TOWERS WEAKENED BY PLANES; BROUGHT DOWN BY FIRE
Recommended areas for study may lead to future building code changes

Jill Dixon
703-295-6404
jdixon@asce.org

Jane Howell
Tel: 202-326-51289
jhowell@asce.org

That finding is significant, said W. Gene Corley, Ph.D., team lead for the ASCE/FEMA Building Performance Study Team, because extreme events of this type, resulting in such substantial damage, are generally not considered in building design, and the fact that these structures were able to successfully withstand such damage is noteworthy.

Among the significant findings from the World Trade Center Building Performance Study: Data Collection, Preliminary Observations and Recommendations are:

• Much of the jet fuel on board the hijacked planes that plowed into the Towers burned off in fireballs outside the buildings. Instead of causing the fires to burn at extremely high temperatures, as was widely speculated,the role of the jet fuel was to ignite other combustible materials over several floors simultaneouslyThose fires eventually weakened the structural steel, leading to the Towers' collapse.
• WTC Building 7, which sustained no significant structural damage, collapsed on September 11 after burning uncontrolled for seven hours, was the first protected steel structure ever known to collapse solely due to fire.
• The team found that some connections between the structural steel beams failed in the fires. This was most apparent in WTC 5, where the fireproofing did not protect the connections, leading to a partial collapse. The design and construction of WTC 5 is typical of many steel-framed high rises. WTC 5 is the first major collapse caused by failure of connections due to fire damage.

Based on its observations of how the Twin Towers and the surrounding buildings performed in the aftermath of the attack, the team recommends that the following six items be considered in the design and construction of buildings deemed likely targets of terrorist attack.

• Buildings should be designed with sturdy, back-up structural supports to bear the weight held by the primary supports when damage to the building occurs.
• Fireproofing needs to adhere under impact and fire-induced steel deformation so that the protective coatings remain on the steel and provide the intended protection.
• The connecting structural elements (nuts, rivets and plates) need to be analyzed to better understand how they fare under sudden impact and fire.
• When sprinkler systems are a critical part of a building's fire protection system, the water supply should be reliable and redundant.
• Stairwells should be evaluated for multiple alternate routes of escapes and strength in order to provide safe and clear evacuation routes when the building is damaged. Transfer floors, stair spacing and locations and stairwell resistance to impact should be reviewed as part of the evaluation.
• Fire protection ratings and safety factors for structural transfer systems should be evaluated for their adequacy relative to the role of transfer systems in building stability.

While the team does not call for immediate changes to existing building codes, the team strongly urges continuing study into the collapse of the buildings, which could eventually lead to revisions in the building codes. The two most significant include a review of current standards for fire resistance of connections and a more comprehensive study into the collapse of World Trade Center Building 7.

The National Institute of Standards and Technology is expected to spearhead the next round of studies into how the buildings performed. Their work will be completed within 24 months.

ASCE's Structural Engineering Institute began formation of building performance teams within hours of the attack on Sept. 11 as part of its Disaster Response Procedure. The goal of the study is to learn how to make buildings safer in the future. Investigations of this type are commonly performed by teams of engineers following other disasters such as earthquakes, hurricanes and blasts, and these investigations have frequently formed the basis for evolutionary development of our nation's building codes. At the Science Committee hearing, draft legislation is expected to be unveiled regarding future investigations into catastrophic building collapses.

ASCE's Disaster Response Procedure has been in place for approximately 10 years. The formation of the team to study the collapse of World Trade Center Towers marked the fifth time in 2001 that the procedure has been used to create study teams. In addition to the study conducted on the World Trade Center complex, ASCE also coordinated a team to assess the performance of the Pentagon after the Sept. 11 attack.

A link to the report and information on ordering printed copies is posted at www.asce.org. Interviews with members of the team can be scheduled by contacting Norida Torriente, (202)326-5129, or Jane Howell, (202)326-5128.

Founded in 1852, ASCE represents 125,000 civil engineers worldwide and is the nation's oldest engineering society. ASCE is now celebrating its 150th anniversary