COVER STORY, JULY 2011
PREPARING FOR THE WORST
Can commercial structures be better designed and built to stand up to tornadoes?
By: Coleman Wood
The devastation brought on by this past spring’s busier-than-usual tornado season provided a perfect example of how we are often powerless to the forces of nature. In the wake of severe tornadoes all across the Midwest, trees were ripped out of the ground, cars were flung around like toys, and buildings were reduced to piles of splintered wood and warped metal. Then, there is the especially high death toll — a result, in large part, of the catastrophic tornado that hit Joplin, Missouri, on May 29.
With advances in building design and construction techniques over the past few decades, could some of the damage from storms like this be prevented? To begin answering this question, it is best to first look at how tornadoes commonly damage buildings.
“The most common damage we’ll see in a commercial structure is on the roofing,” says Dr. Kishon Mehta, a professor of civil engineering at Texas Tech University and a researcher at the school’s Wind Science & Engineering Research Center, which studies destructive wind events such as tornadoes. Mehta explains that in normal circumstances there is a small amount of uplift on the roofs of all buildings. This uplift is even more intense in the event of a tornado. In weaker storms, roofing material may be picked up, but in stronger storms, the entire roof itself can be torn off.
The likelihood of the roof being picked up increases if there are any openings in the building. Years ago, people were advised to open their windows in the event of a tornado to decrease the chances of being hit by broken glass, as well as to keep their houses from collapsing. However, subsequent studies found that when a structure has any openings, whether from open windows or holes created by flying debris, an imbalance is created between the high pressure inside the house and the low pressure created by a tornado outside. Because air pressure always seeks to equalize itself, this imbalance causes even more uplift on the building’s roof, greatly increasing its chances of being torn off by the storm.
“When the roofing is lost, then the walls become vulnerable,” Mehta says. “They are only supported on the ground and at the roofline.” Mehta explains the connections from the walls to the roof and the walls to the foundation are the only things keeping a building standing up. Once the roof is gone, it does not take much more to cause the walls to collapse.
Some commercial structures hold up better than others during a tornado. According to the National Ocean and Atmospheric Administration’s Enhanced Fujita tornado damage scale, a warehouse building can be completely destroyed at wind speeds as low as 131 miles per hour — which equates to an EF2 tornado. This has to do with how the building itself is designed. Warehouses have little structural redundancy in order to provide the most open space possible for users. When the roof is lost, there is little supporting the especially tall walls of the building, which can be up to 40 feet tall in more modern warehouses. In addition, these structures often have large numbers of overhead doors, which can collapse in wind speeds as low as 75 miles per hour.
Because of their similar design, big-box retail structures do not hold up well in tornadoes, either. The roofs of these structures can be removed in wind speeds as low as 114 miles per hour and the entire building can collapse from winds as low as 147 miles per hour (an average F3 tornado).
“In commercial construction, we’re very interested in the economics of the structure, and basically no one wants to pay any more than they have to for a building,” says Stan Peterson president of Topeka, Kan.-based Peterson Architecture Group and a member of the American Institute of Architects’ Disaster Assistance Task Force. “That’s why we have a lot of pre-engineered metal buildings. Structures that are a longer space are difficult to design for the wind loads, and that’s why you see failure in a lot of grocery stores and large-box buildings.”
Multi-story office buildings stand up best in the event of a tornado. Low-rise buildings (ranging from one to four stories) will start to see uplift of lightweight roofs at 114 miles per hour and complete destruction at 161 miles per hour. Mid-rise buildings (ranging from five to 20 stories) can see roof uplift or collapse at wind speeds as low as 118 miles per hour and significant damage to the façade at speeds of 120 miles per hour, but the total destruction of the building will not occur until the wind speed reaches at least 181 miles per hour (an F4 tornado). This has a lot to do with the heavy concrete floors that comprise these buildings as well as the extra care taken in designing them.
“Anything that tall is going to be custom-designed by a structural engineer, while a warehouse is more governed by prescriptive requirements,” Mehta says.
There are several ways to design and construct a commercial building to stand a better chance of surviving a tornado. The first is to simply design the building to code.
“If everything is designed in accordance to standard, it will survive almost 60 to 70 percent of tornadoes,” Mehta says.
This means that all of the building connections, especially the connections between the roof and the walls, must be secured correctly. One must also make sure the load path from the roof to the ground is strong. A building may still be damaged in a storm, but the chances of the building collapsing are reduced. While there is a uniform building code in place that can help buildings survive a severe storm, it is very important to make sure that architects and builders follow it.
“What we find in the Midwest that is a little different from the East Coast or West Coast is that we have more rural communities, and often the rural areas never adopted a building code and don’t have an inspection department,” Peterson says. “So, a lot of structures that are built in rural areas are basically not to code because they don’t know about it.”
One specific building design that causes concern is ballasted roofs. During a tornado, the winds can cause severe uplift in the roof if it is not secured to the walls properly. In addition, the ballast, which is often pea gravel of patio stone, can easily be picked up and become an airborne missile that can be far more damaging than hail. The same goes for veneer materials such as brick and stone if they are not properly secured to a building façade.
However, sometimes a tornado is simply too powerful for a building to withstand. They are also very unpredictable, which prevents builders from simply reinforcing a specific side of the building.
“Tornadoes are creatures of their own,” Peterson says. “They can work outside of the box, so it’s hard to design for a typical tornado, because as soon as you do, they change their mind.”
Every building mentioned in the Enhanced Fujita scale is likely to fail in the event of a strong F4 or F5 tornado. It is not economical to try and design an entire building for tornado resistance, either, so in cases like this, builders are advised to construct a safe area within the building.
In larger buildings, a safe area can be a masonry structure such as a stairwell. Smaller structures are advised to create a community safe room. A safe room only needs an average of five square feet of space per person, so existing rooms within the building can often be used. The doors and walls need to be reinforced and there should be no windows. In addition, the ceiling of a safe room should be independent of the building’s roofing system. The Federal Emergency Management Agency offers detailed instructions for constructing individual and community safe rooms.
It is important to keep the topic in perspective. The likelihood of a single building being hit by a tornado — even in the Midwest — is still very small. There are many architects and builders who do not address this issue at all when developing a project. Still, it is worth thinking about when focusing on a new project, especially in the tornado-prone Midwest, and if a minute of forethought can save a life later down the road, then it is well worth it.
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