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Failure of Welded Floor Truss Connections During Collapse of the World Trade Center Towers



Stephen W. Banovic, Thomas A. Siewert


As part of the National Institute of Standards and Technology World Trade Center Investigation, failure modes of the connections attaching the composite floor system to the exterior wall of WTC 1 and WTC 2 were surveyed. Metallographic analyses of intact and failed welds of the main load-bearing truss seats complemented the survey to identify the location of metallurgical failure for these connections. Above the aircraft impact floors, the failure modes were randomly distributed. However, over 90 percent of floor truss connections at or below the impact floors of both buildings were either bent downwards or completely sheared from the exterior wall suggesting progressive overloading of the floors below the impact zone following collapse initiation of the towers. Depending upon joint geometry, detachment of the main truss seats occurred either by fracture in the heat affected zone of the base material, where the standoff plate detached from the spandrel, or through the weld metal, where the seat angle detached from the standoff plate. Failure in both cases was the result of a shear mechanism due to an overload condition. Exposure to fires prior to the collapse was not found to have an effect on the failure mode of the floor truss connections.
Welding Journal


collapse, floor truss connections, weld failure, World Trade Center


Banovic, S. and Siewert, T. (2007), Failure of Welded Floor Truss Connections During Collapse of the World Trade Center Towers, Welding Journal (Accessed April 12, 2024)
Created September 3, 2007, Updated February 17, 2017