On a muggy evening on March 28, 2000, an F2 tornado ripped through downtown Fort Worth, Texas, and in 10 minutes, left 100 injured, two dead and $450 million in damage.
Several of the city’s landmarks, such as the 35-story Bank One Tower, the Mallick Tower and the Calvary Cathedral, were substantially damaged. The Bank One Tower alone lost 80 percent of its windows.
In the midst of tornado season in Tornado Alley, how likely is it the central business district of Oklahoma City could be hit? If it did happen, how might downtown fare?
No one knows when and if a tornado will hit, and downtowns in cities are not immune. According to the National Oceanic and Atmospheric Administration, Oklahoma City is the most commonly hit area in the United States, with more than 123 tornadoes on record up until 2008.
But don’t pack up and move your home or office just yet.
National Weather Service records show the closest tornado to downtown occurred about 35 years ago in June 1974, when an F3 that formed over Will Rogers World Airport got as close as S.W. 22nd Street and Robinson Avenue. The one before that, in September 1965, came closer ” N.W. Fifth Street and Hudson Avenue ” but registered an F0, and caused only light damage.
“(Downtown Oklahoma City) is basically no more or no less prone to tornadoes than other portions of the metro area,” said Doug Speheger, meteorologist for the National Weather Service of Norman. “It’s just been luck of the draw so far “¦ but we’ve had some close calls.”
So if you or someone you love works downtown, what building standards are in place to protect them?
Riding the storm outAccording to Roy Dean, structural engineer with Oklahoma City firm Trumble Dean, which has worked on renovations for the Skirvin Hotel, current building codes require mainframe structures to be able to withstand winds of 90 mph, or the winds of EF1 (Enhanced Fujita scale) tornadoes. By comparison, storm shelters are usually designed to withstand wind forces eight times greater than building code standards.
Dean said the Federal Emergency Management Agency (FEMA) has information that architects and engineers can use to design walls, roofs, doors and windows to resist windblown debris.
If that doesn’t reassure you, don’t panic.
According to the “Minimum Design Loads for Buildings and Other Structures” published by the American Society of Civil Engineers (ASCE), gust speeds are less than those associated with basic wind speeds (defined as 90 mph for most of the continental United States, including Oklahoma) in about half of the recorded tornadoes, said Benjamin Wallace, director of structural engineering for Frankfurt-Short-Bruza.
Wallace said that although the typical downtown building would have been designed for the code-required wind at the time of construction, previous versions of the code would have been similar. He added that buildings that are considered more important or represent a substantial hazard to human life in the event of failure are required to be designed for a wind force 15 percent higher than normal.
Such facilities are certain schools, day care and health care facilities, fire and police stations and others needed for emergency response, and operation after a storm.
Also worth considering are the aesthetics and cost of construction if buildings were built like storm shelters.
“Designing all buildings to resist the forces imposed by the 250-mph wind speed would be very uneconomical,” Dean said, speaking of the wind speed to which FEMA recommends storm shelters be built. “We typically design a small portion of the building to be a storm shelter whenever requested by the owner. The rest of the building is designed to resist the 90 mph winds.”
Dean said prior to the May 3, 1999, tornado that hit south of OKC’s central business district ” killing 36 people and leaving $1.2 billion in damage ” it was rare for commercial developers to be interested in built-in storm shelters. But that has changed.
“I always ask if the owner wants the shelter to be designed to satisfy the 250 mph wind speed recommended by FEMA,” Dean said. “Most buildings constructed in accordance with the local building codes can resist wind forces from EF1 tornados and near-misses by a small EF2 tornado.”
An EF2 has wind speeds ranging from 111 to 135 mph, and he said a “near-miss” means passing within a quarter-mile of the building.
Wallace’s firm, FSB, was the original architect and engineer for the Edward L. Gaylord Downtown YMCA and the state Capitol dome addition. FSB also worked on renovations for the former Kerr-McGee tower (now SandRidge Energy) and the former Mid-America building (the current Devon Tower).
Most cities have building design standards adopted from a specific issue of the International Building Code, which relies heavily on the ASCE to define minimum design loads for buildings, Wallace said. The document includes procedures to calculate minimum wind and seismic loads, as well as minimum-load capacities for floors and roofs. These minimum loads are considered adequate for the “usual” storms.
Standing strongThese codes, for the most part, seem to work, allowing downtown buildings hit with even severe tornadoes to hold.
Fort Worth’s Bank One Tower was repaired and converted to a mixed-use condominium project, called The Tower.
An F5 that hit downtown Lubbock, Texas, in 1970 killed nearly 30 people and cost $1.2 billion in damage. It did not, however, flatten any skyscrapers, although several were severely damaged.
“I don’t know of any high-rise structure which has been ‘blown down’ by a tornado,” said Wallace, recalling the Lubbock tornado’s havoc on the former Great Plains Life Building. “That building was permanently bent to one side to some extent, but did not collapse and was later re-occupied.”
According to Maj. Dean Findley, emergency manager liaison for the city’s emergency management team, downtown Oklahoma City falls into regions six and eight of the Oklahoma Homeland Security evacuation plan. But since the area is also comprised of Oklahoma, Cleveland, McClain, Pottawatomie, Canadian, Lincoln and Logan counties, it does not have a neighborhood or section-specific public evacuation plan.
“The plan is very general in nature,” Findley said. “(It) suggests that people shelter-in-place when a tornado is approaching.”
In other words, head below ground or move to the lowest floor of the building, toward the center and away from windows or large panes of glass.
Since downtown Oklahoma City’s buildings are responsible for their own evacuation plans, many have extensive measures in place.
Corporate Tower, for instance, conducts a drill every year in which all occupants must evacuate to the basement. Designated floor marshals as well as searchers and monitors from each tenant are responsible for assisting people to the basements and taking head-counts.
“If the civil-defense alarm sounds, we come on the intercom system of the building and tell people to evacuate to the basement at their own discretion,” said Mary Lou Derrick, business manager for Corporate Tower.
Findley said that, if requested, the city’s emergency management office does assist businesses to develop evacuation plans, although it is not mandatory.
“Each building downtown should have developed some type of evacuation plan that directs occupants to the safest location,” Findley said.
To Wallace, designing buildings to be as tornado-resistant as possible always seems to boil down to what people are willing to pay for while still having a building that looks visually appealing.
“Many people consider absolute tornado proof to mean underground construction only,” Wallace said. “Most of us are not willing to live and work in a cave all the time just to avoid a relatively rare hazard.”
photo Structural engineer Roy Dean said building codes allow for 90 mph winds. Photo/Mark Hancock