characteristic of building materials is that they move, both in
response to applied loads and in sympathy with changes in ambient
moisture and temperature.
Accomodating this requires a ballance of flexibility and strength and underlies
sound building design. Much can be
learnt from traditional detailing.
The rate of
movement of building materials varies.
Typically, heat causes expansion and cooling causes contraction. Materials do not necessarily expand
uniformly with changes in temperature.
For example, the dilation of a fibre-cement sheet for a 1oC
temperature rise depends on its initial temperature. Nonetheless, within the temperature ranges
ordinarily experienced in buildings and for most building materials thermally induced movements can be calculated from their coefficients of linear expansion.
which can absorb water, wetting typically causes expansion and drying
contraction. For most common man-made building materials hygral
movements are small compared to thermal movements. Wood, and composites containing wood,
typically move more on wetting and drying than heating and cooling (provided
they are not burnt).
Providing chemical changes do not occur, these hygral and thermal movements are reciprocal - if
moisture and temperature are returned to their original values the original
sizes are restored. This does not
necessarily preclude accumulative movements.
Temperature changes, dilations and contractions may occur more rapidly
in some materials than others. This, in
combination with variations in the restraints to which parts of buildings are
subject, can cause accumulative movements.
materials will dilate or contract less than free materials but will correspondingly be subject to stress fluctuations - potentially creating a failure mechanism, which
needs to be anticipated and limited if cracking is to be prevented.
A material such
as wood may shrink on heating, as heat may dry the wood and thus cause
contraction. Concretes expand on
heating, unless sufficiently hot for the heat to drive water out of the cement
matrix so as to alter its ‘phase’. Bearing in mind such exceptions, within
normal limits, the effect of changes in the ambient temperature due to
fluctuations in the weather is to cause swelling on heating and shrinkage on
Good detailing accommodates the normal range of
reciprocal movements in the specified materials.