PROGRESSIVE COLLAPSE ANALYSIS OF A REINFORCED CONCRETE FRAME BUILDING

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1 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14) ISSN (Print) ISSN (Online) Volume 5, Issue 12, December (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJCIET IAEME PROGRESSIVE COLLAPSE ANALYSIS OF A REINFORCED CONCRETE FRAME BUILDING Shefna L Sunamy, Binu P, Dr. Girija K 1, 2 (Civil Engineering Department, Sree Narayana Gurukulam College of Engineering, Kolenchery, Kerala, India) 3 (Civil Engineering Department, Govt. Engineering college, Bartonhill, Thiruvananthapuram, Kerala, India) ABSTRACT Progressive collapse in a structure occurs when major structural load carrying members are removed suddenly, and the remaining structural elements cannot support the weight of the building. This failure usually occurs in a domino effect and leads to a progressive collapse of the structure. The basic characteristic of progressive collapse is that the end state of destruction is disproportionately greater than the failure that initiated the collapse. This paper describes linear static analysis of a multi-storeyed building using SAP 2000 by static removal of a major structural element and nonlinear dynamic analysis by removing a single column. Keywords: Acceptance Criteria, Column Removal, Demand Capacity Ratio, Progressive Collapse, Nonlinear Analysis. 1. INTRODUCTION The progressive collapse of building is initiated when one or more vertical load carrying members (typically columns) is removed. When a column is removed, (due to a vehicle impact, fire, earthquake, man-made or natural hazards) the building s weight (gravity load) transfers to neighboring columns in the structure. If these neighboring columns are not properly designed to resist and redistribute the additional gravity load that part of the structure fails. The vertical load carrying elements of the structure continue to fail until the additional loading is stabilized. As a result, a substantial part of the structure may collapse, causing greater damage to the structure than the initial impact. Progressive collapse occurs when a structure has its loading pattern or boundary conditions changed such that some members are loaded beyond their intended capacities. The residual structure is then forced to seek alternate load paths to redistribute the out-of balance loads from damaged members. As a result, other neighboring members surrounding the residual structure may also fail shedding some applied loads. The redistribution of loads is a dynamic process and will continue until a new equilibrium position is reached by the residual structure, either through finding a stable alternate load path or through further shedding of loads as a consequence of collapsed members 2. LITERATURE REVIEW Rakshith K G and Radhakrishna, (2013), studied about progressive collapse which is such a disproportional failure, which refers to the condition when the failure of a local component (or localized region) leads to global system failure. The progressive collapse of reinforced concrete structures is initiated when one or more vertical load carrying members are removed due to man-made or natural hazards. The building s weight transfers to neighboring columns in the structure, leads to the failure of adjoining members and finally to the failure of partial or whole structure system. In which the collapsing system continually seeks alternative load paths in order to survive. The adequate reinforcement 93

2 provided in extra to beams which are unsafe can develop alternative load paths and prevent progressive collapse due to the loss of an individual member. Lanhui Guo and Shan Gao, Feng Fu, (2013) studied about Partial or full range progressive collapse of structures which is triggered by a local damage due to abnormal events such a gas explosion, bombing attack or vehicle collision may lead to terrible causalities and severe economic loss. It is mainly because that the loads on superstructures cannot be transferred downwards when a vertical load carrying component fails. However, in the process of column failure, catenary action plays an important role in redistributing the internal load and preventing progressive collapses of the structure. Rigid composite joints exert great influence in catenary action. Therefore, an experiment related to a 1/3 scale progressive collapse resistance with the use of rigid composite joints was conducted, and the results of the experiment were analyzed. In catenary stage, catenary action evidently enhanced the resistance to the progressive collapse of the frames. The steel concrete composite frame with rigid connections designed in accordance to current design standards showed a good resistance to progressive collapse. It is also found that horizontal restraining stiffness of the frame exerted great influence on the resistance in catenary stage. 3. ACCEPTANCE CRITERIA An examination of the linear elastic analysis results shall be performed to identify the magnitudes and distribution of potential demands on both the primary and secondary structural elements for quantifying potential collapse areas. The magnitude and distribution of these demands will be indicated by Demand-Capacity Ratios (DCR). Acceptance criteria for the primary and secondary structural components shall be determined as: where, D.C.R= Q UD / Q CE (1) Q = Acting force (demand) determined in component or connection/joint (moment, axial force, shear, and possible UD combined forces) Q CE = Expected ultimate, un-factored capacity of the component and/or connection/joint (moment, axial force, shear and possible combined forces) Using the DCR criteria of the linear elastic approach, structural elements and connections that have DCR values that exceed the following allowable values are considered to be severely damaged or collapsed. The allowable DCR values for primary and secondary structural elements are: DCR < 2.0 for typical structural configurations DCR < 1.5 for atypical structural configurations 4. SAP2000 MODELLING AND ANALYSIS The building considered for the study is twelve storey symmetrical R.C. building. The structure consists of six bays of 5 m in the longitudinal direction and four bays of 5 m in the transverse direction. Typical floor-to-floor height is 3.1 m and for the first story it is 3.4 m. Wall having 115 mm thickness is considered on all the beams. Slab thickness considered is 150 mm. Beam size is taken for twelve storey s as mm. Column sizes are 500x500, 600x600 & 900x900 mm are considered for building. Loading considered on the building for the study are as follows. Dead load Live load Seismic loading as per IS: 1893 Fig.1: Plan of the building 94

