Progressive Collapse Analysis of Buildings and Its Risk Estimation Using Key Element Index

Abstract

In case of emergency such as intense fire or collision of some objects to buildings, there is a risk of progressive collapse; a phenomenon which occurred to the World Trade Center (WTC) towers during the 9.11 terrorist attacks. The official statements released by the Federal Emergency Management Agency (FEMA) in 2002, and also by the National Institute of Standards and Technology (NIST) in 2005 and 2008, concluded that the details of the failure process after the decisive initial trigger that sets the upper part in motion were very complicated and their clarification would require large computer simulations. In this study, the collapse behaviors of steel framed buildings are simulated using the Adaptively Shifted Integration (ASI) - Gauss code to investigate the relation between a key element index, which indicates the contribution of a structural column to the vertical capacity of the structure, and the scale of progressive collapse. Collapse was initiated by removing specific columns from the models designed based upon different axial force ratios. Some patterns of removed columns, of which the locations were restricted to a single floor, were investigated. The total potential energy values of structural members after the collapse were used to estimate the collapse scale of the buildings. By evaluating the numerical results using the key element index, it was found that the larger the integrated value of key element index the higher the risk of progressive collapse; however, some peculiar tendencies were observed in the cases of removed columns with extremely symmetrical or asymmetrical locations. The critical integrated values of key element index to cause a large-scale progressive collapse tend to depend on the strengths of buildings, and they cannot be uniquely decided. This is due to the fact that the key element index is a parameter which does not relate to the strength of the building itself, and it cannot be compared relatively between buildings with different strengths. Therefore, the key element index may be used to predict and compare the risks of progressive collapse, only in the same building, but even when the locations of removed columns are variously assumed.