Tuesday, 28 September 2010

Designing for Movement

One characteristic of most common building materials is that they move, both in response to directly applied loads and in sympathy with changes in ambient moisture and temperature levels. Designing for this means allowing buildings to be essentially flexible, and accommodating less flexible or brittle materials – such as glass – by suitable provision for differential movements and detailing to prevent this becoming a problem. Whilst seemingly a small issue, taking full account of this when designing fundamentally changes the way one thinks about building, especially in combination with the issue of ‘breathability’. In this, much can be learnt from traditional detailing.

The rate of movement of building materials varies. Typically, heat causes expansion and cooling causes contraction. For each material which expands uniformly with temperature rise, the relationship between the change in temperature and change of size is a constant – called the ‘coefficient of linear expansion’. 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, the coefficient of linear expansion provides a useful guide to thermally induced dilations and contractions.

For materials which can absorb water, wetting typically causes expansion and drying contraction. Materials which are impermeable to water tend to be dimensionally stable when wetted. 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).

These hygral and thermal movements are reciprocal: if the 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.

A cladding system designed to accommodate differential movements between an aluminium frame and metal-faced insulation-cored cladding panels experienced gradual displacement of building components as the gaskets snaked their way along the grooves between shuffling panels.
The panels, being small, elongated less on heating than the relatively long frame sections. The connection between the two was a combination of shelf brackets and clamps, with polymer gaskets inserted tightly into grooves between the panels to complete the weather-sealing. Long continuous gaskets ran horizontally; vertical gaskets were shorter and discontinuous at each horizontal joint.

When the system heated up and expanded, the panels and vertical gaskets tended to move upwards. When it cooled, the panels and gaskets contracted but did not uniformly return to their original positions. The consequence was a gradual displacement of parts of the system relative to one another. This opened gaps at the butt joints between horizontal and vertical gaskets and, in places, drove the vertical gaskets into the horizontal gaskets, deforming them. Those panels on the elevations which received most sunshine moved progressively out of alignment.
The design was intended, by avoidance of rigid fixings, to allow reciprocal movements without distress. But this lack of rigidity allowed each reciprocal movement to cause slight relative displacements in the panels and gaskets, the accumulation of which over time reduced weather resistance and marred appearance.

Restrained materials will dilate or contract less than free materials. The restraint of naturally occurring dimensional changes causes the build-up of stress within the restrained material. This creates a propensity to fail, 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 cooling.

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

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