Volume 48, Issue 12- December 2013
In Light of Recent Storms, Experts Weigh in on Wind Speed vs. Wind Pressure
More than 80 tornados ravaged the Midwest in mid-November, leaving six dead and hundreds of destroyed homes in its wake. The storm marked the second high-profile incident of wind-related devastation, following the havoc left in the Philippines the week before by Typhoon Haiyan. Laminated glass might have helped lessen the damage in both cases, but even that’s unlikely given the vast amount of wind-borne debris in both incidents, experts say.
Tyler Mallard, the project manager for Architectural Wall Systems, the Des Moines, Iowa-based firm that designed the hardened windows of the new Joplin, Mo., hospital whose windows are built to withstand winds of up to 250 miles per hour (mph), didn’t sound optimistic about a similar solution.
A slew of major structural changes to every home would have to be in place before any such remedy could be attempted. The price would be steep.
“It’s possible,” Mallard says, “but I just don’t know from a construction standpoint if it would be feasible.”
Rick De La Guardia, the president and founder of DLG Engineering, says it is the pressure rather than the wind speed that is most critical.
“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?’” asks De La Guardia.
“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.”
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 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.