04.03 Movement

Categories: Structural Systems

Introduction
Movement within the façade may be categorised as follows

• Thermal movement
• Moisture movement
• Elastic movement – as a consequence of loading
• Main building movement.

The consequences of any movement within the façade components will be dependent upon whether such movement is constrained or not.  Any ‘movement’ that is restrained will result in additional stresses within affected components that should be quantified and incorporated within the design. If these additional stresses are ignored it is possible for components to become over-loaded which may lead to failure or additional deformation which further exacerbates the situation.

Unrestrained movements may lead to excessive overall deformations, perhaps exceeding defined limits, or opening up of gaskets and seals between components leading to leakage. In the case of repetitive cycles of movement cyclic fatigue maybe induced.

This Section will first give a qualitative overview of façade movement and its consequences.  A quantitative theoretical approach will follow thereafter.
 

Thermal Movement
Thermal movement comes about through changes in temperature and will be dependent upon local climate conditions. Not only will the cladding components be subject to thermal movements, but so too will the supporting structure. Movement of the main supporting structure can become of critical importance, particularly in atrium situations where large expanses of glazing can give rise to large temperature changes coupled with a lack of thermal currents to displace warm air. Within the design of the cladding envelope accommodation of movement requires detailed consideration of the junctions with the correct choice of fixing detail essential. With this in mind, it is worth considering the temperature range that needs to be accommodated, particularly during construction. It may be considered unlikely that fixings during erection will be subjected to the full in service temperature range of the external exposed surfaces.

Typical UK installation temperatures are

Minimum       5^oC
Maximum    70^oC

When considering the likely surface temperature range for design purposes, due account needs to be taken with regard to the materials being used and their colour finish.

Typical UK Surface temperature range –

Heavyweight/light colour                 -20 to 50^oC
Heavyweight/dark colour                 -20 to 65^oC
Lightweight/light colour                   -25 to 60^oC
Lightweight/dark colour                   -25 to 80^oC

Coloured or coated glass                -25 to 80^oC
 

In terms of a range of cladding components assuming an even all round temperature change these would result in typical axial movements –

Aluminium glazing bar (1200mm)                     -2.0mm            +1.3mm
PVC-u glazing bar (1200mm)                           -6.5mm            +4.2mm
Aluminium mullion (3200mm)                           -5.7mm            +4.6mm
Aluminium sheeting (6000mm)                         -8.1mm            +6.0mm
 

Differential temperatures setting up a temperature gradient through the depth of a given section will give rise to thermal bowing or dishing, image.  This can be a problem for fixings that provide restraint and for brittle materials.
 

Moisture Movement
Changes in moisture content can lead to changes in shape. Aluminium, steel and glass are stable materials but other components within the façade may not be, e.g. wood, insulation, etc. The effects of moisture within building materials may be spilt between initial movements during construction, such as movements within concrete during the hydration process, and repeated movement’s resulting from changes in the ambient moisture content Some examples are as follows –

 Typical initial movements due to changes in moisture content

Cement based materials                0.03 to 0.10% (shrinkage)
Lightweight concrete                      0.20 to 0.40% (shrinkage)
Clay or shale brickwork                 0.02 to 0.07%  (expansion)
 

Typical repeated movements due to moisture changes

Cement based materials                  0.02 to 0.10%
Lightweight concrete                        0.10 to 0.20%
Stone                                             0.01 to 0.07%
Wood (along grain)                          0.05 to 0.10%
Wood (across grain)                        0.50 to 4.00%

As with temperature, a moisture content gradient through a material will lead to bowing or dishing, image.  This will need to be considered with regard to any fixing details and for brittle materials.
 

Elastic Movement
Elastic movement of both the façade components and the main structural frame will depend upon the individual circumstances. Factors involved include –

• Applied loading
• Component dimensions
• Restraint conditions
• Material characteristics

The dominant load case influencing deformation calculations for cladding components will normally be wind loading, although impact loads and other imposed loads could also require attention depending upon the situation being considered. A more detailed discussion of elastic analysis follows below.
 

