Glazing innovations mitigate hurricane damage
Current glazing practices for large missile impact mitigation for hurricane resistance are based on glass interlayer materials originally developed in the late 1930s. Despite numerous advances in material science, system design and architecture, as well as more stringent building codes and other requirements for hurricane protection introduced over the past 10 years, laminated glass bonded with polyvinyl butyral interlayer is still the most widely used protection against windborne debris for commercial structures. Within the last few years, however, a new generation of alternative glazing methods and structural interlayers has emerged, allowing for more economical glazing installations and higher windload-carrying capabilities. These innovations will hopefully result in a greater number of protective glazing installations with minimal influence on economic conditions.
This article will re-introduce a glazing method that has been all but forgotten for large missile impact resistance—dry glazing with a structural interlayer—and its advantages over wet glazing with PVB based glass laminates. More specifically, the article will first discuss the impact of Hurricane Andrew on the glazing industry and subsequent changes to the Florida building code to enhance hurricane protection for people and property. It will provide an overview of the problems and challenges of wet glazed PVB systems, detail dry glazed methodology with a structural interlayer and emphasize benefits to the installer and the end-user. It will conclude with a case study that describes a successful dry glazed installation with a structural interlayer at an office building in south Florida.
Hurricane impact protection
In August 1992, Hurricane Andrew made landfall in south Florida as a Category 5 storm and forever changed the performance requirements for the region’s glazing industry. Following the hurricane’s devastation, local building codes required impact protection, and among other mandates, extreme structural windloads. Demand then increased for advanced architectural glazing systems capable of managing impact events and significant loads, such as hurricane-force winds. This forced contractors and installers to carefully consider how to maximize the performance of glazing systems and ensure safe and reliable installations. For example, high-performance, impact-resistant glazing products will only work when properly fitted with a well-designed framing structure.
Rules regarding proper support, clearances, tolerances and care can be found in guidelines established by the Glass Association of North America of Topeka, Kan. Following these practices and installation procedures can result in improved impact protection in all glazing systems.
Wet glazed systems
Wet glazing with PVB-based glass laminates is a decades-old, common solution. It requires bonding the laminated glass to a supporting structure with a high-performing architectural sealant, usually silicone. Although this method generally survives the dynamic loads applied to windows during large missile impact and cycle tests, its performance in the field may vary greatly.
There are two main challenges in using PVB based laminates in a wet glazed framing system. First, the level of precision and skill required to properly wet glaze a hurricane-impact resistant framing system, as well as the required specialized materials, make this an expensive and time-intensive option. If a glass panel subsequently breaks, additional labor is necessary to cut through and remove the existing adhesive, clean the framing system and re-apply new adhesive—a process that takes more time and further increases costs to the end-user. Second, the performance of wet glazed systems in the real world is largely contingent upon skilled workers who understand and follow installation guidelines to ensure a strong structural adhesive bond. If not installed properly, the wet glazed systems that are designed to protect people and property in the event of a hurricane might be compromised.
“Many installations that are wet glazed [in the field] would fail in a test lab scenario due to improper installation of the silicon adhesive,” says Sid Miller, president, Miller Glass & Glazing Inc., Pompano Beach, Fla. “When debris or dust gets in the glass pocket it’s almost impossible to clean correctly, especially if workers are not willing to follow the manufacturer’s directions. Further, this process takes time to complete, which can delay the completion of a project.”
Dry glazing with a structural laminated glass
This differs from other methods because it relies on rigid, structural glazing interlayers in lieu of PVB interlayers. Therefore, it does not require an adhesive structural bond to retain the glass within the opening during the required cyclical tests. A true structural glazing interlayer creates a composite structural glass laminate with multiple components that, when subject to a load, acts as a single element. Instead of bonding the glass to the supporting structure, dry glazing with a structural glass laminate can provide adequate post-breakage stiffness, eliminating the need for the adhesive bonding to the framing system. This feature makes it a more competitive option by eliminating the need for skilled laborers to apply the structural adhesive bond, thereby greatly reducing the performance variability common with wet glazing.
“There is a widely-held theory in the glazing industry that the only way to keep glass in place is to glue it to a supporting frame,” says Doug Penn, director of marketing, YKK AP America Inc., Atlanta. “But dry glazing is proving to be a faster, more cost-effective technology that is just as effective as wet glazing with PVB interlayer.”
Since the dry glaze method does not rely on adhesives, it can deliver several unique advantages that are not possible with wet glazing. First, in terms of cost, dry glazing with a structural glass laminate reduces the number of workers required to install the system, allowing glazing contractors to do more work with less labor in a shorter timeframe. Additionally, since installations, retrofits or repairs can be done without having to remove or replace bonding adhesive, the end-user benefits from reduced expenses.
Second, since dry glazing with a structural glass laminate eliminates the variability and inconsistencies that are common with other systems, it performs as well in the real world as it does in the test lab, which mitigates the possibility of installation errors or system failures.
“Product performance of dry glaze isn’t dependent upon the adhesive seal, so it’s not compromised by the workmanship, like it can be in a wet glazed system,” Penn says.
Third, dry glazing with a structural glass laminate facilitates refurbishment of existing fenestration installations because it does not require the removal and re-application of adhesive bonds.
“Retrofitting can be done in one-third to one-half of the time it would take to restore a wet glazed system,” Miller says. “It’s very easy to deglaze these units, which saves time for the contractor and the end-user.”
Finally, properly designed dry glazed fenestration systems can provide extremely high windload design performance, maximizing performance levels of laminated glass constructions commonly used in the commercial glazing industry.
The Cornerstone office building is a five-story structure in a 130 mph wind zone region in Plantation, Fla. The local building code requires it to be protected from windborne debris. More specifically, it must have large missile protection on the first three stories and small missile protection for the fourth and fifth floors.
Wet glazing with a PVB-based glass laminate would have met these required code specifications, but Miller, the fenestration installer, and the building owner preferred dry glazing with a structural glass laminate for this project. They each cited the faster installation process and ease of replacement as key factors in their decision. Cost also was influential. Using this method, Miller Glass and Glazing provided the complete structure with large missile protection, which exceeded code requirements; however, the owner still realized significant savings on labor and much lower install costs.
“Wet glazing is widely accepted in south Florida as one of the only options for hurricane protection, but success stories, such as the Cornerstone building, prove that dry glazing can be a competitive solution with several distinct advantages over other, more common methods,” Miller says. “I use dry glazing in 70-80 percent of my projects.”
Lab testing and real-world applications have demonstrated that the wet glazing method can impact the reliability of the system. This has prompted the development of new glazing technologies that are emerging as competitive alternatives. One example is dry glazing with structural glass laminates, which relies on gaskets and the stiffness of the interlayer—not an adhesive—to provide windload transfer from the glazing infill to the supporting frame. This technology is proving to be a more economical choice for glazing contractors, who can complete more projects in less time and with fewer workers, and for end-users, who do not have to incur the time or expenses that wet glazing installations or repairs require.
As a result, dry glazing with a structural interlayer proves to be a well-suited option to meet the increasingly stringent building codes in the Southeast and along the East Coast with minimal influence on economic conditions. To date, the technology has been used successfully in several construction projects in south Florida. The rest of the industry also is beginning to discover the performance benefits of this option, such as exceptional strength to withstand hurricane-force windloads. Most importantly, this method provides a cost-effective option to protect against hurricane mitigation, making it easier for building owners and managers to comply with code requirements.