Volume 46, Issue 6 - July 2011


Standing Up to Tornadoes
A Look at This Growing Protective Glazing Segment
by Pat Condon and Valerie Block

The impetus to transform building code requirements and glazing systems in areas of the country affected by hurricanes came in 1992 with Hurricane Andrew. Since then, wind-borne debris protection has become a requirement along the Atlantic and Gulf Coasts and Hawaii and hurricane-resistant glazing has become a staple in these areas. Designers and specifiers now have the ability to choose from an ever-expanding marketplace of tested and certified impact window, door, storefront, curtainwall and skylight systems.

While tornadoes are found in hurricane-prone areas, these potentially devastating weather events can occur in many other parts of the country as well, bringing severe weather with very little notice. This year, from April 14 to 16, tornadoes rampaged across communities from Oklahoma to North Carolina. In total, 155 confirmed tornadoes were reported in 15 states over this three-day period. From April 25 to 28, more people were killed when thunderstorms, high winds and tornadoes with peak winds up to 190 mph hit four southern states. Tuscaloosa, Ala., Mayor Walt Maddox estimated the clean up from the tornado to be $75-$100 million. The tornado that struck Joplin, Mo., on May 22 was the country’s deadliest single tornado in nearly 60 years, leaving more than 100 people dead.

In general, violent tornadoes are capable of causing extreme destruction, including uprooting trees and well-made structures.

Classifying Storms
The classification of a hurricane is based on the Saffir-Simpson Hurricane Scale. This scale is broken into five categories, which categorizes hurricanes by the intensities of their sustained winds. The scale (using 3-second gusts) ranges from Category 1 at 116 mph to Category 5 with winds exceeding 189 mph. Typically, Category 1 hurricanes do not cause significant structural damage. A Category 5 hurricane, on the other hand, such as Hurricane Andrew, can cause extensive damage to buildings.

In 1971, Dr. Tetsuya Theodore Fujita of the University of Chicago introduced the Fujita Scale (F-Scale) to classify tornadoes. The scale was divided into six categories from F0 (Gale) to F5 (Incredible). Since 1975, the F-Scale has been replaced by the Enhanced Fujita scale (EF-Scale), a set of wind estimates based on damage. The 3-second gust wind speeds are estimated, based on degrees of damage, from the beginning of visible damage to total destruction. For example, there may be only minor damage with an EF0 tornado, but as was seen in Alabama and Missouri with the EF3 and EF4 tornadoes respectively, damage to homes and buildings can be catastrophic.

Standards and Codes for Safe Rooms
The Federal Emergency Management Agency (FEMA) has addressed the design and construction of safe rooms since 1997. The purpose of a safe room is to provide a space that offers a high level of protection during an extreme weather event such as a tornado. While these rooms may not typically include windows, they can. FEMA 320 Taking Shelter From the Storm: Building a Safe Room Inside Your House is applicable to the construction of residential safe rooms. FEMA 361 Design & Construction for Community Safe Rooms is applicable to commercial safe rooms and includes design criteria for community safe rooms. These standards offer information on what is permissible with regard to glazed openings in these spaces.

Safe rooms in homes are located in basements, on concrete slab foundations or in an interior location on the first floor. According to FEMA guidelines, the safe room must be anchored to resist overturning and lift; the walls, ceiling and door of the shelter must be able to withstand wind pressure and penetration from wind-borne debris; the connections between all parts of the safe room must be strong enough to resist the wind; sections of either interior or exterior walls that are part of the safe room must be separated from the structure of the residence so that damage to the residence will not cause damage to the safe room.

In order to ensure the highest quality design and installation of residential storm rooms, the National Storm Shelter Association (NSSA) requires its members to test and comply with FEMA 320 for above-ground storm shelters or to have their shelters tested for debris impact at an NSSA-approved facility and have designs and engineering calculations verified by a third-party engineering company.

A community safe room is defined as a shelter that is designed and constructed to protect a large number of people from a natural hazard event. The number of persons taking refuge in the safe room will typically be more than 16 and could be up to several hundred or more. Community safe rooms can be separate buildings or internal spaces, where the room or area is designed and constructed or retrofitted to be structurally independent of the larger building.

Both standalone and internal community safe rooms may be constructed near or within school buildings, hospitals and other critical facilities, nursing homes, commercial buildings and other buildings or facilities occupied by large numbers of people.

Since 2008, ICC/NSSA 500 Standard for the Design and Construction of Storm Shelters has been available for adoption and use by any jurisdiction. The standard applies to the design, construction, installation and inspection of storm shelters in hurricane- or tornado-prone areas. In 2009, the International Building Code included the ICC/NSSA 500 in the model building code.

