Progressive Collapse Level of Analysis Capacity Evaluation of a Retrofit Scheme

Vol.1 No.3

Year: 2012

 
Issue: Sep-Nov

Title : Progressive Collapse Level of Analysis Capacity Evaluation of a Retrofit Scheme

Author Name: James M. Portea, Jeffrey Laman

Synopsis:

The General Services Administration, Department of Defense, and the National Institute of Standards and Technology have all published and continuously maintained new building design guidelines to lower potential for progressive collapse. In addition, the analysis procedures prescribed by these guides cover a range of options or levels; however, the progressive collapse guidelines focus primarily on new construction. Therefore, research is needed to identify and investigate the effectiveness of novel retrofit methods for mitigating progressive collapse of existing buildings through the available analysis levels. The primary objective of the current study was to investigate and compare the outcomes of four levels of analysis: linear-elastic static analysis (3D); nonlinear static analysis; and dynamic nonlinear analysis (2D and 3D) and to establish an analysis methodology for investigating steel buildings retrofit concepts. The retrofit scheme investigated was devised to improve the progressive collapse resistance of a 5-story steel building subjected to sudden, exterior column loss. Results showed that an a-value of 2 generally leads to highly conservative estimates in the results from 5-story models that did not fail at the progressive collapse design load combination. For the 5-story models that failed at the recommended static load combination, the ratio of peak dynamic end moments to static end moments are only comparable because the full static load was not reached before failure, thus also pointing to a conservative a-value. An assessment of the dynamic amplification factor yielded values of 1.4 to 1.6. A less conservative a-value cannot be specified based on the results of this study alone, but the analysis results indicate that for certain buildings, predictions on performance after column loss that are based on a static analysis with an a-value of 2 may be overly conservative. Although the retrofit did not significantly improve the performance of the 5-story building with moment frames in two directions, the retrofit increased the load-carrying capacity of the model without a moment frame orthogonal to the exterior bay. In this case, the retrofit was more effective because the interior beams on each floor, which would otherwise rotate freely if the cables were not in place, participate in resisting some of the floor loads.