The worldwide application of streamlined flat steel box girder on long-span bridges calls for more knowledge of its temperature distribution. The rapid development of structural health monitoring techniques offers a great opportunity to address this issue. A comprehensive approach of installing monitoring equipment, collecting data, and applying long-term temperature monitoring data to study the temperature distribution of flat steel box girders is developed. As demonstrated through the analysis of 1-year data of a suspension bridge, first, a mapping relation between effective temperature and ambient air temperature is established. Such a relation enables identifying the optimal time to finally join the flat steel box girders at the designed effective temperature based on the easy-to-obtain ambient air temperature. Second, the cycling variation of effective temperature is presented to provide information for design and assessment of expansion joints and bearings, including not only the maximum design displacements but also cumulative displacements related to the long-term durability and remaining life of expansion joints and bearings. Finally, both vertical and transverse temperature gradients are studied to provide some new insights about the temperature characteristics of flat steel box girders. The study suggests that the transverse and vertical temperature gradients should be applied to the bridge cross section individually since the data analysis supports that the two gradients are independent.
