Fragility Estimates for RC Buildings

Abstract

Fragility curves are commonly used to estimate the vulnerability of structures to earthquakes. The seismic fragility of a structure is expressed through a family of „fragility‟ curves, which plot the conditional probabilities of failure against varying intensities of the seismic hazard. The failure probability of the structure can be defined for multiple limit states. The thesis derives such fragility curves for the reinforced concrete frame structure with variation in material strength, accounting for the nonlinear behaviour of the system. A performance-based seismic design is applied to the case study building, which enables to have predefined multiple performance levels. In order to validate the performance-based design method against seismic excitations, seismic fragility curves are developed based on the demand models. “Fragility Estimates for Reinforced Concrete Buildings” is a parametric study, which has been attempted for which experimental pushover data is available. The objective of this research has three main phases. The first is to propose a simplified methodology to assess the expected seismic damage in reinforced concrete buildings.This simplified approach is summarised and applied to reference four storeyed building assumed to be located in zone-IV of IS: 1893(2002). In order to do so, the seismic behaviour of the building was studied by considering variation in material strength. Exhaustive review of literature has been done to understand the state of the art, to identify the points needing further research and then to perform seismic fragility estimates for RC buildings. Often nonlinear pushover analysis of RC building is required for establishing building capacity and fragility curves. This thesis presents a procedure for establishing the required fragility curves for various damage states, in particular for the damage states, based on nonlinear pushover analysis results. A solution is proposed for overcoming the difficulty encountered when determining the median spectral displacements for the damage states. Second phase of the study deals with usage of variation in material strength. The variations in strength of materials were generated considering partial safety factors for material strength. The thesis focuses on the structural fragility of reinforced concretebuildings under monotonic loading. Methodology has been adopted to quantify the effect of variation in material strength on prediction of capacity of structure. The capacity and demand assessment are addressed in detail with regard to RC frame treating concrete as confined and unconfined. Thus, thirty five models of moment resisting frames in each case were produced to represent the RC building stock. Each generated frame was subjected to pushover analyses using SAP2000. After seismic fragility estimates, more discussions are presented with regard to applying the performance based approach. Third phase of work considers the usage of Indian Strong Ground Motion. The variations of material strength for M20 grade of concrete, 20-30 N/mm2 and for Fe 415 steel, 520-600 N/mm2 are taken. Different models were created and non-linear static analyses of the reference building stocks are performed with two different modelling approaches. All the created models are subjected to Indian strong ground motion records. Twenty Indian ground motions are selected, scaled to different levels of intensity represented by peak ground acceleration. The fragility curves, damage thresholds were obtained. A method is defined for obtaining the yielding and collapse capacity of the analyzed structure using curves. The fragility curves for yielding and collapse damage levels are developed by statistically interpreting the results of the non-linear static analyses. In this work, an available test result of a full scale four storeyed RCC structure under monotonic load profile till failure was used. The test was conducted using tower testing facility at Central Power Research Institute (CPRI), Bengaluru along with NITK, Surathkal and in association with Reactor Safety Division, Bhaba Atomic Research Centre, Mumbai that provided a base shear versus roof displacement plot (experimental pushover curve). The result of the study is estimated fragility curves for typical reinforced concrete designed without seismic detailing; the obtained results clearly reveal the deformation capacity. Parametric study shows that probability of failure for the RC buildings varies with variation in material strength. The effectiveness of the results is demonstrated by their application to a structural model. Additionally, the fragility curves related to various damage states were estimated. It isconcluded that the proposed procedure offers a viable alternative to existing approaches. The experimental investigation results were utilized and compared with that of analytical investigation; very interesting conclusions have been drawn.