Structural requirements

The cladding is subjected to various loads, generated by the environment and occupancy, including windloads, self weight and impact loads. Inappropriate design can lead to cladding collapsing or being torn from the structure. Excessive deformation may compromise the effectiveness of weather seals.

Wind flowing round a building causes pressures and suctions on the surfaces.

A building presents an obstruction to wind flow. The kinetic energy of the wind is converted to pressures and suctions on the facades as it flows round the building. The wind loads can be determined by reference to British Standards using a method developed from wind tunnel tests on a large number of standard built forms. For large projects where the geometry of the building is more complex then the wind loads may be determined from wind modelling of the actual building.

The cladding should be designed to have adequate strength, both in the panels and their connections to the structure. Performance criteria are established in various publications.

The structure and the cladding of a building are the two principal components of the fabric of a building.

The connection between the structure and cladding is critical both during installation and in the performance of the envelope. The separation between the structure represents a number of interfaces.

• different building elements and materials
• different trades and skills
• different contractors
•  different designers.

The interaction between the structure and cladding must consider load transfer and relative movement.

The connections of the cladding to structure should recognise the different loads that are being transferred, self weight or wind loads. In curtain walling for example often connections are designed to transfer both self weight and wind loads with others carrying only wind loading.

In the diagram the self weight is transferred directly to the columns. Wind load is transferred at the columns and at the slab edge beam. The connection to the edge beam is designed to allow relative vertical movement between the slab edge and cladding.

Creep (deformation of structure under sustained loads) is a problem for some materials but not steel.

Concrete undergoes additional deformation under sustained loading. This can occur in both beams and columns. in concrete framed structures unintentional load can be transferred onto the cladding leading to failure. Creep does not occur in steel framed structures.

The interaction between the structure and cladding must also consider dimensional variations and ease of construction.

Dimensional variations, representing the degree of accuracy in construction and erection, are inevitable if the production of building components. Pre-fabricated elements are generally more accurately made than on site construction. The interface between structure and cladding must allow for the differing tolerances acceptable for structure and cladding.

steel framed structures	+/- 12 mm
in-situ concrete frame	+/- 18 mm
prefabricated cladding panel	+/- 4 mm

The typical dimensional variations in dimensions.

Consider the following example:

A prefabricated cladding panel is to be inserted between the columns in a concrete structure. The variability in the joint between the cladding panel and frame is 10 - 30 mm. Therefore a suitable joint seal must be developed that can operate effectively over this range.

The process of construction is made easier by careful planning and organisation. The installation of the cladding should therefore be phased to ensure adequate access to crane time, adjustment to line and level, access for operatives to perform critical seals and the provision of protection during construction.