CWCT FACETS

03.06 Fit and appearance

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Categories: Buildability

Introduction
Appearance and fit are important aspects of wall performance.  Appearance is important because the cladding creates the image of the building.  Quality is easily seen in:

  • Flatness of panels
  • Alignment of panels or framing members
  • Matching of colours
  • Matching of textures


Appearance
Good appearance depends on colours and textures and accuracy which is dealt with below.

Colour matching of individual finishes is covered in Section 12.03.  Colour matching of finishes to different materials is difficult.  For instance a paint finish to an aluminium framing member may not give exactly the same colour as a paint finish to a glass or metal spandrel panel.  Furthermore, quality control is difficult if components come from different factories and are first brought together on site.  This can be overcome by using colours that contrast or are markedly different rather than seeking to match colours.

Many materials have a texture or grain to their surface and appear different if viewed from a different direction.  For instance stainless steel panels have a grain.  If this runs horizontally on some panels and vertically on others then the panels may appear to be markedly different from one another.
 


Accuracy
Lack of understanding of building tolerances is a major source of conflict on site. It increases construction costs and can lead to poor quality buildings or even failures if corrective measures are ill-conceived.  This Section discusses the issue of tolerance as it relates to cladding, in order to improve levels of understanding and awareness of how to achieve satisfactory fit and appearance. The guidance given in this Section is based on the principles described in BS 5606 and BS 6954.
 


Definitions
Fit is not merely being able to, for instance, insert one component within another, but also requires that the assembly functions as intended. Satisfactory fit is rarely achieved by accident - it is the result of good planning by all participants.  This requires an understanding of some fundamental concepts, which are defined and described below.

Deviation
The difference between a specified nominal value and an actual measured value.
There are two types of deviation.

Induced deviations
The inevitable result of variations in the manufacturing process and the inability of human operatives and instrumentation to measure with absolute precision.

Induced deviations are permanent and whilst the design needs to take account of their presence at the time of construction, it is not necessary to allow for further induced deviations in service.

Inherent deviations
Inevitable inaccuracies due to the physical properties of materials.

Inherent deviations are post-installation dimensional changes and movements in both the structure and the cladding.  Sources of inherent deviations include moisture and thermal movement, elastic deformations, concrete creep and shrinkage and foundation settlement. Guidance on estimation of inherent deviations is given in BRE Digests 227-9.

Cladding can absorb inherent deviations through allowing freedom of movement, by incorporating discontinuities and designing some fixings to slide.  Alternatively, fixings and panels can be designed to resist the forces induced by restraining movement.  Conversely, movement of the structure may in some instances have to be limited to a level that can be accommodated by the cladding fixings and joints.

Tolerance
Tolerances define the limits of induced deviation for which allowance should be made in design, and within which actual sizes are acceptable.

Tolerances therefore describe how much variability is allowable; not how much is present which is termed deviation (see above). Tolerances may be specified with equal deviation either side of the stated size but can also be specified as the maximum deviation from a stated maximum or minimum value.
 


Tolerances
Tolerances cover two main sets of variables:

  • The location of any point on the wall or roof in relation to its intended position on the building.
  • The relationship between individual components of the wall to provide a satisfactory appearance.

Tolerances on the position of the structural frame will depend on the method of construction and standard of workmanship.  The position and fit of the roof or wall are achieved by allowing sufficient adjustment at the fiixing brackets.

There should be a zone within which the roof or wall system is to be constructed and this should be appropriately dimensioned with tolerances.  It is important that not only is the zone itself dimensioned but that it is also located relative to the vertical building datum.  The drawings should show whether the roof or wall is dimensioned critically to its outside face, inside face, centreline, or is allowed to ‘float’ anywhere within the zone. It is normal practice, where a roof or wall system passes in front of the slab edge, to allow a clearance between the back of the cladding zone and the nominal position of the slab edge.

