Frozen ground is significantly stiffer than unfrozen ground.For bridges supported on deep foundations,bridge stiffness is also measurably higher in winter months.Significant changes due to seasonal freezing in bridge pier boundary conditions require additional detailing in order to ensure a ductile performance of the bridge during a design earthquake event.This paper reports the latest results obtained from a project that systematically investigated the effects of seasonally frozen soil on the seismic behavior of highway bridges in cold regions.A bridge was chosen and was monitored to study its seismic performance and assess the impact of seasonally frozen soil on its dynamic properties.A Finite Element(FE) model was created for this bridge to analyze the impact of seasonal frost.It was found that when frost depth reaches 1.2 m,the first transverse modal frequency increases about 200% when compared with the no-frost case.The results show that seasonal frost has a significant impact on the overall dynamic behavior of bridges supported by pile foundations in cold regions,and that these effects should be accounted for in seismic design. Frozen ground is significantly stiffer than unfrozen ground. For bridges supported on deep foundations, bridge stiffness is also measurably higher in winter months. Significant changes due to seasonal freezing in bridge pier boundary conditions require additional detailing in order to ensure that ductile performance of the bridge during a design earthquake event. This paper reports the latest results obtained from a project that systematically investigated the effects of seasonally frozen soil on the seismic behavior of highway bridges in cold regions. A bridge was chosen and was monitored to study its seismic performance and assessability the impact of seasonally frozen soil on its dynamic properties. A Finite Element (FE) model was created for this bridge to analyze the impact of seasonal frost. It was found that when frost depth reaches 1.2 m, the first transverse modal frequency increases about 200 % when compared with the no-frost case.The results show that seasonal frost has a significant impact on the overall dynamic behavior of bridges supported by pile foundations in cold regions, and that these effects should be accounted for in seismic design.