical portion of the test.
“The system is then mounted on a wall, which is capable of drawing the glass in or pushing the glass out at a prescribed interval and pressure, which is simulating the oncoming and passing hurricane,” she says. “After 9,000 cycles, if the system and glazing are still intact, meeting the opening requirements, it is deemed to pass. This must be repeated a total of three times for a system to be qualified.”
Joshua Huff is the assistant editor of USGlass magazine. Email him at jhuff@glass.com and connect with him on LinkedIn.
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