CWCT FACETS

03.04 Stability

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Categories: Buildability

Introduction
It is necessary to ensure the stability of cladding at all times from fabrication to demolition of the structure. This section is concerned with the construction stage from delivery of components to site to completion of erection.

During the construction stage cladding components may be subjected loads which do not apply or act in different ways when the cladding is incorporated in the structure and the support conditions will also differ. The erection of the cladding may provide additional support to the structural frame and the stability of the frame during erection also needs to be considered.
 


Responsibility
Traditionally the contractor has been responsible for devising safe means of erection which includes ensuring the stability of the structure and cladding. The CDM Regulations place a duty on the designer to consider the risks inherent in construction and maintenance of the building and to minimise them where possible. The designer is also responsible for providing details of particular hazards identified in the Health and Safety Plan. This may include details of requirements to ensure stability of the structure where they may not be obvious to an experienced contractor.

The contractor is responsible for devising methods of erection taking account of hazards and requirements identified by the designer. Where the cladding is erected by a sub contractor the CDM regulations require liaison between the sub contractor and main contractor to co ordinate safety.

To ensure that site staff erecting the cladding carry out the work in a safe manner that ensures stability, the erection procedures should be set out in a method statement. The extent of detail in a method statement will depend on the complexity and size of the project. A simple structure using a standard cladding system may be covered by standard instruction sheets together with site details. Bespoke cladding solutions using heavy panels requiring craneage would require far more detail.
 


Handling of cladding components
When erected in the structure cladding will normally be in the vertical plane however it may be delivered to site horizontally. Lifting and turning units may induce bending stresses and concentrated loads at lifting points which do not apply when the cladding is erected. The effect of lifting operations needs to be considered and lifting methods agreed and set out in method statements.

It is not good practice to use the permanent fixing points for lifting as the cladding panels may then have to be released from the crane before they are securely attached to the structure.
 


Storage of Cladding
Where possible cladding units should be delivered to site and erected immediately however this is not always possible and it may be necessary to store cladding on site. Storage arrangements must ensure stability. A firm level storage area is required, wind restraint may need to be provided where units are stored vertically and the height of stacks may need to be limited if they are stored horizontally. Protection from weather and damage by other site operations also needs to be considered.
 


Exceptional loads
Loads which need to be considered include:

  • Stacking of cladding materials on the partially completed structure may impose concentrated loads for which it has not been designed, or which it is unable to resist in the partially erected state.
  • Accidental impacts may occur from construction plant, delivery vehicles or when moving objects by crane.
  • Wind loads may act differently on a partially completed building. For example the loads on the inner face of the cladding will be different when the structure is only partially enclosed.
  • Many cladding components have a large surface area and are therefore sensitive to wind load until they are fully fixed. This is a particular hazard for lightweight sheet materials.


Tolerances
The design of the cladding fixings should provide adjustment to accommodate deviations in the cladding and supporting structure. If this is not done or if the deviations are greater than permitted it may not be possible to fix the cladding securely. In such cases it may be necessary to provide revised fixing details to give long term stability of the cladding  and temporary propping may be necessary as an interim measure to allow work to proceed. Any amendment to the fixing arrangement must consider the effect on the stability of both the structure and the cladding. The temptation to omit or force fixings to fit must be resisted.
 


Temporary bracing/support
Temporary bracing may be required to ensure stability prior to final fixing. Suitable anchorage points on both the cladding and supporting structure must be provided. Temporary support may be provided by push/pull props which can be used to adjust the position as well as provide support. Props should be connected to the supported panel above mid height and set at an angle of between 45o and 60o to the horizontal.

The details of propping requirements including sequence of installation and removal should be established by the designer and included in the method statement.
 


Sequence of construction
The sequence of construction may affect stability in the following ways.

  • Where a cladding panel relies on an interlocking joint with a previously fixed panel for its stability the construction sequence is critical to the stability of the erected cladding. Alternatively temporary propping may be required.
  • If interlocking panels are erected out of sequence, panels may be ‘forced’ into place damaging joints and impairing the stability of the cladding.
  • If previously erected panels obstruct access for cranes or other lifting equipment cladding panels may be pulled into place and lifting equipment released before the panels have been secured.
  • The erection programme may have to ensure that a section of cladding can be completed before the end of the day or before a weekend break to enable it to be left in a stable condition.
  • Fixings relying on adhesives and cementitious mortars require time to develop strength and must not be loaded too early. Examples include resin bonded fixings in concrete and masonry backing walls for stone cladding.