Main Building Movement
Movement of the main building structural components will be of concern to the cladding design where interactions occur. All facades will come into contact with the main building frame in some way and therefore due attention is required. Main building movements will occur due to

• Thermal movement
• Moisture movement
• Settlement and creep
• Elastic movement

Building movement - Thermal
Main structural frames will also be subject to temperature changes. In general internal frames will undergo smaller temperature swings than external, although as mentioned above, due care needs to be given regarding individual circumstances. Typical temperature ranges for internal and external frames are as follows –

Internal frames                    -5 to +35^oC
External frames                 -20 to +65^oC
 

Building movement - Moisture
Main building frames may also be susceptible to movement due to moisture changes. Timber in particular, but also concrete, will under go repeated reversible changes. As with temperature, external frames will normally see a greater range of moisture content variation. Most moisture movement of concrete occurs due to the hydration process as the concrete cures and gains strength. Concrete columns typically contract by 3 to 4mm in each storey height during the first 6 to 12 months.
 

Building movement - Settlement
All buildings settle to some extent. The main concern for the cladding designer will be differential settlement between adjacent main structural components to which the cladding components attach or bear on. Differential settlement occurs due to poor foundation design and irregular building shape. Changes in building usage and refurbishment may also change the loading characteristics for which the original foundation were not designed, which could also lead to differential movement. New build extensions will often move differentially and care needs to be exercised when cladding over a movement joint, image. Different heights and/or numbers of storeys in adjacent structural bays leading to different foundation loads will also suggest differential movement, image.
 

Building movement – Elastic
Elastic building movements that will be of interest to the cladding designer include those caused by –

• Floor loading
• Cladding loading
• Column shortening
• Wind
• Earthquake
• Other imposed loads

Floor or edge beam deflection will affect any cladding fixings that attach in these areas, and will influence the manner in which adjacent cladding panels interact.

• Panels fixed at two support points will attemp to follow the floor deflection, image.  If the movement is restricted then the fixings and panels may experience unanticipated loads.
• Panels free to lift at a fixing are likely to remain vertical, image.  However, a single fixing now transfers the full weight of the panel.

Deflections may be of the order of 25mm to 50mm or L/250 to L/500 depending on the type of cladding. The floor dead load does not cause movement of the cladding fixings as they are installed after such movement has occurred, although it may impact upon the tolerance requirements of the fixing detail. Live loading however will cause movement of the fixings.

The dead load of the cladding is built up progressively during installation and may cause movement of the cladding fixings, as will any applied live loads.

The weight of the structure and live loads for the storeys above will shorten the columns at each and every floor. With high rise structures this can become significant for the cladding towards the lower levels. Some of this movement occurs before the joints within the façade are sealed.

Wind loading will cause a building to sway. This gives rise to differential horizontal floor movements. The amount of sway varies greatly from structure to structure and is dependent upon the building’s lateral stiffness, which can be increased by bracing. The amount of sway in terms of maximum floor drift depends upon the building structure -

• Braced concrete frames and masonry walls 3mm.
• Unbraced frames H/300

Although it should be noted that greater values may be used where the structural engineer believes the cladding and fixings will accept them.

The movement of the cladding panels and joints will depend on the form of cladding and the method of fixing the panels. Movement can occur in the following ways:

• Individual panels may shear to follow the supporting frame if they are fixed at all edges or corners, image. This will happen for panels such as profiled metal sheeting.  It is only a problem if the sheeting and fasteners are unable to carry the induced loads.
• Whole panels may rotate if the fixings allow sufficient movement, image.  This has to be the form of movement accomodation for rigid panels such as concrete units and panelised wall.
• Smaller panels may also rotate to accommodate movement, image.  This is the way in which glass moves within a glazing rebate of a stick curtain wall. The degree of rotation possible will depend on the depth of glazing rebate or the movement possible in any fixings.
• Panels may slide past one another, image.  This is a particularly appropriate way to accommodate movement in a rainscreen with open horizontal joints.

