01.09 Building 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 structure.  It describes the nature of the movement, its likely magnitude and factors affecting its occurrence.  Movement of the cladding is covered in Section 01.10.
 

Thermal movement
The structural frame is generally not subject to the same temperature range as the cladding unless it is an external frame.  Internal frames behind or beneath glass may be subject to solar gain and reach high temperatures, particularly in sloping glazing systems where there is no convection cooling.

With the exceptions given above the frame may be assumed to be at the same temperature as the internal building temperature.  The CWCT 'Curtain walling Standard' gives the following values:
 

Moisture movement
Elements of the building envelope and the building primary structure undergo movements as their moisture content changes.  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.  This will be limited for a protected, internal, frame but may be greater for a concrete structure exposed to the weather.

Initial moisture movement takes the form of component contraction, with the exception of clay or shale bricks which may expand.  Shrinkage of the primary structure will lead to a closing of the sealed joints in the building envelope.  Most typically the columns of a reinforced concrete frame may contract 3 to 4mm in each storey height and reduce the width of horizontal joints in the building envelope.  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 attention has to be given to moisture movements in timber constructions; dimensional changes are particularly large in a direction across the grain.
 

Building movements due to loading
Building primary structures deflect under load and impose movement or loads on the building envelope.  The structural loads that cause movement are:

• Floor loading
• Wind sway
• Snow loading
• Seismic loading
• Differential settlement and heave

Allowable deflections based on the UK structural engineering codes are given below.

Sealant joints should be capable of accommodating all movements that occur after the joint has been sealed.  Movements that occur before the joint is sealed may affect the dimensions of the gap to be sealed.  CIRIA Technical Note 107: 1981, Design for movement in buildings, gives guidance on the movements that may occur.  However, anticipated building movements should normally be calculated by the structural engineer.
 

Floor loading
Floor deflections will only give rise to building envelope movements if the cladding or curtain walling is supported from the edge of the floor slab or a floor beam.  Allowable deflections for structures of different materials are given below.  Floor slabs may deflect up to 25 or 30mm in some cases.  Deflections in the range L/250 to L/500 are considered acceptable by the structural engineer dependent on the type of cladding.

Part of the floor deflection will be due to the weight of the cladding and part to the occupation loads.  If the resulting deflection of the building envelope is too great, consideration should be given to supporting the wall elements off a braced frame spanning between columns or stiffening the edge of the floor slab.

If no provision is made to adjust the position of the wall components after fixing and prior to sealing of the joints then the sealant joints will have to tolerate any movement that occurs during fixing due to the weight of the cladding.

Sealant joints should accommodate all movements occurring from loads applied after they have been sealed.  These include dead loads from the construction of internal partitions as well as live loads on the floor.  The differential deflection between consecutive floors is often more critical than the absolute deflection of one floor.  Full consideration of the sequence of loading on each floor and the combinations of loading on two floors may lead to a more economic joint design.
 

Wind sway
Wind loading will cause structures to sway such that each floor experiences a different horizontal displacement.  The joints in the building envelope should be capable of accommodating the differential floor movement that occurs at each storey.  This movement should normally be accommodated within each storey height.

Sway of a building structure will lead to the movement of building envelope panels.  The exact movement will depend on the nature of the cladding and its fixing. Section 01.10 describes how cladding may accomodate sway movement.

Building primary structures may experience very small sway movements if they are braced bay frames or contain shearwalls.

Unbraced frames will experience much larger sway movements.

Braced bay concrete frames and structures containing masonry or concrete shear walls transmit horizontal loads to the ground through large truss or plate structures.  These stiff structures undergo displacements of up to 3mm in each storey height.  Buildings containing lift shafts will generally be of this form.

Moment resisting frames of steel or concrete construction transmit horizontal loads to the ground by flexing of the columns, beams and floor slabs that are connected together by rigid moment resisting joints.  Movements are greater than those for stiffer structures but sway movements are limited to 1/300 of the storey height in any one storey unless greater displacements can be justified having due regard to the type of cladding.  The building envelope and its joints have to be designed to accommodate horizontal movements of 8 to 20 mm in each storey height. Note that industrial buildings may undergo very much larger movements.  Joint design may be simplified and more economic joints may be achieved if the joints, the cladding system and the fixings are designed at the same time.
 

Snow loading
Snow loading will give rise to movements in roofing systems and these may be significant where a flexible support is used such as a steel portal.  This may have implications for the sealing of penetrations through the roof.  Snow loading may also give rise to movement at any joint where the roof interfaces with an adjacent wall or gable.  If movements due to snow loading are believed to be significant the structural engineer should be consulted to ascertain what movement can occur.
 

Vibration
Components of the building envelope may vibrate as a result of traffic, plant, wind and pedestrians.  With the exception of industrial buildings the extent of vibration in the building envelope should have been limited to create a comfortable habitable environment.  Problems may, however, occur when secondary structures such as flagpoles or signs penetrate the building cladding.  Such joints should be sealed with an elastic sealant and consideration should be given to using a conservative joint design.
 

Settlement and heave
In any structure the foundation of onepart may settle more than that of another.  For rigid structures of masonry, or those built on rigid foundations, settlement is unlikely to give rise to distress in cladding systems or sealant joints.

For framed structures of steel, concrete or timber the structural design codes allow for differential settlement of 1/500 of distance between adjacent columns.  This value should be used in the absence of more detailed information from the structural engineer.  The building envelope will then have to accommodate differential vertical movements of 10 to 20mm in any one structural bay.  Note that this is in addition to wind sway.  Section 01.10 describes how cladding systems may accomodate shear movement due to settlement.

For some buildings designed to accommodate large differential settlements, such as mining subsidence, the building settlement movement may be greater than 1/500 of distance between columns.  The advice of the structural engineer should be sought in these cases.  It will often be necessary to design the joints, cladding and roofing panels and their fixings at the same time.

If a primary structure contains movement joints to accommodate differential settlement then the building envelope must have corresponding movement joints.  These will have to accommodate concentrated localised movement of the envelope.
 

Creep
Long-term loading on the building may lead to creep in some members of the supporting structure or cladding.  This problem is pronounced for timber structures but also affects concrete structures.  Under a constant load a structure will first undergo elastic deformation.  Creep will then allow the structure to deform further over a relatively long period of time although no further load is applied.  If movements due to creep are likely to occur the structural engineer should be consulted to determine the likely movement.
 

Seismic sway
Earthquakes cause buildings to sway creating building envelope movements of the same type as those due to wind loading.  These movements are likely to be greater than wind sway movements and 1/200 of storey height is an often-used limit for differential horizontal floor movement.  Section 01.10 describes how a cladding system may accomodate sway movements.

It is necessary to establish with the structural engineer and client the movements to be accommodated by joints in the building envelope.  It is acceptable for joints to lose their integrity at a lesser movement than that which causes the loss of structural integrity of the building envelope.
 

Prediction of structural movements
The structural loads that cause movement may be summarised as:

• Floor loading
• Wind sway
• Snow loading
• Seismic loading
• Differential settlement and heave

Such loads may give rise to an overall tension or compression or shear displacement of the joint.  As such, these loads must be carefully considered in combination when designing the joint.  The structural engineer will normally be responsible for calculating deflections of the the primary structural frame.  Structural preformance is covered in greater detail in Package 04.

The table below shows typical building movements for different structural materials as allowed by UK standards.  Note that sway will be reduced for structures containing braced bays of shear walls.