Fragility Curves of the Simple Single-Degree-of-Freedom Model of the Design Example of the Civil 404-100 Code

Fragility Curves of the Simple Single-Degree-of-Freedom Model of the Design Example of the Civil 404-100 Code

At present, the nonlinear pushover analysis based on the capacity spectrum method is used as a detailed seismic evaluation method for existing buildings in Taiwan. However, for mid-rise buildings, owing to the contribution of high modes, this analysis may yield incorrect conclusions. Nonlinear incremental dynamic analysis (IDA) may be used to correctly simulate a structure’s response to earthquakes, so it is more suitable for evaluating the seismic performances of mid-rise buildings. A structure’s fragility curve can be constructed using the results of IDA and may be used to examine whether the performance target will be achieved under a design-level earthquake. If the life safety standard is used as a performance demand under a design earthquake with a 475-year return period, then the probability of collapse of the structure must be less than 10%. However, due to the large amount of computation time required for the nonlinear dynamic analysis, this method is not generally accepted by engineers. In this paper, the original multi-degree-of-freedom (MDOF) model is replaced by the equivalent single-degree-of-freedom (SDOF) model to process the IDA and fragility analysis. Although the SDOF model will be less precise, it can effectively reduce the required computation time and may therefore be easily utilized to perform detailed seismic evaluations for mid-rise buildings. In this paper, a sample building based on the Civil 404-100 concrete engineering design code is used to determine how to establish the backbone curve of an equivalent SDOF model. This curve is determined from the capacity curve of the pushover analysis, IDA and fragility analysis of this SDOF are utilized for comparison with the results obtained using the original MDOF. The results show that the difference between the fragility analysis results of the equivalent SDOF system and those of the original MDOF system is highly dependent on the lateral-force distribution model that is used. Using the equivalent SDOF to process the IDA and fragility analysis can effectively reduce the computation time required for the nonlinear dynamic analysis. They can be used as a tool for detailed seismic evaluation of mid-rise buildings. However, this model will underestimate the probability of the GI state, and the performance requirement can be set to a GI state probability of less than 5%.