Volume 38, Issue 8, August 2003
No Such Thing?
When it Comes to Hurricane-Resistance, It’s Not the Glass, It’s the System
by John Bush
One of the biggest misconceptions about the hurricane-resistant glazing market is the belief that there is such a thing as hurricane-resistant glass. Yes, you read correctly—hurricane-resistant glass does not exist. What does exist is laminated glass. Installed properly in a suitable frame, which is anchored correctly into the opening, laminated glass is a major component in a glazing system that is resistant to windborne debris and cyclic wind pressure, or, in other words, a hurricane-resistant glazing system.
The main components of a hurricane glazing system are the frame, laminated glass, structural silicone, fasteners and anchors and, for operable windows, hardware. Changing any component without first testing to confirm the change presents a security risk. You would not put a Ford engine in a Ferrari because it would not perform. Similarly, you cannot use an untested component, such as a cheap sealant from a local hardware store. It might look the same, but the hurricane glazing system will not perform as intended or as tested.
Any hurricane glazing system must meet the requirements of five separate tests. Three of these tests—static structural load, air infiltration and water infiltration—are common to non-hurricane glazing systems. One specimen can be used for all three tests. Be aware that the maximum test pressure is 150 percent of the design pressure. Unfortunately, the two pressures are often confused. It is advisable to ask whether a specified pressure is a design pressure or a test
For hurricane-resistant glazing systems, two additional tests are required. These tests are performed on three identical units and all three must pass. The first of these involves either a large or a small missile that is fired from an air cannon to impact both the glass and the frame at specified locations. These impacts usually crack the glass and often damage the frame. Large missile testing is required for glazed openings within 30 feet of the ground. Depending on the building type and geographical location, there are a of large missiles used. However, most often it is a 9-pound, wooden 2x4 traveling at 50 feet per second. The small missile, required on glazing systems used at elevations greater than 30 feet, involves 30 2-gram steel balls traveling at 130 feet per second.
The second test, cyclic pressure loading, is carried out on the system after it has been damaged by the missile impact. While missile impact is the dramatic part of the testing procedures, the pressure cycling test is the toughest. Pressure cycling consists of a total of 9,000 cycles each taking between one and three seconds. The test lasts a minimum of two and half hours, during which the air pressure loading attempts to pull the glass out of the frame and the frame out of the opening. A good analogy might be bending a credit card. After the first bend, the card just springs back, but repeated flexing could eventually lead to failure. The glazed system must have no significant openings after completion of 9,000 cycles to achieve a pass. Table 1 (on page 50) shows how the 9,000 cycles are performed.
Full details of the test method can be found in ASTM E 1886 – Test Method for Performance of Exterior Windows Curtain Walls, Doors and Storm Shutters Impacted by Missile(s) and exposed to Cyclic Wind Pressures; and, ASTM E 1996 – Standard Specifications for Performance of Exterior Windows, Curtain Walls, Doors and Storm Shutters Impacted by Windborne Debris in Hurricanes.
Precise testing parameters are critical to ensure performance standards are met. Substitution of a window system size greater than the maximum tested size is unacceptable. For example, if a test report shows that the system has been tested to a design pressure of 60 psf at a size of 48 inches x 60 inches, it is not possible to calculate or assume that a unit measuring 48 inches x 72 inches could be used, even at a lower design pressure.
There are several different types of laminated glass interlayers each with different performance capabilities. It is critical that the correct laminated glass is used in the window system. Using a different glass requires further testing. All fenestration system manufacturers provide copies of their test reports or product approvals that detail exactly what type of laminated glass should be used in the system. Glass laminators, however, regularly get purchase orders from glazing contractors without specifying the interlayer type and thickness. Each lite of glass should have an indelible mark indicating the glass fabricator and the interlayer type being used. This enables building inspectors to confirm that the correct glass has been used.
Building inspectors have the authority to make glazing contractors rip out all the glass and replace it with the correct type.
The architect or the structural engineer is responsible for calculating the design pressure for all structural components on the building, including windows. This can be done either by using ASCE 7 or wind tunnel testing. This pressure should be noted on the drawings. Beware of a request for windows to meet, say, 110 mph winds, as this is not enough information. A single-family home may have a design pressure of 70 psf, while a high-rise a few blocks away may have a design pressure of 90 psf.
Table 1: Cyclic Pressure Loading
|No. of Pos. Cycles||Negative Pressure||No. of Neg. Cycles|
|0.2 - .05 Ppos||3,500||0.3 - 1.0 Pneg||50|
|0.0 - 0.6 Ppos||300||0.5 - 0.8 Pneg||1050|
|0.5 - 0.8 Ppos||600||0.0 - 0.6 Pneg||50|
|0.3 -1.0 Ppos||100||0.2 - 0.5 Pneg||3350|
In summary, there are five key points worth emphasizing:
• The window system is tested, not just the glass;
• There can be no substitution of components;
• Large or small missile tests must be conducted depending on elevation;
• There are many different glass interlayers; and
• Examine the test report for maximum size.
John Bush is the director of laminated glass products and development for Oldcastle Glass. He is based in Fort Lauderdale, Fla.
|Shutter and Film Industries Face Off in the Wake of Hurricane Season
by Penny Beverage
Protection from the wind: it’s on the minds of all who live and build in hurricane-prone areas. But what’s the best protection from flying debris in the midst of a windstorm? Well, in the world of glass and windows, opinions are divided. While film manufacturers and dealers have designed films specifically for hurricane protection, there are those in the shutter industry who say shutters are the only way to go. Likewise, the controversial Dade County Standard, which is part of the Southern Building Code Congress International’s (SBCCI) structural building code for new construction, specifies many shutters as meeting the highest windborne debris impact resistance—but not window film.
Spotlight on Hurricane-Resistant Glazing Products
Oldcastle Glass of Santa Monica, Calif., has introduced StormGlass™, a high-performance, laminated product for hurricane impact-resistant glazing.
“StormGlass has been developed to resist very high design pressures for large glazed openings in both the residential and commercial building markets,” said Ted Hathaway, chief executive officer. “With its unique, exclusive composite interlayer, StormGlass provides architects and designers with the flexibility of using large glass sizes while satisfying hurricane-related building code requirements.”
According to information provided by the company, StormGlass incorporates an advanced interlayer technology that features high elastic modulus for adhesion to glass.
To demonstrate the performance of StormGlass, two independent Florida test labs subjected a number of window units to wind pressures at 80 psf and 107 psf. The 40-square foot units, which featured both non-captured and captured glazing systems, met the design pressures required by the wind zones of South Florida, according to Oldcastle.
Info www.oldcastleglass.com or call 1-866-653-2278.
|Safe and Sound
Glasslam N.G.I. Inc. of Pompano Beach, Fla., has available a number of products to meet safety-glazing needs, including its Safety-Plus® II hurricane-impact glass. According to Flavio Schonholoz, the company’s marketing director, interest in the Safety-Glass Plus II has been tremendous, as it has passed large missile tests in all glass thicknesses of annealed, heat-strengthened and tempered glass, both monolithic and insulating.
Glasslam also offers its Supreme window system. Schonholoz said the system can be used as a storefront, ribbon or punched-opening system. It accepts glass thicknesses of 3/8- to 2-inches without changing sections. It also accepts insulating glass. Likewise, the system passes Metro-Dade County, SBCCI and ASTM large missile impact and cycling tests.
Info www.glasslam.com or call 954-975-3233.
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