Abstract
Introduction
Catastrophic disasters of large-scale framed structures occurred recently
are mainly caused by sudden, extreme external loads such as aircraft collision,
explosion, large seismic excitation, and big fire. Dynamic codes are generally
used to investigate such phenomena. However, strong nonlinearity in the
deformation of structures and rapidness of the external loads often generate
higher hurdle in the analyses. The authors have developed an adaptive finite
element code which provides higher computational efficiency than the conventional
code in such analyses, and enable us to cope with dynamic behavior with
strong nonlinearities including phenomena such as member fracture and elemental
contact.
Methods
An adaptive finite element code using linear Timoshenko beam elements called
the ASI (Adaptively Shifted Integration)-Gauss technique (Lynn and Isobe,
2007) have been applied to various collapse analyses of framed structures.
Contact release and re-contact algorithms are also developed and implemented
in the code to realize complex behaviors of structural members during impact
and collapse sequence.
Results
One of the various collapse simulations to be presented is a fire-induced collapse analysis of a high-rise tower, which is carried out for an investigation seeking for the true cause of the total collapse of New York World Trade Center (WTC) towers, which collapsed in 2001. The results clearly show the effect of the weak member joints, which were reported to be 20 to 30 % of the strength of the members in WTC towers, as well as the effect of the strength reduction due to elevated temperatures. The effect of outrigger truss system on roof top is also verified. The models with outrigger truss systems tend to withstand longer in time by catenary action only if their load paths are protected. Other simulation results to be presented include some controlled demolition analyses of a high-rise hotel, and impact collapse analyses of neighboring buildings under strong seismic excitation.
Conclusion
The adaptive finite element code with the ASI-Gauss technique can be effectively
used to investigate various collapse problems of framed structures. It
is confirmed that the contact release and re-contact algorithms developed
in this study also help us to understand complex behaviors of structural
members during impact and collapse sequence.