3 4.1. Linear static progressive collapse analysis To evaluate the potential for progressive collapse of a twelve storey symmetrical reinforced concrete building using the linear static analysis three column removal conditions is considered. First building is designed in SAP 2000 for the IS 1893 load combinations. Then separate linear static analysis is performed for each case of column removal. Fig. 2: 3D model generated in SAP Calculation of Demand Capacity Ratio Demand capacity ratio for flexure at all storeys is calculated for all three cases of column failure..capacity of the member at any section is calculated as per IS456:2000 from the obtained reinforcement details after analysis and design. Demand capacity ratio after removal of column is found out considering the member force for the load combination as per GSA guidelines. Member forces are obtained by analysis results carried out in SAP Graphical Representation of DCR After getting all the DCR values for critical cases of column removal, for all zones graph is plotted DCR Vs Storeys. Fig.3: Graphical representation of DCR when Long side column removed 95

4 Fig.4: Graphical representation of DCR when Short side column removed Fig.5: Graphical representation of DCR when Corner column removed 5. NONLINEAR STATIC PROGRESSIVE COLLAPSE ANALYSIS Nonlinear static analysis is widely used to analyze a building for a lateral load and is known as pushover analysis. It increases applied loads step-by-step until maximum load is attained (load controlled) or maximum displacement is attained (displacement controlled). This method can be used to determine the ductility measure of the structure for lateral loading. Ductility is measured as a ratio of maximum displacement and yield displacement. Generally, the ability of the structure to attain large ductility results in better performance under earthquake loading. For nonlinear analysis automatic hinge properties and user-defined hinge properties can be assigned to frame elements. When automatic or user-defined hinge properties are assigned to a frame element, the program automatically creates a generated hinge property for each and every hinge. There are five default hinge options are available, Axial (P), 96

5 Torsion (T), Moment (M2 or M3),Shear (V2 or V3), and Coupled (P-M2-M3). The hinge properties are calculated by the program for the cross section and reinforcement details provided. For default moment hinges, SAP2000 uses Tables 6-7 and 6-8 of FEMA 356. Fig.6: Pushover curve: 3D Frame without column removed case 6. CONCLUSIONS Fig.7: Pushover curve: 3D Frame when long side column removed Seismically Designed building has inherent ability to resist progressive collapse. Nonlinear static analysis reveals that hinge formation starts from the location having maximum demand capacity ratio. To avoid the progressive failure of beams and columns, caused by failure of particular column, adequate reinforcement is required to limit the DCR within the acceptance criteria. To mitigate progressive collapse an alternate load path has to be provided. The alternate load path like, providing bracing at floor level and increasing size of column at the outer face can be adopted advantageously. REFERENCES [1] Weifeng Yuan and Kang Hai Tan,Modeling of progressive collapse of a multistorey structure using a spring mass damper system, Structural Engineering and Mechanics, 37, 2011, [2] Rakshith K G, Radhakrishna, Progressive collapse analysis of Reinforced Concrete framed structure, International Journal of Research in Engineering and Technology, 2013, [3] Lanhui Guo, Shan Gao, Feng Fu,Experimental study and numerical analysis of progressive collapse resistance of composite frames, Journal of Constructional Steel Research,89, 2013,

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