Designing for Movement

One 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.

For materials 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.

Restrained 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’[1].  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 cooling.

Good detailing accommodates the normal range of reciprocal movements in the specified materials.

[1] In this context ‘phase’ is used to describe a physically distinct and homogeneous form of a substance characterized by its composition and state and separated by a bounding surface from other forms.  If the water removed from the concrete is free, drying does not alter the phase of the cement.  If the water removed is released by extracting the water of crystallisation from hydrated cements or otherwise is extracted by decomposing compounds in the concrete, phase changes may be said to occur and heating and cooling may not cause dilations and contractions respectively.