Progressive collapse has been of an increasing concern in the structural engineering community, especially since the collapse of the World Trade Center towers in 2001. As a result of increasing catastrophic events in recent years, the prevention of progressive collapse is becoming a requirement in building design and analysis. A large number of studies have been performed to improve the design of the building against progressive collapse and to evaluate the progressive collapse potential of existing and new buildings by using computer programs and analytical tools. However, experimental evidence is still necessary to validate the computational analysis tools to better simulate the progressive collapse of structures.
In this research, both experimental and analytical assessments of the progressive collapse potential of existing buildings were conducted. Two actual steel frame buildings, the Ohio Union building in Columbus, Ohio and the Bankers Life and Casualty Company (BLCC) building in Northbrook, Illinois were tested by physically removing four first-story columns prior to buildings’ scheduled demolition. During the field tests the changes in column axial forces were measured, and the recorded strains were compared with the analysis results from computer models. A commercially available computer program, SAP2000 was used to model and analyze the test buildings, following the General Services Administration guidelines (GSA, 2003). Two-dimensional (2-D) as well as three-dimensional (3-D) models of each building were developed to analyze and compare the progressive collapse response. Also, two different analysis procedures were evaluated for their effectiveness in modeling progressive collapse scenarios; linear static and nonlinear dynamic procedures.
The measured strain data compared relatively well with the analysis results of SAP2000. In particular, 3-D model was more accurate than 2-D model, because 3-D models can account for 3-D effects as well as avoid overly conservative solutions. 3-D model had lower DCR values and vertical displacements than 2-D model, which was probably due to inclusion of transverse beams in 3-D model. 3-D model is believed to be more realistic than 2-D model for the progressive collapse analysis. Linear static analysis showed higher vertical displacements than nonlinear dynamic analysis for both 2-D and 3-D models. The amplification factor of 2 required for the dead load in linear static analysis may lead to very conservative analysis results.
This research is an initial step in developing analysis tools and design guidelines that could be easily and effectively used to evaluate the progressive collapse potential of new and existing buildings. It is expected that the field experiments and SAP2000 analyses performed in this research could provide structural engineers with both practical and fundamental information on the progressive collapse response of buildings.