Designers should be aware of the cumulative nature of these dimensions.  For instance, assume the situation of a 150 mm roof or wall zone dimensioned critically to the outside face.  The zone is located with a nominal clearance of 50 mm from the slab edge and the slab edge is subject to a +/- 40 mm tolerance on location.  A specialist contractor proposing a system which is 125 mm thick overall would have to allow for bracketry to span a potential gap of anything between 35 mm (150-125+50-40 mm) and 115 mm (150-125+50+40 mm).  This is not exceptional.

Drawings should show any other critical dimensions or locations of components.  When dimensioning the following should be noted:

  • Dimensions should be given as +0/-n or +n/-0 where these are critical and not automatically as +/-n.
  • Tolerances shall be realistic and take account of the various building systems involved.
  • Drawings shall clearly identify critical faces or locations and these shall be dimensioned to the relevant grid lines.

For instance, it may be more sensible to dimension the absolute outer building line as +0/-x from the centreline of the building whereas the more obvious dimension would be +0/-y from the edge grid line.
 


Accuracy of erection of a complete roof or wall
The actual dimensions of a roof or wall can be affected by any of the following criteria:

  • Accuracy of instruments used for measurement;
  • Location and accuracy of the setting-out points and datums;
  • Tolerances of the building structure;
  • The fabrication tolerances of differing components and materials;
  • The temperature at which the cladding system was erected and/or measurements taken;
  • The thermal expansion and contraction of the constituent components and materials;
  • The design of the fixing systems.


Accuracy of finished cladding
The tolerances on the completed cladding are generally governed by the need to achieve satisfactory appearance. Tolerances for curtain walling are given in CWCT Standard as follows

  • Line +/- 2mm in any one storey height, or structural bay width, and +/- 5mm overall;
  • Level +/- 2mm in any one structural bay, and +/- 5mm overall;
  • Plumb +/- 2mm in any storey height, and +/- 5mm overall;
  • Plane +/- 2mm in any one storey height, or structural bay width, and +/- 5mm overall;

These tolerances are intended to give satisfactory appearance, which is affected by relative positions of the visible cladding elements rather than absolute position is space. They should therefore be measured with respect to the plane of the cladding or in more practical terms a series of adjacent reference lines. There will be an additional deviation in the setting out of the reference lines with respect to the basic reference points for the site.

These tolerances are intended to be applied to curtain walling which is made up from smooth straight and flat elements and therefore the permitted deviations are relatively small. The tolerances for other types of cladding will vary according to the nature of the material involved. References to appropriate standards or guidance documents are given in Section 03.07.
 


Accuracy of junctions between components
Static joints between components should be constructed to the following permitted deviations:  This limits the permitted deviation of one component relative to another in the interests of the overall appearance of the roof or walling system.

  • Within the length of any joint (including any in-line continuations across transverse joints) the greatest width shall not exceed the least width by more than 10 per cent, any variation to be evenly distributed with no sudden changes.
    For example, a nominal 20 mm joint could vary from 18.2 mm to 20 mm or from 20 mm to 22 mm.
     
  • The offset in elevation between nominally in-line edges across a transverse joint shall not be more than 10 per cent of the width of the transverse joint.
    For example, across a nominal 20 mm joint, the offset would not be more than 2 mm.
     
  • The offset in plan or section between flat faces of adjacent panels across any joint shall not be more than 10 per cent of the width of the joint.
    For example, across a nominal 20 mm joint, the offset would not be more than 2 mm.

Permitted deviations for movement joints between components should be agreed prior to the installation of any roof or wall.  The permitted deviations will depend on the nominal joint width, the degree of anticipated movement, and the conditions prevailing during installation.
 


