UI profs survey tornado destruction for design clues
OKLAHOMA CITY — When a massive EF5 tornado struck the Oklahoma City suburb of Moore, Okla., on May 20, it left a path of debris a mile wide and 17 miles long.
Poking around in the wreckage last weekend, alongside those involved in cleanup efforts, were two University of Illinois civil engineering professors, Jim LaFave and Larry Fahnestock, hunting for clues about how buildings hold up during a devastating storm.
They surveyed homes, hospitals and other public and commercial buildings to see what they could learn about construction practices and disaster preparedness.
The engineers from the Mid-America Earthquake Cen- ter based at the UI try to understand how the safety of air- ports, schools, bridges and other structures can be improved to protect against all kinds of hazards — terrorism, earthquakes, tornadoes and the like.
Funded by a Rapid Response Grant from the UI Department of Civil and Environmental Engineering, the two structural engineers wanted to survey the damage while it was still relatively undisturbed. They arrived in Oklahoma late Thursday and returned Sunday.
They saw a 2-by-4 embedded in the roof of a medical center, one of the many projectiles hurled by the twister. They saw a steel support beam bent by the winds, steel roof panels peeled up like sardine cans, and homes, businesses and strip malls flattened by the force of the tornado. They also heard remarkable stories of survival, including three employees at a collapsed Dollar General store who took shelter in a central office and somehow crawled out of the wreckage unhurt.
The information that LaFave and Fahnestock gathered will form the basis for continued research to better understand and design for strong winds and "windborne missile" debris during tornadoes. Down the road, their findings could be incorporated into new building codes.
LaFave was also part of a team that visited New York two weeks after the 9/11 attacks to survey damage to buildings adjacent to the World Trade Center towers, which hurled debris into neighboring structures when they collapsed.
One emerging area of research involves above-ground "safe rooms," which seem to have performed well during the Oklahoma tornado, LaFave said.
The UI team didn't see any in Moore, but they toured several homes with storm shelters built into their garages, sort of a high-tech version of storm cellars. While most houses there don't have basements, many newer homes have a concrete pit in the garage, roughly 3 feet wide, 6 feet long and 7 feet deep, with a sliding steel door across the top, he said.
He met one homeowner who had used the shelter along with several other people, and they all emerged safely even though his home was "beaten up pretty badly."
Older neighborhoods have concrete storm shelters built into the ground in their back yards with heavy steel doors.
"In one case, a house was completely wiped out," but the storm shelter was intact.
The engineers' first stop was the Moore Medical Center, which sustained extensive damage and will likely be razed, LaFave said. Patients and staff huddled in the middle of the hospital during the storm, and "other than a few scratches, people got away in pretty good shape," he said.
The professors saw "lots and lots of damage and debris" there, but most was confined to the facade, windows, roof and a poorly designed carport, he said. The uplift generated by the winds peeled the roof system off its supporting structure "like a sardine can." It's possible more stringent guidelines could prevent that in the future, he said.
They also found beams and other parts of nearby houses embedded in the side of the medical center, but the overall structure held, he said.
A nearby bowling alley and strip mall didn't fare so well, essentially collapsing in the storm. They were made of pre-engineered steel, commonly used for one-story strip malls or some "big-box" stores and not as heavy as a building designed for a specific site, he said.
"These are extremely strong winds we're talking about," he said. "On the other hand, in an area like this, or central Illinois, the goal should be to design structures that at least can remain standing even if there's a lot of damage."
Nearby was the Dollar General, where LaFave ran into two of the women who had ridden out the storm in the store's office.
"We couldn't even tell what it was until we got really, really close," he said.
LaFave said researchers may want to look at the lateral force-resisting system for such buildings, which are typically braced with fairly lightweight steel rods. Just as a building's floor must tie into support beams and eventually the foundation to hold weight, a structure needs a clear "load path" to resist the force of strong winds, he said.
Touring residential areas, researchers also noticed that garage doors had often failed during the storm. Regardless of which direction a house was facing, once the garage door was breached, the wind got up under the roof, and it collapsed, even if the rest of the house suffered little damage, he said.
Florida and other areas prone to hurricanes, have much stronger requirements for garage frameworks and doors, so they won't get breached during storms, and also require clips that tie the roof framing to the walls, he said.
Everywhere was a "flurry of activity," with residents replacing roofs and making other repairs, he said.
"You just hear hammers all the time," he said.
A few blocks away, other residents were sifting through the rubble of what was once their homes "trying to claim whatever last odds and ends they could" before trucks arrived to remove the debris.
"They're going to have to just start over," LaFave said.