A fabric-covered, steel frame practice facility owned by the National Football League's Dallas Cowboys collapsed under wind loads significantly less than those it should have resisted under applicable design standards, according to a report released today for public comment by the National Institute of Standards and Technology (NIST).
Located in Irving, Texas, the facility collapsed on May 2, 2009, during a severe thunderstorm. Twelve people were injured, one seriously.
The Cowboys facility was designed as a series of identical, rib-like steel frames supporting a tensioned fabric covering. Assumptions and approaches used in the design of the building led to the differences between the values originally calculated for the wind load demand and structural frame capacity compared to those derived by the NIST researchers. For instance, the NIST researchers included internal wind pressure due to the presence of vents and multiple doors in their wind load calculations because they classified the building as "partially enclosed" rather than "fully enclosed" as stated in the design documents. The NIST researchers also determined that the building's fabric could not be relied upon to provide lateral bracing (additional perpendicular support) to the frames in contrast to what was stated in the design documents and that the expected wind resistance of the structure did not account for bending effects in some members of the frame.
Based on data acquired during a reconnaissance of the collapsed facility, the NIST study team developed a computer model of a typical structural frame used in the practice facility and then studied the frame's ability to resist wind. NIST worked with the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory to estimate the wind conditions at the time of collapse. The researchers determined that, at the time of collapse, the wind was blowing perpendicular to the long side of the building. Maximum wind speed gusts at the time of collapse were estimated to be in the range of 55 to 65 miles per hour—well below the design wind speed of 90 miles per hour in the national standard for wind loads. A microburst (a small, intense downdraft which results in a localized area of strong winds) was centered about one mile southwest of the structure at the time of collapse.
According to the NIST and NOAA researchers, the wind field in the vicinity of the Cowboys facility at the time of collapse was consistent with design standards and not unusual.
The NIST report provides an analysis of the facility's collapse, including the most likely sequence of events, and recommends that other fabric-covered steel frame structures be evaluated to ensure adequate performance under design wind loads. These evaluations, says NIST, should determine whether or not the fabric covering provides lateral bracing for structural frames considering its potential for tearing; the building should be considered partially enclosed or fully enclosed based on the openings that may be present around the building's perimeter; and the failure of one or a few frame members may propagate, leading to a partial or total collapse of the structure.
The draft report is available online at www.bfrl.nist.gov/investigations/investigations.htm. Comments on the draft report and recommendation must be received by noon Eastern time on Nov. 6, 2009. Comments may be submitted in writing via e-mail to structuralsafety [at] nist.gov (structuralsafety[at]nist[dot]gov); fax to (301) 869-6275; or surface mail to the attention of Stephen Cauffman, NIST, 100 Bureau Dr., Stop 8611, Gaithersburg, Md. 20899-8611.
Once the final report is published, NIST will brief and provide technical support on its recommendations to the American Society of Civil Engineers (ASCE) committee currently developing a building standard specifically for tensioned fabric structures. NIST also will brief the appropriate committee of the International Code Council (ICC) on its recommendations for use in improving provisions in ICC's model building code.
For more details, see the NIST news release "Draft NIST Report on Cowboys Facility Collapse Released for Comment."