Where strong stiff panels are used with weak but stiff panels all of the movement will occur at the weak panels which may fail, image.  This frequently happens in buildings subjected to seismic shaking.

Deformation and Movement Calculations
Quantifiable deformations from structural analysis will deal with

• Elastic movements – due to loading actions
• Thermal movements

Moisture movements due to the absorption or drying out of materials leading to changes in shape require appreciation of the internal chemical composition and how this relates to the external environment.  Detailed assessment methods for quantifying these movements are outside the scope of this work, and if the general information in the preceding section is not adequate then further advice should be sought from a materials scientist associated with the material in question.
 

Elastic Analysis
All structural deformation calculations are based on the theory of Elasticity, a prerequisite is a relationship between the loads applied and the resulting displacements, or stress and strain, this being the ‘stiffness’. Thus a measured stiffness parameter that identifies the material load/deformation relationship is required. There are numerous such stiffness parameters available for the structural engineer depending upon the situation and complexity required. One of the most common stiffness parameters is that represented by the Youngs Modulus [E], which presumes a linear stiffness relationship. This is generally a simplifying assumption, more or less valid depending upon the material in question in relation to the loads/deformations being considered.  It is beyond the remit here to delve into the intricacies of this parameter, suffice to say that the nature of the material should be well understood to ensure that the use of a single linear stiffness parameter within a simplifying elastic analysis is appropriate to the condition being considered.

Typical numerical values of E for the principal materials used within facades are as follows –

Steel                                 210 kN/mm²
Aluminium                           70 kN/mm²
Glass                                  70 kN/mm²
 

Elastic deformation analysis deals with a materials response to loading such as tension, compression and bending. Twisting, torsion and shear may also require attention in some situations.
 

Direct Tension and Compression
Deformations in these cases are directly related to the applied load via the stress – strain – stiffness expressions, which are given below.

s  = P / A

e  = dL / L

E = s / e

From which  dL = PL/EA

Where,

s   = tension or compressive stress
P  = applied load
A  = cross sectional area
e   = strain
dL = change in length, tension +ive, compression –ive
L   = original length
E  = elastic stiffness modulus, Youngs modulus
 

Example:
Consider an aluminium mullion 3m long, with cross sectional area 250mm², subjected to an axial tension load of 5kN.

Thus from the expression dL = PL/EA, the bar would extend,

dL = (5 x 3000 )/ (70 x 250) = 0.86 mm

If the mullion had been in compression the same numerical result would be obtained, but the bar would shorten. Care needs to be exercised when elements are subject to compression to ensure that the element does not buckle.
 

Bending
Bending occurs when an element is subjected to both tension and compression within the same cross-section. Bending gives rise to curvature, and the nature of the curvature is identified by the terms Sagging and Hogging. Sagging describes curvature that puts the upper surface into compression and the lower in tension, whereas hogging puts the top into tension and the bottom into compression, image. Elements that span continuously over intermediate supports will undergo reversals of curvature, image.

Curvature itself results from one surface shortening and the opposite lengthening, thus giving rise to the compression and tension stresses respectively. Since the strain changes from compression (-ve strain) to tension  (+ve strain) there must be some point within the cross-section that remains unstrained (zero strain), and this is termed the neutral axis, image.

The response of sections subject to bending is modelled by the standard bending equation, which is:–

M / I = s / y = E / R

Where,

M = applied moment or moment of resistance
I = second moment of area
s = bending stress at distance y from neutral axis
y = distance from neutral axis to position of stress being considered.
E = elastic stiffness modulus
R = radius of curvature

From the bending equation various relationships can be developed. In terms of deflection we consider

M = EI / R

Which relates the moment to curvature and hence to displacement. It is from this relationship that all elastic deformation values are calculated, some of which are stated below.

