Sunder: Thermal expansion is key.
The National Institute of Standards and Technology, which released on Aug. 21 a draft of its final report on the collapse of Seven World Trade Center on September 11, 2001, is recommending that procedures and practices used to ensure fire endurance of structures be enhanced to prevent global collapse in the event of "infrequent" fires that occur when life safety systems fail or fires are not fought.
The fire at Seven WTC was triggered by debris from the collapse of One WTC. The 47-story building collapsed after burning for seven hours, unattended by firefighters. The primary cause was thermal expansion of the steel girders due to heat of the fires, says the report.
A girder from perimeter column 44 to interior column 79 on floor 13 lost its connection to column 79, in the core that supported long spans on the east side of the building. That caused floor 13 to collapse. A "cascade" of floor failures followed down to the fifth floor. The collapse left the critical column unsupported over nine stories. A rapid succession of structural failures then ensued and the entire building collapsed.
Shyam Sunder, lead investigator for the three-year inquiry, says no other factors, including explosives, fires caused by the fuel supply tanks in and under the building and damage from debris from One WTC, caused the collapse of Seven WTC.
The 50-member team that wrote the report says it "believes" structures can be enhanced to reduce the likelihood of collapse from burnout in a "cost-effective" way. NIST says the long-span structural systems are of concern. The authors suggest minimizing collapse caused by thermal steel expansion by using more robust connections and framing systems, frames designed to resist progressive collapse, better thermal insulation, improved compartmentation, and thermally resistant windows.
Thermal expansion of steel is not considered in current building codes, says Sunder.
NIST has invited comments on the findings until noon EDT on September 15. The report will be finalized during October. The report is available at http://wtc.nist.gov.