A thermal load is a load resulting from a temperature differential in the structure between a member and the ambient temperature.  A thermal load may also result from a temperature gradient through the depth of a member.

To apply a thermal load to a member

§    Select the member or members to be loaded

§    Choose Thermal Load from the Load menu or the short cut menu

A dialog box will appear with fields for the magnitude and depth of the thermal load.

§    Choose whether the temperature varies through the depth (web or y' axis direction) or the breadth (flange or z' axis direction) of the member

§    Type in the temperatures at the top and bottom of the member

These two temperatures will be the same if the member is at a constant temperature.  If the load is applied in the z' direction top refers to the positive z' side while bottom refers to the negative z' side.

§    Type in a value for the thermal coefficient for the member material

The thermal coefficient is entered in units of microstrain per degree.

§    Type in a value for the depth of the thermal load

§    Click on the OK button

All the temperature values are variations in degrees from the ambient temperature.  For example, a bridge deck with the top of the beams at 45° and the bottom of the beams at an ambient of 20° would have top and bottom temperatures of 25° and 0° respectively.

The thermal gradient is assumed to be symmetric about the neutral axis of the member.  That is, the top temperature occurs at half the depth above the neutral axis and varies linearly down to a point half the depth below the neutral axis. Note that the depth of the load does not have to be the same as the depth of the member.