Central beam deflections for various load cases:

Centrally loaded simply supported beam                                    PL³/48EI
Centrally loaded fixed ended beam                                            PL³/192EI
Uniformly distributed loaded simply supported beam                5wL⁴/384EI
Uniformly distributed loaded fixed ended beam                          wL⁴/384EI
 

Second Moment of Area [I]
The second moment of area [I] is a numerical representation of how the area within the cross-section is distributed about some axis. There are an infinite number of I values for any given section, but for engineering purposes we are normally most concerned with the I values associated with two mutually perpendicular axes, the major and minor axes. The major axis being that axis about which the section is stiffest in response to bending and the axis about which the section would be expected to be orientated to provide maximum resistance to bending deflection, image. For an I value to be of any use, the orientation of the axis about which it is calculated MUST be known. Within current British Standards and numerous texts the subscripts Ixx and Iyy are commonly used to identify the major and minor orientations respectively. However the proposed Euro codes will change the axis system.
 

Calculation of I
The determination of an I value follows from the following equation –

I = SAr²

Where,

A = increment of area within cross-section
r  =  distance to area from axis about which I is being calculated.

For a rectangle of depth d and breadth b, image, this results in a value of I about the major axis of

I_xx = bd³ / 12

Which in conjunction with the parallel axis theorem allows for the practical determination of I values for most cross sections. The parallel axis theorem allows for the determination of an I value for an element of area displaced some distance from, but parallel to, the axis required, image.

I_axis = I_na + Ay²

Where,

I_axis = I required about some designated axis
I_na  = I value of area about own neutral axis
A  = area
y  = distance from own neutral axis to axis required

Example:
Referring to this image;

Area = 90 x 5 + 50 x 5 + 2(100x3) = 1300 mm²

The position of the neutral axis is found from Ay = Sy DA

Thus

y = { 2.5(90 x 5) + 55(100 x 3)} / 1300 + { 55(100 x 3) + 107.5(50 x 5)} / 1300 = 47 mm

Using the standard result for a rectangle I = bd³ / 12 and the parallel axis theorem, the I value about the major axis is as follows –

I_xx = 90 x 5³ / 12  + 90 x 5 x (47 – 2.5)²
      + 2 { 3 x 100³ / 12  +  100 x 3 x (55 – 47)²}
      + 50 x 5³ / 12   +  50 x 5 x (107.5 – 47)²

       =  2.35 x 10⁶ mm⁴
 

Most CAD packages will be able to calculate I values and manufacturers literature will be available for all proprietary products. Critical issues relate to orientation and an appreciation of where area should be put or reduced for maximum benefit.
 

Thermal Movement
Changes in temperature will lead to tension, compression, bending and possible twisting. The quantity of movement will be determined by the coefficient of expansion [a] of the material being considered. In a similar manner to Youngs modulus for elastic bending, the simplification of the thermal response of the material to a single number presumes a linear analysis. Typical values are:

a for aluminium approximately 24 x 10^-6/ ^oC
a for glass approximately 8.4 x 10^-6/ ^oC

The thermal strain due to a change in temperature is then

e_t  =  a Dt

where Dt = change in temperature.

Where an element is subjected to an all round even temperature change a linear expansion or contraction will result. If a temperature gradient is present across the member then bowing will occur.

Example:

For an aluminium transom with an initial length 1200mm subjected to an all round temperature change of 85^oC, the change in length will be, image;

e_t = a Dt = 23 x 10^-6 x 85 = 1.95 x 10^-3

DL = L e_t = 1200 x 1.95 x 10^-3 = 2.3mm
 

Thermally induced Stresses
If a member is restrained such that it is unable to respond to thermal movements by expanding, contracting, bending etc, then additional stresses will be generated within the section in order to maintain its shape. These stresses may be calculated with reference to the elastic equations presented above.

Example:

Consider that the thermal movement calculated in the previous example was restrained then the additional stress imposed within the section would be as follows.

s  = e_t E = 1.95 x 10^-3 x 70 x 10³ = 136.5 N/mm²