Abstract
Introduction
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.
Methods
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.
Results
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.
Conclusion
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.