Thanks to adrenaline-infused action films and an endless supply of urban myths, there is a general consensus that:
1) There is such a thing as “bullet proof glass” (in actuality, it’s “bullet resistant plastic,” usually acrylic, polycarbonate, or fiberglass), and
2) That “bullet proof glass” can shrug off a 9mm bullet, an RPG, raging seas, enraged dinosaurs, and an F3 tornado with equal ease.
This isn’t entirely irrational. Bullet resistant glass is one application of the “protective materials” that keep astronauts, fire fighters, soldiers, and scientists safe in the most inhospitable environments. For example, the storm-chasing “Tornado Intercept Vehicle” used by Sean Casey to film his popular television show Storm Chasers and the IMAX film Tornado Alley is armored with a number of materials familiar to bullet resistant glaziers. As TIV driver Marcus Gutierrez told a reporter with CBS11, the vehicle’s windows have been replaced with custom bullet proof glass, and the body itself is armored with layers of “ DuPont™ Kevlar® fiber, aluminum, steel, rubber, polycarbonate, rubber, and then steel again. So, it’s a sandwich of absorbent . . . materials.”
Bullet Proof Glass Has Its Limits
It’s important to note that Casey’s TIV is a small, mobile target, not a building: A moving vehicle offers less of a profile to tornado-force winds; when struck by tornado-driven debris it absorbs much of the energy of that projectile by rocking, sliding, or rolling into a corn field. Hardening a Dodge Ram 3500 to survive a twister is a far cry from securing a building.
Nonetheless, manufacturers occasionally claim to offer “tornado-proof windows” made from layers of laminated tempered glass or glass-clad polycarbonate. While some polycarbonate products carry UL-verified ratings for forced entry, blasts, and even gale-force winds, these are forces an order of magnitude milder than a tornado. Read the fine print on one manufacturer’s “tornado-proof windows” and you’ll discover that these are tested with 2x4s going just 35 miles per hour. While this is indeed a mild gale-force wind, the lowest winds registered by the Fujita tornado scale are twice as fast.
According to the Federal Emergency Management Agency: “Testing indicated that glass windows in any configuration (even laminated or ‘bulletproof’ glass) were not acceptable for tornado shelters.” And FEMA wasn’t just making a guess; from their website:
The standard missile tested was a 15-pound, 2-foot by 4-foot piece of wood (about 12 feet long) traveling at 100 miles an hour horizontally and 67 miles an hour vertically. This is considered the representative missile for a tornado with wind speeds up to 250 miles an hour. In testing, this missile was propelled into typical building material at a 90-degree impact.
Glass-clad polycarbonate will stop a burst from an AK-47, but nothing can challenge the fury of storm-driven lumber.