Progressive Collapse Analysis of Steel Structure

Abstract

Progressive collapse occurs when a structure has its loading pattern or boundary conditions changed such that structural elements are loaded beyond their capacity and fail. The residual structure is forced to seek alternative load paths to redistribute the load applied. As a result, other elements may fail, causing further load redistribution. The process will continue until the structure can find equilibrium either by shedding load as a by-product of the failures of other elements or by finding stable alternative load paths. In the past, structures designed to withstand normal load conditions were over-designed and were usually capable of tolerating abnormal loads. Modern building designs and construction practices enabled engineers to build lighter and more optimized structural systems with considerably fewer over strength characteristics. It is estimated that at least 15 to 20% of the total number of building failures are due to progressive collapse. Progressive collapse became an issue following the Ronan Point collapse. Shortly after the Ronan Point collapse, British Standards emphasized general tying of various structural elements of a building together, to provide continuity and redundancy. Eurocode recommended tying the building together and defined values for tie forces. The National Building Code of Canada contains a general statement about the need for structural integrity. After the collapse of World trade center (WTC) towers, many government and private authorities worked on developing design guidelines for progressive collapse resistant structures. In the U.S., the prominent documents are: Progressive collapse analysis and design guidelines developed by the General Services Administration (GSA 2003) and Design of Buildings to Resist Progressive collapse developed by the Department of Defense (DoD 2005). Among all the available guidelines GSA and DoD guidelines are widely adopted for progressive collapse analysis. The present study includes various case studies of progressive collapse of structures around the world. It includes the evolution of various guidelines published by many government authorities and their comparison. Specifications of GSA and DoD guidelines are discussed in detail. 4-Storey and 9-storey moment resistant steel buildings are considered for evaluation of progressive collapse potential. Four analysis procedures are suggested by the vi guidelines to evaluate the potential of progressive collapse namely linear static, linear dynamic, nonlinear static and nonlinear dynamic. Linear static and linear dynamic procedures are carried out using structural analysis programme SAP2000 to find out demand capacity ratio (DCR) of beams and for highly stressed near by columns after the removal of load carrying elements from different locations. DCR found using linear static analysis are compared with the DCR calculated from linear dynamic analysis at each storey for different column removal cases.Displacements found under the column removal points by linear static analysis are compared with linear dynamic analysis for each column removal case. Nonlinear static analysis procedure is carried out to understand the extent of damage in form of hinges in the structure at yield point and at collapse load. The graph of percentage of vertical load Vs. deflection is drawn after nonlinear static analysis procedure for different column removal cases and plastic hinge rotations are found out for maximum collapse load as per GSA and DoD guidelines. Nonlinear dynamic analysis procedure is also carried out to understand the behavior of the structure accurately. Nonlinear dynamic analysis is carried out to find displacement ductility, maximum support rotation and plastic hinge rotation for all the column removal cases. Report also includes the mitigation strategies to resist or to reduce the chances of progressive collapse of an entire structure. Three retrofitting strategies are suggested for mitigation of progressive collapse. Comparison is made between the values of DCR, collapse load, displacement ductility and plastic hinge rotations for building without retrofitting and with retrofitting strategies.