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Frame piers of double-deck viaducts (DVFP) have been widely studied and applied due to their high levels of space utilisation and significant engineering benefits, but the uncertainty of their information on modelling significantly affects the accuracy of calculation results for structural seismic demand and seismic capacity, which leads to challenges regarding the accuracy and reliability of seismic fragility analysis. In this paper, an accurate non-linear finite element model of DVFPs was established based on the OpenSees open platform. The seismic fragility analysis of DVFPs is carried out by introducing the endurance time method, and the tornado diagram and central point methods were performed to investigate the sensitivity and uncertainty of a seismic fragility of DVFPs to nine modelling parameters, respectively. The results show that the logarithmic standard deviation of fragility when slight damage, moderate damage, significant damage, and collapse occur on DVFP after considering modelling uncertainty will increase by 6.4%, 16.1%, 13.3%, and 20.1%, respectively. Therefore, it is necessary to consider the influence of modelling uncertainty in seismic fragility analysis. [ABSTRACT FROM AUTHOR]

This paper investigates the seismic behavior of a seismic-damaged double-deck viaduct frame pier (DVFP) strengthened with CFRP and enveloped steel, four strengthened DVFP specimens with different degrees of initial damage were tested under quasi-static cyclic loading. Based on the test results, the hysteretic behavior, the stiffness and strength degradation, crack propagation, and failure mechanism were firstly analyzed. Then, the damage indexes of the tested specimens were calculated with different models to evaluate the seismic strengthening performance. Results of this study show that CFRP and enveloped steel strengthening could effectively improve the strength and ductility of pre-damaged DVFPs. The ultimate load, the failure displacement and the displacement ductility of the moderately damaged specimen after being strengthened were found to increase by 120.74%, 35% and 32.33%, respectively. For the severely damaged specimens with CFRP and enveloped steel strengthening, the figures were 105.36%, 25.98% and 31.41%, respectively. The research results can provide reference for the hybrid strengthening application of seismic-damaged DVFP. [ABSTRACT FROM AUTHOR]