There are two types of thermal activity that occur in bridges that are of interest for
design. The flrst is the annual variation of the average temperature of the bridge. This
variation causes expansion and contraction of the bridge's length over the course of the year.
This effect is well understood, and designers know how to accommodate it.
The second effect is the daily variation of the thermal gradient in the bridge. The
daily rising and setting of the sun as well as other climatic factors tend to heat and cool a
bridge's superstructure unevenly through its depth. During the day, the solar radiation causes
the exposed deck of the bridge to heat up. However, concrete is a relatively poor heat
conductor. Thus, the top of the cross section tends to get much warmer than the lower parts
of the cross section. At night, most of the heat is dissipated through the external perimeter of
the cross section causing the interior of the cross section to be much warmer than the
exterior. These phenomena are known as nonlinear thermal gradients. Little is known about
the magnitudes of these gradients and what effects they have on bridges. This section will
discuss thermal effects on bridges in two parts: the shape of the thermal distribution in the cross section, and then the structural effects from the application of the temperature
distribution to the superstructure cross section.