Glass Can Withstand 200 mph Winds, But What about Flying Debris?November 18th, 2013 by DWM Magazine
Finding a glass that can withstand 200 mile-per-hour (mph) winds is one thing, but somehow finding the kind that can also withstand the wind-borne debris that accompanies such a powerful storm is another matter entirely, say several experts.
The topic is a timely one now, given the swath of destruction left in the Philippines last week by Typhoon Haiyan. The powerful storm, which brought sustained winds of just under 200 mph and gusts as high as 235 mph, devastated the South Pacific Island nation, leaving thousands dead and millions more without food and shelter.
Rick De Le Guardia, president and founder of DLG Engineering, says laminated glass might have helped, but not even that could have withstood the fury of the many projectiles filling the air during a storm such as Haiyan.
“The answer is yes – glass can sustain the uniform pressures associated with wind speeds of 200 miles per hour,” De La Guardia says. “But the real question is ‘Can glass sustain the impact forces of wind-borne debris associated with a wind speed of 200 mph and the subsequent continued loading after impact?’
“The answer to that is probably not, at least not in a cost-effective manner.”
Bob Ziders, an engineer with the National Certified Testing Laboratories, echoed similar thoughts.
“Glass can stand [200 mph. winds],” he says. “It’s the debris that causes it to break and that’s the problem.”
There are glazing products available that have been built to withstand such extreme winds. The new hospital being constructed in Joplin, Mo., will feature windows that can withstand up to 250-mph winds.
De La Guardia is quick, however, to point out the big distinction between wind pressure and wind speed. The pressure, which is the actual load imposed on a product by the wind, is a function of wind speed among numerous other factors.
“It is the pressure imposed on the glass,” De La Guardia says, “that is critical and not the wind speed.”
De La Guardia says that glass is not designed to support wind speed, but rather wind pressure, which is defined as pounds per square foot of pressure or force applied to the surface of the glass as it relates to the wind speed.
Other factors that determine varying levels of wind pressures include height above grade, pitch of roof, proximity to building corners, surrounding terrain, etc. Factors that affect the strength of the glass include the lite’s overall size, support conditions and load duration among others, says De La Guardia.
It’s for that reason that he says it’s misleading at best for any manufacturer to boast of having a product that can sustain high winds in tornados or more powerful storms such as Haiyan unless they provide the “pressure” associated with that particular wind speed and the glass parameters.
At worst, he says, it can lead people into a potentially fatal false sense of security.
“For example,” De La Guardia says, “yes, a piece of glass one-foot long by one-foot wide located on the ground floor of a one-story building may be able to support the pressures associated with the wind speeds of 200 mph, but perhaps not if that same glass specimen is installed on the 20th floor, which would be subject to much-higher pressures under the same 200 mph wind.”
De La Guardia also notes that a glass specimen of two feet long by two feet wide located on the same ground floor of the same one-story building would not be able to sustain the pressure associated with the same 200 mph winds due to its increased size.
The point, however, is moot if the glass isn’t protected from the debris strewn through the air by destructive winds of that nature.
“That’s the killer,” Ziders says.