Why do we need accuracy?
Absolute accuracy is not achievable although the level of inaccuracy can be reduced by greater care and more refined construction techniques. The reduction in inaccuracy will generally be accompanied by increased cost and specifying unnecessarily tight tolerances is therefore undesirable. It is necessary to achieve a balance between the cost of achieving accuracy and the costs resulting from inaccuracy which arise from the effects of inaccuracy on:

  • Structural behaviour,
  • Weathertightness,
  • Durability,
  • Appearance,
  • Buildability,
  • Legal requirements.

Structural behaviour
Deviations in the shape of building components, and the assembled structural frame, potentially have a significant effect on structural behaviour. Hence the tolerances specified in BS 5950: Part 2 for structural steelwork are intended to ensure compliance with the design assumptions that underlie the design procedures in Part 1 of the standard.

On lightweight structural frames, tolerances may be affected by torsional rotation of edge beams. Care therefore needs to be taken at the outset to ensure that edge beams are designed with sufficient rigidity and any anticipated torsional rotation is duly quantified.
The effect of tolerances on the design of brackets may be much greater than for normal structural design and design calculations should therefore be carried out with forces and bending moments derived for the most adverse arrangement of dimensions.

Weathertightness
Variability in building components will lead to variation in width of joints, which may lead to ineffective seals. Joints that are too narrow may be impossible to seal properly due to restricted access. If joints are too wide, excessive quantities of sealant may be required and problems of slumping are likely to be experienced. Large variations in joint width over short distances lead to stress concentrations in the seal.

Durability
Lack of weathertightness resulting from excessive variability will have an adverse effect on durability. Durability may also be affected if unforeseen or increased stresses are induced as a result of excessive variability.

Appearance
The appearance of the completed structure will be affected by the variability of components and construction. These effects are subjective and may be disguised to some extent by careful detailing. Conversely when design features which emphasise defects are required by the designer greater accuracy may be required.

Buildability
Buildability relates to the ease with which a building can be constructed. In respect of fixing cladding it requires connections which have sufficient adjustment to overcome variations in both the supporting frame and the cladding. The connections should be simple to install and adjust at all stages of the cladding erection sequence.

Excessive variations arising from out of tolerance components or construction of the frame may require special fixings to be fabricated increasing cost and delaying the erection programme.

Legal requirements
Some dimensions are set by legal requirements, for example the height of handrails. Excessive variability may result in these legal requirements not being met.
 


Design requirements
Traditional cladding, such as brick-, block- or stonework, is flexible in terms of building layout for two reasons.  Firstly, the construction process occurs on site and therefore provides the opportunity to accommodate actual irregularities.  Secondly, the product comprises a large number of small components with relatively wide joints which provides the means to accommodate in-plane deviations and therefore to fit between or enclose the building frame.  However, both these features reduce the level of accuracy and rate of construction of the cladding.

Modern forms of cladding involve the rapid enclosure of a site-erected frame with a factory-made skin.  Lead times for cladding do not allow the cladding contractor to take actual measurements of the structure before proceeding with manufacture or fabrication.  Prior to detailed design work commencing, it is essential that the tolerances to which the structure will be built are reconciled with those that the cladding specialists have planned to accommodate. It is commonplace for such co-ordination to be overlooked, with serious implications.

Satisfactory fit of modern cladding therefore has to be planned. There are two issues to be considered:

  • Specification of tolerances for the completed cladding,
  • Accommodation of tolerances in the supporting structure and cladding components to achieve fit.

Techniques that may be used to reduce the visual impact of variations include:

  • Placing the least satisfactory cladding panels in less conspicuous locations such as the top or back of the building,
  • Lack of straightness in horizontal features is most noticeable when viewed at an angle. Landscaping may be used to restrict viewing angles,
  • Deviations in the alignment of profiled metal sheets are more noticeable when the sheets are laid horizontally rather than vertically,
  • Coarse or irregular panel finishes with less definite edges hide misaligned joints
  • Chamfered edges at joints help to hide variations,
  • Where joint widths change at intersections of horizontal and vertical joints, ensuring that there are equal offsets on both sides of the joint.