Progressive Collapse Analysis of Tall Buildings with Different Structural Systems

Abstract

Structural engineers are facing new challenges in designing safe structure due to increase in terrorist attacks which causes damage and harm to the safety of the people. The main role of structure engineer is to understand the loads that building can carry throughout its life span and ensuring that integrity of the structural system remain intact. Conventionally, the buildings are designed for gravity and lateral loads but any abnormal loads are generally not considered in design. As a result structure undergoes failure, when subjected to abnormal loading. One of the mechanism of failure during such abnormal loading is known as “Progressive Collapse”. Progressive collapse is a situation where, localized failure spreads to the larger portion of structure. The main objective of the present study is to evaluate progressive collapse potential of R. C. building with outrigger structural system and steel building with diagrid structural system. Outrigger system is mainly used to reduce the core bending moment and drift of the building. Outriggers are stiff horizontal member provided at one or more location connected between perimeter columns and interior core. Diagrid system is most economical and flexible system as lateral loads are resisted by axial force in diagrids. Diagrid is a particular form of space truss which consists multiple diagonal elements that form a diagonal grid in the periphery of the structure. Diagrid system does not require core. Outrigger system and Diagrid system are effective in lateral load resistance. However, their performance during progressive collapse needs to be assessed. In this study, progressive collapse potential of R. C. building having outrigger structural system & steel building with diagrid structural system is evaluated. Nonlinear static analysis & Nonlinear dynamic analysis are carried out by following U. S. General Service Administration (GSA) guidelines under removal of column for outrigger system & removal of pair of diagrids for diagrid system. Modeling, Analysis and Design is carried out using MIDAS Gen (Modeling, Integrated Design & Analysis Software). The nonlinear static analysis also known as pushdown analysis performed as a displacement control and behavior is measure in terms of load factor, which is the ratio of load carrying capacity of building in each step of pushdown analysis to the original capacity of building. In general, Load factor indicates robustness of the structure. Nonlinear dynamic analysis is complex and time consuming analysis. The result of dynamic analysis represented by plotting displacement at dynamically removed column. R. C. building with outrigger structural system, Progressive collapse potential is evaluated under two different column removal scenario i. e. corner column & side face column. Effect of different location of outriggers along the height of building on progressive collapse resistance is also investigated. Outriggers are separately provided At top, At 2 3 rd height, At mid height, At 1 3 rd height, in case of building with only one outrigger. For two outriggers, it is provided at various locations such as At top & At mid height, At top & At 1 3 rd height, At top & 2 3 rd height, At 1 3 rd height & 2 3 rd height. From the analysis it is observed that building with one outrigger is having maximum resistance against progressive collapse under corner column removal when outrigger is provided at mid height and under side face column removal when it is provided at top. Higher percentage of hinges are formed with greater load factor when outrigger provided at mid height under corner column removal scenario, which indicates more number of members are participate in force redistribution. Similarly, higher percentage of hinge formation with higher failure displacement & greater load factor is observed when outrigger is provided at top under side face column removal scenario. Similar for nonlinear dynamic analysis, at mid height and at top location of outrigger gives a higher joint displacement, which correspondence to the higher failure displacement in nonlinear static analysis when corner and side face column removed respectively. For all the cases of one outrigger ductile failure observed. Building with two outrigger locations, it is observed that outriggers is having maximum resistance against progressive collapse under both the cases corner and side face column removal when outrigger provided at top & at mid height. Higher percentage of hinges are formed with higher failure displacement & greater load factor when outrigger provided at top & at mid height under corner and side face column removal scenario which indicates more members are participate in force redistribution. But two outriggers are not more effective when compared to building without outrigger as the load factor values and values of percentage of hinge formation are nearer. Similar, for nonlinear dynamic analysis the joint displacement value higher for at top & mid height location of outrigger correspondence to higher failure displacement in nonlinear static analysis. For all the cases of two outrigger location ductile failure observed. For the diagrid system, progressive collapse resistant capacity is assessed for different diagrid angles i.e 49.40, 66.80, 74.05, 81.87 under removal of pair of diagrid form one of the corner from bottom storey. It is observed that as diagrid angle increases, progressive collapse resistance decreases as less number of member are participate in force redistribution. The building having 66.80 angle of diagrid shows higher load factor. The building having 49.40 angle of diagrid shows ductile failure with evenly distributed hinge on both the side of removed diagrid. For 66.80, 74.05 and 81.87 angle of diagrids, the brittle failure observed with hinge formation at bottom storey of the building as per nonlinear static analysis. For nonlinear dynamic analysis, with increasing the angle of diagrids higher joint displacement observed which means system becomes more flexible. Validatery check for dynamic amplification factor 2 is carried out by taking the ratio of displacement measured from nonlinear dynamic analysis to linear static analysis. From the results, ratio of displacement obtained from nonlinear dynamic analysis to linear static analysis is observed less than 2, which indicates that dynamic amplification factor 2 suggested by guidelines, for static analysis is conservative.