Using Numerical Simulations and Engineering Reasoning under Uncertainty: Studying the Collapse of WTC‐1

Abstract

A series of numerical aircraft crash simulations and thermal behavior analyses were made at Purdue University to study the response of the World Trade Center Tower 1 (WTC-1) on September 11, 2001. The process included accuracy verification for the computational tools using available experiment data. Numerical models for the Boeing 767-200ER aircraft and the structural system for the top 20 stories of WTC-1 were developed for the simulations. A second aircraft model, simpler yet comparable in effect, was developed and used for a parametric sensitivity analysis. Results from these simulations and published by other researchers indicate that while the observed impact damage to tower exterior framing can be estimated accurately, the unseen impact damage to the core structure of the tower could not be estimated with high confidence. Although the computational tools helped in developing an understanding as to what might have happened as the aircraft penetrated and disintegrated into the structure, they were not able to reduce the uncertainty in the core damage estimate. However, reflecting insight from the behavior of the Pentagon building under the impact loads it received on the same day and studying the effects of elevated temperature on mechanical properties of steel in light of experimental data, the uncertainty in the core structural damage estimate was found to be of negligible importance with regards to the ultimate fate of the tower. It is demonstrated that through use of numerical simulations and engineering reasoning, a dominant factor in the collapse of the tower could be proposed with confidence. It was the loss of fire-proofing in the tower core during aircraft impact * To whom correspondence should be addressed. Email: ayhan@ purdue.edu. that left the core vulnerable to ensuing thermal loads and resulted in the eventual collapse of the tower.

Design of High Rise Steel Buildings against Terrorist Attacks

“…The design against aircraft impact can be performed in a similar way as that for a vehicle impact. Nevertheless, the impact of civil airliner or military aircrafts at typical high rise buildings will usually cause local failure of the affected structural elements due to its enormous kinetic energy (Irfanoglu, 2012). In such cases the building should be planned with the Alternate Path procedure.…”

Structural fire analysis of simple steel structures by using LS-DYNA explicit solver

“…LS-DYNA is a commercial general-purpose finite element software [19] and is one of the mostly common used explicit integration software [8]. It has been used for the aircraft impact, progressive collapse analysis of World Trade Center (WTC) by NIST [17,18]. Rackauskaite et al [8] benchmarked four selected testing cases to study the model parameter sensitivity of structural fire analysis in case of fire using Hughes-Liu beam element by LS-DYNA explicit solver.…”

Response of reinforced concrete beams to high‐velocity fluid impact. Part I: Experiments

“…Of the 47 columns in the building core area, a total of 25–30 column lines were destroyed in stories between 94 and 99, where heavy damage or destruction took place in the core . Even though the impact may have caused significant damage in the core area, the researchers found that the uncertainty in the damage level became immaterial when the effects of elevated temperature, due to ensuing fire and missing steel thermal insulation, on the strength and stiffness of steel members were taken into consideration . The damage inflicted by aircraft impact on the insulation of the core framing was the dominant factor in the collapse of the structure …”

“…11,12 Even though the impact may have caused significant damage in the core area, the researchers found that the uncertainty in the damage level became immaterial when the effects of elevated temperature, due to ensuing fire and missing steel thermal insulation, on the strength and stiffness of steel members were taken into consideration. 12,13 The damage inflicted by aircraft impact on the insulation of the core framing was the dominant factor in the collapse of the structure. 14 Impact simulations by National Institute of Standards and Technology showed that had the aircrafts that impacted the WTC towers were devoid of fuel, their wings would have produced significant damage to the exterior columns but would not have failed the columns regardless where they might have hit the towers.…”