01.10 Cladding movements

Categories: Envelope Sealing

Introduction
For each structure and its cladding the major movements should be identified.  They should each be described by magnitude and type and all considered in the design of joints.  They include:

• Thermal movement
• Moisture movement
• Floor loading
• Wind loading
• Snow loading
• Vibration
• Settlement and heave
• Creep
• Seismic sway

This section describes the causes of movement in the building envelope.  It describes the nature of the movement, its likely magnitude and factors affecting its occurrence.  Movement of the building structure is covered in Section 01.09.

Movement caused at joints can be characterised as slow, intermediate or rapid and as tension, compression, shear or combined direct and shear movement.  Unloading as well as loading on a building makes demands on the sealant joints.
 

Thermal movement
Elements of the building envelope will undergo variations in temperature.  During the day, radiation from the sun will raise surface temperatures well above the ambient air temperature.  By night, radiation from the surface can reduce the temperature.  The temperature change of a component is dependent on its orientation on the building, any localised shading of the component, its colour and surface finish (its emissivity), its thermal mass and its connection to the other building elements, particularly any insulation behind it.  For rainscreens and other walls containing cavities near to their outer surface there is little conduction of heat to or from the building structure and the temperature swing is accentuated.  Similarly the recent move towards better insulated buildings very often leads to designs in which the cladding is insulated from the rest of the elements, has little thermal mass and again the temperature range experienced by the cladding is accentuated.  Typical service temperatures are given below.

Each facade of a building will experience a different temperature range depending on the building orientation and shading.  In particular, north facing facades in the UK will not experience extreme solar heating.  It may be appropriate to use different joint designs on each facade.

Increases in the temperature of a component will cause it to expand, but if it is prevented from expanding either a compressive stress will develop in the component or it will buckle or ripple to relieve the expansion.  Note that corrugated surfaces will resist rippling in the direction parallel to the corrugations while in the other direction movement will be absorbed with little visual effect.

Corrugated surfaces and other curved surfaces will allow shortening to take place in one direction.  Where no mechanism to relieve contraction exists then the components will carry large tensile forces.  A sealed joint between such components will tear apart unless adequate mechanical fixing of the component is provided.
 

Installation temperature
The amount of contraction or expansion that occurs at a joint will depend on the temperatures at which the components are installed, their size, the method of fixing and the temperature at which the joint is made.  Component temperatures are the important criteria, not air temperatures.  Component temperatures will differ widely from air temperatures.  This is particularly the case for components heated by the sun.  Sealant joints should not be made at component temperatures below 5^oC; this limits the temperature rise that can occur after sealing.  Sealant joints can be made between materials at elevated temperatures but care should be taken to set an upper temperature limit for installation if the design of a joint is critical and the material is designed to be fully extended in service.  Setting the upper component temperature limit for installation at 40^oC will reduce the temperature fall that can occur after the joint is made and so limit the opening movement or force at the joint.

Expansions and contractions are given in the table below for some typical components on the assumption that the sealant in the joints is applied with component temperatures in the range 5^oC to 40^oC in the UK.  The method for calculating thermal movements is given in below.
 

Curved panels
Curved panels when heated or cooled move normal to their surface.  This arises because a uniform expansion or contraction leads to a change of radius.  If the straight edges of the panel are restrained against movement a further movement occurs due to induced bending in the panel.
 

Thermal bowing and dishing
Surface temperature change can cause thermal bowing of elements.  For components that are good conductors the temperature of the component will quickly become nearly uniform through its thickness and only expansion or contraction will take place.  For components that are good insulators the inner surface of the component will experience a smaller temperature change than the outer surface.  The different rate of expansion or contraction of the two faces leads to bowing of the component.  This can be a particular problem for composite insulated panels and plastic infill panels.  Note that for glazing frames, even thermally broken frames, the inner face of the component is likely to be warmed by incident radiation passing through the glazing.

Thermal bowing of components is not generally a consideration in the sealing of joints as it gives rise to an out-of-plane movement that is normally restrained at the panel edge.  Sealing of panels edge to edge gives little problem but sealing joints between dissimilar panels should take account of possible differential movement.  Flashings and sills that are fastened with an adhesive sealant may bow and break loose if not adequately restrained.

Thermal dishing may occur when a panel is wamer on one side than another, image.
 

