Influence of input motion uncertainty in developing slope-specific seismic fragility curves based on nonlinear finite element simulations

In the seismic fragility assessment for geotechnical structures, input motion selection for nonlinear dynamic finite element (FE) analyses has solely relied on the method from an incremental dynamic analysis (IDA) although methods from cloud analysis (CA) and multiple stripe analysis (MSA) have been introduced in structural engineering. This study investigates uncertainties in the seismic fragility curve of slopes arising from input motion selections used in nonlinear dynamic FE analysis for developing a probabilistic seismic demand model (PSDM). We consider a single FE slope model and four different sets of input motions. Sets 1, 2, and 3 are generated based on the methods used in CA, IDA, and MSA, respectively. Five PGA levels are considered in Sets 2 and 3. Set 4 also adopts the MSA method, but the five PGA levels are equally spaced on a logarithmic scale whereas Set 3 chooses the five PGA levels on a linear scale. Comparisons of the seismic fragility curves from Sets 2, 3, and 4 relative to the curves from Set 1 reveal that Set 4 (with a sample size of 9, involving 45 analyses) is more computationally efficient than Set 2 (with a sample size of 50, totaling 250 analyses) and would yield the curve stochastically closer to the one from Set 1. Our findings may provide the MSA-based method (Set 4 with logarithmically spaced PGA levels), as a practical alternative to the IDA-based set, which can improve the precision of the PSDM given the computational challenges with the CA-based method.