Bridge resistance deteriorates over time, impacting safety and time-dependent reliability due to non-stationary degradation and increasing load frequency and intensity. This paper investigates the reliability of reinforced concrete structures, focusing on these factors. A Gamma process models the non-stationary degradation of bridge resistance, validated through a finite element experiment with a simply supported RC beam, where tensile steel reinforcements were reduced to simulate deterioration. Two time-dependent reliability equations were derived from the Gamma process and verified via Monte Carlo Simulation. Results show that the sensitivity of load intensity growth, frequency growth, non-stationarity degradation of resistance and environmental affection occupies 56.1%, 0.03%, 40.5% and 3.37%, respectively. Load intensity growth declines the safety of aging structures most, while the non-stationarity of resistance degradation should be given extra attention, as the analysis did not set expectations for its growth.