Essential facilities are required to remain operational during an extreme storm event, including tornadoes. The design choice is to either follow the ICC/NSSA 500 requirements for the entire facility or to evacuate inhabitants to an area that qualifies as a Community Safe Room.

Protection against storms, including tornadoes, requires three components:

1) adequate wind load design,

2) unbreakable anchors and

3) debris resistance.

Impact Testing for Tornadoes
Protection against storms, including tornadoes, requires three components: 1) adequate wind load design, 2) unbreakable anchors and 3) debris resistance. Impact-resistant glazing systems intended for use in coastal areas are tested according to the large and small missile requirements cited in the building code. The most common large missile testing (below 30 feet) is done with missile D (a 9-pound wood 2-by-4 traveling at 34 mph), according to ASTM E1996, Standard Specification for the Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems Impacted by Windborne Debris in Hurricanes. Enhanced protection for emergency facilities may utilize missile E, a 9-pound wood 2-by-4 traveling at 55 mph.
Missile criteria for tornado storm shelters are based on the design wind speeds. For instance a shelter designed around 130 mph (3-sec gust) would need to resist impact from a 15-pound wood 2-by-4 traveling at 80 mph. A shelter designed around a design wind speed of 250 mph would have to resist the impact of a 2-by-4 traveling at 100 mph.

FEMA’s Thoughts on Product Performance
Commentary in Chapter 7 of FEMA 361, the second edition of which was published in August 2008, addresses testing of doors and windows:
• Both steel and wood doors were tested for missile impact resistance. Steel doors with 14-gauge or heavier skins were able to prevent perforation of the missile. No wood door successfully passed either the pressure or missile impact tests using the design criteria for 250-mph winds.

• Polycarbonate sheets in thicknesses of 3/8-inch or greater were able to prevent missile perforation. However due to the high elasticity of the material, it was difficult to attach the material to a supporting window frame. Large deflections were observed during testing, and the glazing often popped out of the frame.

• Glass-clad polycarbonate and all-glass laminates constructed with PVB also were tested. According to FEMA, in both cases, glass shards were produced and propelled at great distances and at speeds considered dangerous to safe room occupants.

Since the FEMA 361 tests occurred, however, there have been technical advances in laminated glass with stiff ionoplast interlayers that improve the ability of fenestration products to survive wind storms, including wind loads and debris impacts. A small number of tornado-rated windows now are available (see “AAMA Releases Voluntary Tornado Specification,” see below, for more on tornado-rated windows).

What’s Next?
As with windborne debris requirements related to hurricanes, the impetus for change in states and local municipalities most likely will be tied to building code adoption of the ICC/NSSA 500 Building Code. At this time, only Alabama has adopted the ICC/NSSA 5050, as a vehicle to provide school storm shelters. Once building code requirements related to tornado protection are in place, the expansion of this market segment, with the production of strong new products, is likely to occur.

Patrick Condon, PhD, LEED AP BD+C, is the owner of West Tampa Glass in Tampa, Fla., and can be contacted via email at pcondon@westtampaglass.com. Valerie Block, LEED® AP, is a senior marketing specialist at DuPont in Wilmington, Del., and can be contacted via email at valerie.l.block@usa.dupont.com.

AAMA Releases Voluntary Tornado Specification
The American Architectural Manufacturers Association (AAMA) has released a voluntary specification for testing and rating building components that will be exposed to tornadoes and similar extreme wind and rain conditions.

AAMA 512-11, Voluntary Specifications for Tornado Hazard Mitigating Fenestration Products, uses existing test methods and other procedures to qualify windows and other glazed fenestration products for tornado hazard mitigation. The newly released document provides a system for rating the ability of windows to withstand impact, pressure cycling and water penetration, which are generally associated with tornado conditions.

The specification outlines that different levels of protection apply to different buildings such as, but not limited to, hospital emergency rooms, community shelters and police/fire headquarters. These levels of protection are specified based on requirements of the authority having jurisdiction, and each level corresponds to different testing requirements. The level of testing required for each of these types of facilities also depends on the FEMA performance zone where the building is located, as the weather conditions and likelihood of a tornado varies depending on the part of the country.

The Testing and Performance Requirements section of the specification outlines the necessary test conditions for adhering to AAMA 512. According to the specification, test specimens should be tested for anchorage, missile impact, water testing and cycling (for windows used in hurricane-prone zones only).

AAMA 512-11 is available for purchase through the AAMA Online Publication Store.

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