Incremental Dynamic Analysis of Seismic Fragility in RC Columns with Varying Reinforcement Ratios

This study evaluates the influence of longitudinal reinforcement ratios on the model-predicted seismic fragility of reinforced-concrete (RC) cantilever columns. By holding geometry, materials, loading, and modeling formulation constant, four 300 × 300 mm, 4.0 m-high numerical columns with varying reinforcement ratios (6.55%, 4.19%, 2.79%, and 2.18%) were analyzed in SeismoStruct to isolate the comparative influence of steel content on drift-based fragility. Seven horizontal records from the 1989 Mw 6.9 Loma Prieta earthquake were incrementally scaled using peak ground acceleration (PGA) as the intensity measure. Maximum drift ratio served as the engineering demand parameter, with lognormal fragility functions fitted for 0.5%, 0.7%, 1.5%, 2.5%, and 5.0% drift thresholds. Results demonstrate that the mean PGA required to reach a 5.0% drift decreased from 1.85g for the most heavily reinforced column to 1.42g, 1.07g, and 0.95g as reinforcement decreased. While performance differences between the columns were small or non-monotonic at lower thresholds, they became clear at severe drift levels, accompanied by increased record-to-record dispersion for the least reinforced model. Ultimately, this controlled comparison highlights that reinforcement effects depend significantly on the selected drift severity and fitted dispersion, providing simulation-based, relative threshold-exceedance trends rather than evidence of experimentally validated physical collapse.

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