Dr. Barbara Lane, an expert in structural fire design solutions with London-based global consulting and engineering firm Arup, has presented the firm's findings that the collapse of the WorldTradeCenter towers due to fire could have occurred even without the loss of structural fireproofing caused by aircraft impact.
Dr. Lane presented the results of Arup's detailed structural fire collapse study at a two-day National Institute of Standards and Technology (NIST) technical conference on the FederalBuilding and Fire Safety Investigation of the World Trade Center (WTC) Disaster. She spoke at the public comment period of the session on Structural Fire Response and Collapse Analysis on Sept.15.
Following a three-year investigation and analysis of the WTC collapse, NIST is in the final stages of preparing the results of its study and recommendations for improvements to tall building design and management procedures.
Arup commended the work of NIST to model the WTC collapse—a vast undertaking. However, Arup's review of NIST's findings and its own analysis led it to conclude that NIST has not satisfactorily demonstrated its main conclusion but that the impact-induced loss of fireproofing was the deciding factor in the collapse.
Quantifying the performance of the structure in real fire scenarios is key in designing structures to withstand progressive collapse. For several years, Arup has been working with the University of Edinburgh to model the performance of structural frames in realistic building fires using finite-element analysis. This approach has been used by Arup to model a building with very similar structural design and fire characteristics to WTC Tower 1.
Arup’s analysis concluded that the effect of thermal expansion on the perimeter columns of the towers—even without the airplane impact—could have led to collapse due to the severity of fire occurring on multiple floors and the resulting thermal expansion of structural elements, particularly the floor systems. The Arup analysis conclusively illustrates that even with code-approved fire protection, a severe fire—without aircraft impact—could still lead to collapse.
Thermal expansion, an integral parameter of Arup’s modeling of the event, was not included in the NIST model—a likely reason for the differing conclusions. Arup supports the widespread application of such in-depth structural detailing for future tall building design and construction, as opposed to more prescriptive code- or materials-based solutions. This form of analysis can bring additional robustness to a structural design. Quantifying the response of a structural design subjected to fire allows a designer to determine the strengths and weaknesses of the design and make alternative detailing or other alterations to the structure to improve its performance.