Progressive Collapse Analysis of Steel Braced Frame Buildings

Abstract

The analysis and design of structures is usually carried out by considering the gravity and lateral loads. The abnormal loads like vehicular impact, gas explosion, blast, air-craft impact, etc. are generally not considered for analysis and design of normal structures. The abnormal loads acting on the structure, for which structure is not designed, results in failure of one or more load-bearing elements like columns, load-bearing walls, etc. which may lead to failure of a large part of the structure, disproportionate to initial local failure. Such failure phenomenon is referred to as “Progressive Collapse”. Progressive collapse is a situation where the local failure of primary structural elements leads to the collapse of adjoining members due to redistribution of additional forces, which in turn leads to the spread of collapse. The progressive collapse of any structure causes catastrophic failure of structure and substantial loss of human lives as well as natural resources. In order to reduce risk of progressive collapse, it is necessary to design and detail the structure to develop alternate load path in the event of extreme loading.` This work includes study of different causes of the progressive collapse and various case studies of structure experience progressive collapse in the past. Many government and private agencies worked on the development of guidelines for progressive collapse analysis and design of structure, among which guidelines issued by the U. S. General Services Administration (GSA) and the Department of Defense’s (DoD) Unified Facility Criteria (UFC) are widely followed by researchers and practicing engineers. The U. S General Services Administration (GSA 2016) guidelines are used in this study. In the present study, the progressive collapse potential of 9-story steel braced frame building is evaluated and its performance is compared with the steel Moment Resisting Frame (MRF) building. To begin with, two different types of bracings i.e. chevron bracings (also known as inverted V bracings) and 2-story X bracings (also known as a combination of V and inverted V bracings) are considered to examine the effect of the provision of bracings on progressive collapse resistance of steel buildings. In both, the types of braced frame buildings, six different cases of bracing arrangements provided in different bays in longitudinal and transverse direction throughout the building height are considered. The progressive collapse potential is evaluated by performing three analysis methods i.e. linear static, nonlinear static, and nonlinear dynamic analysis, as suggested in GSA guidelines using SAP2000 software, under three different column and/or brace removal scenarios from the ground floor of the perimeter frame. Based on the results of linear static analysis, the Demand Capacity Ratio (DCR) at few critical locations are calculated for beams, columns, and bracings. The results of DCR for Inverted V Braced Frame (IVBF) and 2 Story X Braced Frame (2XBF) buildings are compared with DCR of MRF building. One most effective arrangement of bracing members is identified for both IVBF and 2XBF building, based on DCR results obtained through linear static analysis. The non-linear static and dynamic analyses are performed for the identified effective arrangement of bracing members for both IVBF and 2XBF buildings. Pushdown curves and displacement-time history plots at column removal locations are developed from nonlinear static and dynamic analysis, respectively in addition to a comparison of results obtained for MRF building. Further, the provision of bracings members are optimized by 23% for the 9-story steel building considered for the study by providing bracings in combinations i.e. horizontally as well as vertically. For the optimized bracing arrangements, four different types of bracing configurations are considered i.e. Chevron (Inverted V) bracing, 2-Story X bracing, cross bracing and diagonal bracing. Linear static, nonlinear static, and nonlinear dynamic analyses are performed for three different column and/or brace removal scenarios from the ground floor. DCR and pushdown curves are obtained from linear static and nonlinear static analysis, respectively. Based on nonlinear dynamic analysis, displacement-time history plots, column axial force Impact Factors (IFs), column internal forces time history plots, brace axial force time-history plots are developed in addition to the calculation of ductility demand. Incremental Dynamic Analysis (IDA) is also carried out to identify the ultimate load-carrying capacity of buildings and their behavior at ultimate load. From the analysis, it is found that the provision of bracings in the building is an effective way to increase the progressive collapse resistance. From the load transfer mechanism after column and/or brace removal, it is observed that the bracings are effectively acting as an alternate path and safeguard the beams and column from being failed to prevent the collapse of the building. From the results of various cases of bracing arrangements and optimization of bracings, it is evident that the bracings provided at different locations behave differently. So, the arrangement of bracings should be such that, it increases the resistance of building against progressive collapse globally. From the results of various types of bracing configurations, it is proved that all types of bracing configurations considered in the study are effective to reduce the risk of progressive collapse. However, the performance of Chevron (Inverted V) braced frame building Chevron (Inverted V) brace configuration is more effective as compared to 2-Story X bracings, Cross bracings, and diagonal bracings under the different column and/or brace removal.