In addition to the dead and live traffic loading normally considered in design, bridges and other structures. exposed to the elements may experience significant thermal movements and stresses depending on the materials used, type of construction, geometry, support conditions, and the range and variation of temperature within the structure. The present AASHTO specifications, recognizing that provisions should be made for thermal movements, provide temperature ranges for use in design; however, no direct provisions are made for temperature distributions within the structure or the methodology needed to find thermal stresses resulting from these distributions.

The temperature distribution within a structure subjected to the elements is time dependent. The primary environmental parameters affecting the distribution are ambient air temperature, solar radiation and. air velocity. An understanding of the erratic manner in which air temperature and solar radiation vary with time is essential to the thermal analysis of structures exposed to the environment.