04.02 Loads

Categories: Structural Systems

Introduction
Wind loading is considered in Section 04.04. This section concerns all other forms of load
 

Dead load
Dead loads of the curtain walling and any permanent fixtures attached to the curtain wall should be calculated in accordance with BS 6399 Part 1 and the 'Standard and guide to good practice for curtain walling' published by CWCT.
 

Live loads
Live loads may arise from the use of ladders and cradles during maintenance work.  The CWCT Standard for Curtain Walling clause 2.5.3 states that allowance for a load of 500N over a square of 100mm side should be made.

Requirement K2 of the Building Regulations 1991 requires protection to be provided at the edges of floors to prevent the risk of falling. This protection may be provided by the curtain wall, although in some situations barriers or guardrails will be provided. Section 3 of Approved Document K of the Building Regulations gives the loads to be resisted for different categories of building.

BS 8200 gives requirements for impact resistance on the external face of external walls. These loads are given in terms of impact energy rather than static force and compliance must be judged on the basis of test results rather than design calculations.
 

Snow load
For sloping surfaces or where the façade incorporates horizontal ledges snow loads may need to be considered. Snow loads are given in BS 6399: Part 3.  See Section 04.10, Roofs.
 

Induced loads
Loads may also be generated by restraint to movement, however it is normal to make provision for movement so that significant forces do not arise.
 

Combined Loads
Although all the above loads need to be considered, it is not normally necessary to consider all loads acting at the same time and it is only necessary to include realistic load combinations. For example wind load is based on the 50-year return wind. It is unlikely that maintenance would be undertaken in such conditions and building occupants would probably keep clear of the façade due to the fear of breaking glass. In the UK, snow loading is unlikely to be associated with winds from the direction of the maximum wind load. It will normally be sufficient to allow for wind load in conjunction with the dead load but this needs to be confirmed for each situation.
 

Limiting conditions
As with most other structures the purpose of structural design is to ensure that the curtain wall is strong enough to withstand the loads to which it is subjected safely and without impairing performance. Structural performance may be considered in terms of both strength and stiffness. Strength deals with ensuring rupture does not occur whilst stiffness is concerned with deflection and movement ensuring that the components remain serviceable during use. These two conditions of strength and stiffness are increasingly referred to within the Limit State design code protocol as the Ultimate Limit State and Serviceability Limit State respectively.

When dealing with the numeric aspects of strength analysis both elastic and plastic methods are available. For the Ultimate Limit state plastic analysis is becoming more widespread as smaller sections result, but it needs careful attention to detail and a sound understanding of the characteristics of the material being used. Elastic analysis maintains continuity between stress and strain and thus provides the only mechanism for determination of deflections. Thus when dealing with the Serviceability Limit state elastic analysis offers the only means of calculation. Generally, within the façade, due to the loads involved and materials used the Serviceability criteria normally provide the limiting condition, thus it is normally appropriate to use elastic design methods for both strength and deflection conditions.
 

Safety
To satisfy safety requirements the structure should be able to carry the design loads with an adequate margin of safety. In practice this is achieved by limiting the stresses on the components under a design load which is unlikely to be exceeded in service.
 

Serviceability
Although the safety requirement ensures that the structure can carry the design loads, the overall performance may be impaired. Excessive deflection of the framing under the action of horizontal loads may result in inadequate support to the infill panels and cause alarm to building occupants. The performance of the curtain wall may also be impaired by failure of seals or drainage channels.

The CWCT Standard for Curtain Walling sets limits to the allowable deflection of curtain walling members as follows:

• General limits

The maximum frontal deflection shall not exceed 1/200 of the span of the member measured between points of attachment to the building, or 20mm, whichever is the lesser.
 
• Limit related to infill supported

This limit depends on the type of material supported.  For double glazing units the limit is 1/175 of the length of the unit measured along the supported edge, or 15mm, whichever is the lesser, or more restrictive limits if set by the unit manufacturer.

These limits are illustrated here for a stone cladding panel supported by a framing member.  The governing limit in a given situation will depend on the relationship between the overall span of the framing member and the size of the infill units.