Natural disasters have long been a formidable foe for the design and construction industry. Building codes are evolving continually to minimize damage and injuries from hurricanes, tornadoes, earthquakes, and floods. Of all these, wind is probably the worst.
Joseph E. Saliba, provost at the University of Dayton and former Dean of the School of Engineering, says that while all natural disasters provide a challenge, designing for wind “is the ultimate test for an engineer. You have to design for wind from every direction because if there is a weak link, wind will find it. And once it finds its way inside, you’re going to have problems.”
Wind, however, is a force that the tilt-up concrete industry has withstood with considerable success. As early as the 1970s, tilt-up concrete walls were playing a significant role in new construction throughout Florida and much of the southeastern U.S. Given this region’s inclination for hurricanes, tilt-up buildings quickly proved able to survive these storms thanks to the monolithic concrete panels that span greater lengths and widths. Tilt-up also offered more flexibility in structural systems and aesthetic treatments, and grew as a popular construction method for delivering a wide range of building types.
Tilt-up construction is a method of constructing a concrete wall system on the jobsite. Often referred to as site-cast precast, tilt-up involves casting a concrete element in a location other than its final destination. Once the panels are erected with a crane, they are braced temporarily until the final structural elements of the building’s force-resisting system are in place. The performance of the wall panels both during construction and in the final structure is dependent upon decisions of the structural engineer.
This year, storm damage has once again visited the U.S., with some of the largest storms on record affecting Missouri, Alabama, Minnesota, and Kansas. But one major construction project with tilt-up concrete walls near Lambert-St. Louis International Airport escaped historic winds with no damage.
On April 22, an EF4 level tornado with estimated 200 mph winds—the St. Louis area’s most powerful tornado in 44 years—tore through the airport and a densely populated suburban area. The storm destroyed up to 100 homes, shattered hundreds of panes of glass at the main airport terminal, and blew a shuttle bus onto a rooftop. A nearby four-story office building under construction with tilt-up concrete wall panels, however, remained standing and unharmed.
The four-story Express Scripts Building 3 is an office and data center. At the time of the tornado, approximately 95% of the structural steel was in place on the building and 75% of the floor decks were poured, although the roof was not yet in place. The yearlong construction on the 234,000-square-foot facility is expected to finish in November 2011.
“This was the second time this project was faced with a high-wind event,” says Marko Borovic, project manager for Concrete Strategies, the St. Louis-based company performing the tilt-up work on the project. The site was hit with more than 70-mph winds in an earlier phase of the project when the walls were much more vulnerable. “Work was scheduled on welding the connection plates but the panels had just been erected,” Borovic says. “The majority of the panels were held up by temporary bracing used to support the panels during the construction process prior to the erection of supporting structural members.”
Scott Collins, PE, assistant chief engineer for Tampa, Fla.-based Meadow Burke, the company that supplied the concrete lifting systems and accessories, says the majority of the braces for the panels were anchored to Meadow Burke’s Badgers—a helical ground anchor system. Installed to the outside of the building, these anchors and braces leave the inside of the building open for the erection of structural steel.
Requirements differ for the support of panels during construction from when the building is occupied. The requirements during construction are based on the reasonable assumption that life safety is not a significant concern after a certain wind speed, because the contractor will have removed the crew from the jobsite.
Collins cites the Wind Bracing Guidelines published by the Tilt-Up Concrete Association (TCA) as the standard followed. “Using the TCA guideline, we designed the structure to withstand 90-mph winds, similar to that of permanent structures, but modified by a statistical factor of 0.8 to represent the short time period panels are supported temporarily. This factor reduces the actual wind speed used for design of temporary bracing to 72 mph,” he says.
He notes that temporary bracing transfers enormous loads to the anchor point of the brace foot under high-wind conditions. Although engineers sometimes are asked to increase the maximum wind speed for the temporary bracing condition, Collins states that was not the case on this project.
“Recommended safety factors per TCA Wind Bracing Guidelines are 1.5:1 for braces and 2.0:1 for brace anchors. Braces used for the tall 60-foot panels on the Express Scripts Building 3 project have a 16 kip ultimate strength, and were fortunately not loaded to their maximum capacity,” says Collins. “The braces were anchored to helical anchors that were installed in reasonably stable soils, resulting in above average ultimate strengths of nearly 25 kips. I anticipate that the largest panel with the highest brace loads should have theoretically been able to withstand approximately 96-mph winds.”
According to Jim Baty, TCA technical director, the stucture’s outstanding performance in a high-wind event is not uncommon. “There are numerous cases every year of buildings constructed using tilt-up walls panels that have resisted significant storm forces,” he says, “whether on a coast from hurricanes or inland from tornadoes.”
David Tomasula, PE, a structural engineer and principal of LJB Inc., Dayton, Ohio, noted that LJB and its design/build construction partners have designed and constructed more than 1400 buildings using tilt-up construction in the past 40 years. According to Tomasula, many of these buildings were designed to resist extreme forces—such as hurricane force winds, seismic activity, even blast forces—and he is confident in the ability of tilt-up to perform as intended.
Unfortunately, tornadoes are not limited to the Midwest. Premier Beverage, a 580,000-square-foot building in Tampa, Fla., with 45-foot-tall panels, experienced a direct hit from an EF2 tornado.
“The primary damage came from the large box gutter being ripped from the west wall,” says Randy Simmons, chairman of R.R. Simmons, Tampa, Fla. “The metal gutter in this area bounced across the roof creating some punctures to the membrane, which is easily repaired.”
Florida is home to some of the nation’s most stringent building codes, strengthened following several significant events, including Hurricane Andrew in 1992. For Premier Beverage, the building was designed for a higher wind load than normal as a precautionary measure to protect the contents. “We had an above-standard membrane roof that was very resistant to the wind forces,” says Simmons. “The building had very few rooftop penetrations that further limited the horizontal forces on the roof to tear away. With the stiffer roof design, the potential for panel failure was limited.”
Having used the tilt-up system for decades and seen its exceptional performance in high-wind events including Category 5 hurricanes, Simmons has tremendous confidence in the building method. He notes that responsible users or owners of buildings with high-value contents should recognize the value in enhancing the structural integrity of their roof systems. “Without much exception,” he says, “the wall systems rarely fail unless they are pulled down by a weakened roof system. The shear mass of the tilt-up system makes it an exceptional system for resisting hurricane level forces.”
Ed Sauter is the executive director of the Tilt-Up Concrete Association (TCA), Mount Vernon, Iowa.