This thesis investigates two critical aspects of bridge engineering: the fatigue life of bridges under traffic loads and the impact of cold temperatures on their ultimate capacity for composite I-girder bridges. The first part of the study examines the fatigue resistance of continuous steel I-girder bridges, analyzing the influence of girder profiles and stress concentrations on fatigue damage. Monte Carlo simulations indicate that optimizing girder geometry can reduce the risk of fatigue damage by up to 35% over a 75-year service life. The second part of the study explores the structural behavior of steel I-girder and reinforced concrete deck bridges under cold temperature conditions. Experimental testing using a scaled bridge model subjected to four-point bending tests demonstrated a 10.3% increase in ultimate load resistance in cold conditions compared to tests conducted at normal temperatures. However, this improvement came at the cost of a 44.5% reduction in maximum deflection and a 55.4% decline in ductility for the most deformation-sensitive girder (G1). While cold temperature exposure resulted in a modest increase in strength, these findings provide valuable insights for designing bridges in cold climates.