Seismic fragility curves of steel structures including soil-structure interaction and variation of soil parameters

Abstract

The current paper focuses on calculation of the seismic fragility curves for steel structures representing special moment frames conventionally used in highly seismic regions. To be able to draw results more resembling the reality, flexibility of the underlying soil is taken into account. Moreover, since soft soils exhibit variations and uncertainties in their mechanical characteristics, their properties are assumed to be probabilistic parameters in this study. For this purpose, 4, 8, and 12-srory special moment frame steel structures are adopted. The collapse spectral acceleration of the buildings is calculated using the incremental dynamic analysis under concurrent horizontal components of 11 consistent earthquakes. Assuming a Log-Normal distribution, the probability of exceedance of the collapse threshold is calculated at each spectral acceleration value, as a fragility curve. The soil-structure interaction is modelled using the theory of beam on the nonlinear Winkler foundation. Using the Tornado analysis, the most influencing soil parameter on collapse is identified to be its shear modulus. The Monte Carlo analysis is utilized to account for uncertainty of the soil characteristics. It is shown that variation of the soil parameters affects the collapse behavior of shorter buildings more considerably both in terms of relative variability and amplitude. The relative difference between the 16 and 84 fragility percentiles reduces from 14% for the 4-story to 6% for the 12-story building. Soil-structure interaction increases the collapse probability at the same spectral acceleration between 11 and 21% for the shorter structure and between 3 and 13% for the taller building.