Progressive Collapse Analysis of R.C. Buildings

Abstract

Natural hazards like hurricane, flood, earthquake and terrorist attacks on buildings lead to great human loss as well as economic consequences. The failure of structural members under Blast, Impact, Wind pressures and Earthquake is a highly dynamic phenomenon. This type of event exposes abnormal loading on the building structure. Generally members of building are not able to resist this type of abnormal loading and results in to failure. One of the mechanisms of failure during such event is referred to as \Progressive Collapse". Progressive collapse is a dynamic event. Progressive collapse of building structures is initiated when one or more vertical load carrying members particularly columns are seriously damaged or collapsed during any of the abnormal event. Once a column is failed the building's gravity load transfers to neighboring members in the structure. If these members are not properly designed to resist and redistribute the additional load that part of the structure fails. As a result, a substantial part of the structure may collapse, causing greater damage to the structure than the initial impact. Thus it is necessary to prevent the progressive collapse of the important building structures. The aim of present study is to evaluate the potential for progressive collapse of Asym- metrical reinforced concrete building designed for seismic loading. Various causes for progressive collapse along with different types of progressive collapse are presented. Based on characteristic features six different types of progressive collapse are discussed i.e. Pancake-type collapse, Zipper-type collapse, Domino-type collapse, Section-type collapse, Instability-type collapse, Mixed-type collapse. The interest of Structural engineers is increasing to prevent the progressive collapse of the building. After the collapse of World Trade Center (WTC) Tower many gov- ernment and private authorities worked on developing design guidelines for progressive collapse resistant structures. Among all the guidelines the U.S. General Service Administration (GSA) and Department of Defense (DoD) is most widely used by structural engineers. Various criteria to be considered to perform progressive collapse analysis as specified by these guidelines are discussed in this report. These guidelines have suggested three analysis methods: Alternate load path method, Tie force method and Local resistance method. Four analysis procedures are suggested to evaluate the potential of progressive collapse like linear static, linear dynamic, nonlinear static and nonlinear dynamic. Comparison between various guidelines is also presented. Progressive collapse analysis is a process to determine the potential of progressive collapse of building. One 4-storey and 10-storey Asymmetrical reinforced concrete (RC) building is studied for progressive collapse analysis. Evaluation of progressive collapse potential of seismically designed building is carried out by following U.S. General Service Administration (GSA) and Department of Defense (DoD) guidelines. Linear static and dynamic analysis is performed using structural analysis program SAP2000 by following alternate load path method to find out the Demand Capacity Ratio (DCR). The DCR found using linear static analysis are compared with the DCR calculated from linear dynamic analysis at each storey. The displacement at the col- umn failure point, as obtained from static analysis is compared with dynamic analysis. To understand the nonlinear behaviour of building structure nonlinear static and dynamic analysis are performed. Formation of hinge pattern is obtained from the nonlinear static analysis. DCR calculated by linear static analysis are compared with the pattern of hinge formation obtained from nonlinear static analysis. Nonlinear dynamic analysis is performed by considering both material and geometrical non- linearities. Displacement observed from nonlinear dynamic analysis under column removal point is compared with displacement obtained by linear dynamic analysis. It is very important to mitigate the susceptibility of progressive collapse of build- ing if it is having high potential of progressive collapse. In this study three different alternatives are explored to mitigate the potential of progressive collapse. The content of major project is divided in to various chapters. Chapter 1 includes general introduction of progressive collapse. Chapter 2 presents the review of litera- ture for progressive collapse analysis. A criterion for progressive collapse analysis by GSA guidelines is discussed in chapter 3. Various analysis procedures suggested by Unified Facilities Criteria (UFC) along with comparison between various guidelines to evaluate the potential of progressive collapse is presented in chapter 4. In chap- ter 5 and 6 progressive collapse analysis of 4-storey and 10-storey Asymmetrical RC building is illustrated respectively. All the four analysis is performed to evaluate the potential of progressive collapse. Procedure to find out the DCR for beam in exure and shear and DCR for column is also presented. Three different mitigation strategies to prevent progressive collapse is illustrated in chapter 7. Summary, conclusions and future scope of work are included in chapter 8.