Building materials move by varying amounts when they heat up and cool down. This is due to differing coefficients of thermal movement of the materials.
Thermo-couples in electric irons and ovens exploit the differing coefficients of thermal movement, by using them to switch on and off the flow of electricity. In this instance the potential stress generated by the differing coefficients is allowed to dissipate through the movement of the bimetallic strip.
Where movement is not allowed, or is restricted, stress builds up. In buildings problems arise particularly on metal roofs where there are large thermal changes due to solar gain followed by radiation on cold nights, this is known as thermal cycling . In the summer this can be very rapid caused by hailstorms.
When roofs are insulated, temperature cycling is more extreme as the heat gained in the surface material is unable to dissipate to the interior materials which act as a heat sink (see thermal capacity).
Many buildings have failed due to thermal stresses caused by differential thermal movement. The risk can be reduced by
Differential Thermal Movements between adjacent materials can be estimated by calculating the temperature gradient through the structure and knowing the thermal coefficients.
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