Numerical Investigations on Effects of Structural Parameters in Fire-Induced Collapse Behaviors of High-Rise Buildings


Official reports on the total collapse of the New York World Trade Center (WTC) towers regarding the 9/11 incident, released by the Federal Emergency Management Agency (FEMA) in 2002 and ones released by the National Institute of Standards and Technology (NIST) in 2005, had suggested that the differences of aircraft impacted locations between the two towers and the ranges of fire that followed had made the collapse initiation times different. Moreover, the reports had concluded that the outrigger truss systems in upper stories of the two towers, which provided stiffening of the frame against wind loads, gave effects on delaying the collapse initiation. We conducted some fire-induced collapse analyses to investigate how fire range, structural weakness of member joints, axial force ratio and outrigger truss system give influence on collapse behaviors, as well as on the collapse initiation time of high-rise buildings.

An adaptive finite element code using linear Timoshenko beam elements called the ASI (Adaptively Shifted Integration)-Gauss technique (Lynn and Isobe, 2007) is applied to a 30 stories-7 spans framed tube structure model with some different cases of fire patterns, strength of member joints, stiffness of outrigger trusses, as well as axial force ratio of columns. Fracture, contact, contact release and re-contact algorithms are developed and implemented in the code to realize complex behaviors of structural members during collapse sequence. Reduction curves of elastic modulus and yield stress of steel related to temperature shown by NIST are adopted to represent the structural effects of fire. Thermal expansion of materials is also considered.

The results of the fire-induced collapse analyses of high-rise buildings show influences of the factors mentioned above to the collapse initiation time as well as to the collapse behaviors of the buildings. The strongly designed models tend to withstand high temperature without any sign of collapse, whereas the weak models, especially with asymmetric range of fire, start to collapse at comparatively lower temperature. The outrigger truss systems placed on roof tops help elongating the collapse initiation time only if the lower structures are strong enough.

Fire-induced collapse behaviors and collapse initiation times of high-rise buildings are investigated using an originally developed numerical code. It is shown that the collapse initiation time of high-rise buildings are elongated only if the load paths to/from the systems are sufficiently protected.