Prediction of thermal movements
Linear changes in material dimension due to changes in temperature can be calculated directly from:

DL = L  x  a   x  T

where
DL = Change of size
 L = Length of dimension
 T  = Temperature change
 a  = Coefficient of thermal expansion

Typical service temperature ranges  and coefficients of thermal expansion are given below.
 

Thermal induced stresses
If contraction or expansion is restrained, the resultant stress in a component can be calculated from:
s  = a  x  E  x  T

where
s  = Stress
a  = Coefficient of thermal expansion
E = Modulus of elasticity
T = Change in temperature

Typical values of modulus of elasticity are given above.

Hence for a 20m long flint gravel aggregate concrete panel subject to a 35^oC change in temperature,

DL = (20) x (12 x 10^-6) x 35
     = 0.0084 m
     = 8.4mm

This assumes a worst credible situation with fully exposed members and sustained temperatures to allow full cooling.
If this change in length were restrained, it would result in a stress,

s  = (12 x 10^-6) x (30 x 10³) x (35)
    = 12.6 N/mm²

If such a stress were tensile and superimposed on other existing tensile stresses, a risk of cracking would be present.  The level and distribution of cracking in reality would depend on the amount, form and distribution of the reinforcement
 

Moisture movement
Elements of the building envelope undergo movements as their moisture content changes.  Durable impervious materials are not subject to moisture movement.  Movements occur as one-off movements immediately following construction or as repeated movements.  One-off movements occur as the water used in mortars, concrete and render dries out from the fabric and generally leads to shrinkage.  Repeated movements occur as a result of seasonal, or more frequent changes in moisture content of the building fabric.  Calculation of moisture movement is described below.

Initial moisture movement takes the form of component contraction, with the exception of clay or shale bricks which may expand.  Shrinkage of components in the building envelope will lead to an opening up of any sealant joints that are not fixed against movement.  Note that for fixed joints contraction of components will cause a tensile load in the joint and possibly shrinkage cracking of the component.  Most of the initial movement occurs in the first 6 to 12 months of the building’s life and should normally be assumed to occur after the joints have been sealed.  Particular problems may arise where a brick panel expands within or over a concrete frame that is shrinking and appropriate joint design is necessary.  A rule of thumb is that all masonry panels on concrete frames should have a 15mm wide expansion joint at each storey height.  However, there is no theoretical basis for this figure.  After initial movement, shrinkage proceeds more slowly and reversible movements due to moisture become more significant.

Expected moisture movements are of similar magnitude to thermal movements and joints should be designed to accommodate moisture movement of stone, brick and concrete elements.  Particular attention has to be given to moisture movements in timber constructions and for timber elements of the building envelope; dimensional changes are particularly large in a direction across the grain. As with thermal movement, bowing and dishing may occur due to any differential moisture expansion through the thickness, image.
 

Prediction of moisture movements
Changes in size due to moisture may be either one-off movements (e.g., irreversible movements such as shrinkage) or repeated movements (e.g., reversible movements due to changes in moisture content in service).

Percentage changes in size can be calculated from:

(factor x dimension)  / 100

Where the appropriate factor may be obtained from:
 

It is worth noting that, in most cases, an additive combination of thermal and moisture movements will be over pessimistic.  The ‘correct’ level is a matter for engineering judgement but may best be established by estimating the size of the dominant movement and then assessing the extent to which the other mechanism would increase (or reduce) that movement.
 

Shear and sway movement
Wall and roof systems can accommodate shear and sway movement by one of five mechanisms:

• Individual panels distort over a storey height.  This implies that individual panels are flexible in shear or can accommodate movement by rotation of a rigid panel within a flexible mounting, image.  This may be a glazing rebate with gaskets.

 
• Individual panels rotate and the vertical joints between them accommodate shear movement, image.

 
• Individual panels rotate and both the vertical and horizontal joints between them accommodate shear movement, image.  Note that care is needed in the detailing of the cruciforms of the joints.

 
• Panels do not rotate but translate relative to each other.  The horizontal sealed joints between the panels then have to accommodate shear movement, image.

 
• Parts of the storey height are relatively rigid in shear and movement is accommodated in a more flexible band of the building envelope.  This may be a band of ribbon glazing that is designed to accept movement of the glass units within their frames, image.

Sway movenment can be induced by wind loading and seismic events, Section 01.09.