<p>In this study, a Copula based framework is developed to derive seismic fragility functions at both component scale and system scale for double column pier bridges. A pier damage index based on curvature is proposed by combining moment curvature analysis with the frequency distribution of peak axial forces obtained from nonlinear time history analyses, which addresses the time varying axial force effect under transverse excitation. Bearing displacement, pier curvature, and tie beam plastic hinge rotation are adopted as component damage indices, and probabilistic seismic demand models are established using incremental dynamic analysis and log linear regression. The joint failure probability of key components is then evaluated using multivariate Copulas so that system fragility can be quantified while explicitly considering component dependence rather than assuming independence or perfect correlation. The proposed approach is demonstrated on a three span prestressed concrete bridge and is verified by comparison with first order and second order bound methods. The results show that, for a selected Copula family with estimated parameters, the proposed formulation yields a single valued system fragility curve that improves interpretability relative to bound based intervals and provides a practical tool for fragility assessment of double column pier bridge substructures.</p>