The destruction of the World Trade Center (WTC) towers had an emotional impact on everyone. As structural engineers, we too felt the pain of the lives taken, those who gave their lives in service and the loss of a great achievement. It has come to our attention that "conspiracy theories" regarding the collapse of the WTC are being perpetuated on the Internet by individuals who are not structural engineers. As students of the art, we would like to present a few facts to dispel some of these "myths."
On Feb. 23, Leslie Robertson, structural engineer for the WTC, gave a presentation entitled "The Merging of Structural Engineering and Architecture." The WTC was not the subject of his talk, though he generously answered a question regarding tall structures and terrorism. In his opinion, modern structures should not be designed to withstand intentional aircraft impacts. No amount design would yield a structure that is both 100 percent indestructible and cost effective.
The following comments are our response to the letter to the editor entitled "Sept. 11 collapses not explained," which appeared on Feb. 24.
The WTC was constructed according to national standards and codes in place during its design. The original analysis considered the impact of a Boeing 707. However, the assumption was that the plane would not be fully fueled, would be low flying and would not have the intent of crashing into the building (as in 1945 when a B-25 bomber crashed into the Empire State building under similar circumstances).
The fires that occurred in the WTC were initially sparked by jet fuel, which ignited other sources (desks, carpet, etc.). In 2004, NIST recreated the conditions and found temperatures reaching 1800 degrees Fahrenheit. The steel used in construction was tested under ASTM E119 (a controlled laboratory test, not a certification procedure) and could withstand such temperatures without melting. However, temperatures of this magnitude would cause the steel to undergo thermal expansion, deforming under the existing load. Deformations from thermal expansion create extremely large stresses in the entire assembly, which would cause connections to fail.
The design of the WTC was revolutionary. It consisted of an outer tube around an inner core. The inner core was not designed to withstand wind loads. It was designed to carry gravity loads from the floor system. This "truss theory" supposedly states that the floor system rigidly connected the outer tube to the inner core. In reality, the floor system was not rigidly connected because the core was not designed to withstand the loads imparted on the outer structure. The failure of the WTC was a result of the compromised outer tube undergoing large thermal stresses.
The collapse of the WTC was a result of progressive failures. Many believe visual evidence of the buildings collapsing vertically indicates the buildings were brought down by a controlled demolition. In reality, the buildings collapsed vertically because the structure could no longer withstand the force of gravity. There were no other forces on the building at the time to cause it to fall any other way.
The FEMA report is comprehensive and does not represent a government conspiracy. Conspiracy theorists who state that the FEMA report is wrought with lies serve only to harm the reputation of engineers who went beyond the call of duty on Sept. 11 and since. What should be remembered about the WTC is that it withstood the initial impact so that thousands of people could evacuate.
The FEMA report can be found at http://www.fema.gov/library/wtcstudy.shtm.
The NIST report can be found at http://wtc.nist.gov/.
Kyle Butler
Graduate Student
Structural Engineering Graduate Students of Notre Dame
CE 461/561 Structural Systems Class
Feb. 27
