Avoiding the fallout
Following Hurricane Wilma in 2005, glass fallout in Miami’s high-rise office buildings—like the Colonial Bank pictured—continued for weeks.
The 2005 hurricane season has had far-reaching effects. While Katrina's devastation along the Gulf Coast and New Orleans is perhaps the most memorable, Hurricane Wilma brought its own brand of destruction to Florida. And the aftermath of that storm has changed the approach to glazing today. The resulting new configurations benefit not only hurricane zones, but also any areas in which fallout protection is required or desired.
Hurricane Wilma made international news as images of significant glass breakage in downtown Miami and Fort Lauderdale's business districts were broadcast around the world. However, the hurricane itself was just the beginning; glass fallout in Miami's high-rise office buildings continued for weeks. Many streets were closed for months, and some sidewalks were covered or closed for more than a year. In one 42-story building, more than 8,000 pieces of glass had to be replaced.
While Hurricane Wilma made news for its glass damage in Miami, Houston experienced similar problems during Hurricane Ike, although the damage was confined to a smaller geographic area and not as widespread.
Codes and configurations, pre-2005
Surprisingly, many of the hurricane-damaged structures were built to meet South Florida's stringent building codes. Prior to the hurricanes in 2005, the Florida Building Code, including the High Velocity Hurricane Zone of Miami-Dade County, simply required the entire glazing unit to pass impact and pressure testing.
Buildings using an insulating glass configuration would typically put a "sacrificial" ply – a single lite of annealed, heat strengthened or tempered glass – to the outside and a laminated glass lite to the interior. To enhance energy performance, a low-E or other high-performance coating was typically applied to the #2 surface.
However, this sacrificial ply was no match for wind-borne debris, easily shattering on impact and sending glass falling to the ground. This broken glass became wind-borne debris, causing further damage. Much of the damage in Wilma was the result of flying glass from nearby buildings. For example, breakage of the heat-strengthened exterior lites in the 42-story building in Miami was caused by broken tempered glass on the outer ply of an insulated glazing unit in a code-compliant building upwind.
According to Paul Beers, president of Glazing Consultants International LLC, a glazing consulting firm that worked extensively in Miami after the storms: "Glass was a primary debris source. If you eliminated the debris source, you wouldn't have had such severe breakage."
Codes and configurations going forward
After the hurricanes of 2005, the Florida Building Code was changed to require safety glazing on the outboard lite in the High Velocity Hurricane Zone of Miami-Dade County. The outboard pane of glass could be either tempered or laminated. Although tempered glass is a code-approved option, it does not address the hazard of falling glass. Laminated glass, on the other hand, does. This fact prompted building owners, architects and glaziers to consider flipping glass units around: putting the laminated glass ply to the outside with a conventional lite to the interior.
The challenge with the new configurations is the coatings. Today's high-performance coatings perform best when they are touching an air space. The air space helps dissipate the solar heat a coating may collect rather than passing it directly into another lite of glass. If the laminated glass lite is toward the outside in the new configuration, this means the coating would need to be applied to the #4 surface. Applying the coating to the #2 or #3 surface of the laminated glass lite would result in significantly reduced performance, because the coating would not have an air space to dissipate heat. Applying the coating to the #5 surface would result in even lower performance, because the coating is further away from the sun and solar heat would be able to pass all the way to the interior lite before being reflected.
Until recently, a fourth surface coating was difficult for glass manufacturers to produce. Coatings are applied before lamination, so manufacturers had to create systems that could coat, then laminate the glass without damaging the applied coating. However, thanks to careful engineering, the fourth surface coating is now possible.
Fourth surface benefits
This configuration offers superior energy performance. Energy codes and sustainability requirements are growing in importance in building projects across the country. In Southern climates, where many projects used to use monolithic configurations, insulating glass configurations enhance energy performance and significantly reduce condensation issues. This new configuration adds fallout protection to the benefits of insulating configurations.
The result of a fourth surface coating is significantly enhanced U-values that can help meet energy codes across the country and an outboard laminated lite that offers superior fallout protection. In fact, enhanced solar performance helps building owners earn a better return on investment with this configuration (See chart, right).
The configuration, essentially, does not create any more cost than a design with the outboard sacrificial lite. The configuration simply flips the unit, putting the laminated glass to the exterior.
For buildings incorporating colored interlayers into a façade, placing the coating on the fourth surface has additional benefits. Colored interlayers are incorporated into the laminated ply. In typical insulating glazing configurations with a sacrificial lite to the exterior, the interlayer is filtered by whatever high-performance coating is applied to the second surface in the configuration. Even coatings with low reflectivity will impact the color of the interlayer when viewed from the street.
When the laminated unit is put to the exterior, with a coating on the fourth surface, the coating does not impact the colored interlayer. This allows the full color of the interlayer to be viewed from the exterior.
Fourth surface challenges
While flipping the laminated glass lite to the exterior might not add significant cost from the glass fabricator's perspective, it does represent a framing challenge. "Flipping the units changes the anchoring conditions, which means manufacturers will need to get their systems retested," Beers says.
On large projects in which custom systems are being tested, this cost should not have a significant impact on total project costs. For standard systems, manufacturers might need to bear the cost of retesting. While some manufacturers might opt to retest current systems, many could simply incorporate new configurations into the next generation of systems they design, and do all testing on those new systems.
Another challenge with flipping the configuration is large-missile protection. Currently, the Florida Building Code requires large missile-resistant glazing systems on buildings from grade to 30 feet above grade. Small missile protection is required above 30 feet. When two- or four-sided structural glazing systems are used for large missile glazing, the outboard lite of an insulating unit cannot be laminated. This is because there is no framing exterior to wet seal the laminated ply to the frame to allow the unit to pass cycle testing. Products are currently evolving to enable this design, but today, insulating two- and four-sided systems utilize a ¼-inch sacrificial exterior lite of either tempered or heat strengthened glass and an inboard laminated ply for impact.
Although this new glazing and coating configuration might have originated in hurricane regions in the United States, it has applications that reach far beyond the Gulf Coast. Having an energy-efficient unit with laminated glass to the exterior is beneficial to any regions or buildings that need extra fallout protection in seismic or blast events. For example, Beers says he has seen interest in this configuration in hotels to avoid the possibility of glass raining down on patrons using the pool or other public areas. In any region that has hurricanes, earthquakes or other natural or manmade phenomena that have the potential to break glass, this configuration could save lives as well as mitigate the need for immediate board up.