04.01 Structural Systems

Categories: Structural Systems

Introduction
Curtain walling is not generally expected to act as a structural element of the building; it is only required to support its own weight, wind loads and horizontal live loads arising from use of the building. It must be able to transfer these loads to the main structural frame of the building.  This Section describes the structural design procedures that may be used to ensure that the curtain wall can carry these loads without impairing the safety or serviceability of the structure.
 

Design Methods
Design may be carried out using one of the following methods:

• Design calculations,
• Testing,
• Design charts.

Design calculations
There are several stages to design by calculation as follows:

• Establish the structural configuration(spans, joint types etc);
• Establish the loading(s);
• Calculate moments and forces which occur in the element as a result of the applied loading;
• Determine either the size of section required to carry these loads within the permitted stress and deflection limits or whether a proposed section is capable of carrying the load.

The first two stages will also be required when designing using tests or design charts. Calculation of the moments and forces in the components may be carried out by standard structural analysis techniques.

Codes of practice give formulae that can be used to calculate the capacity of structural components and limiting values for stresses. BS 8118 gives guidance on the design of aluminium structures and BS 5950 covers steel structures.

When carrying out calculations it is necessary to make assumptions. Depending on the design of the fixings, the behaviour of the mullions and transoms lies somewhere between that of a member with pinned ends (free to rotate) and that of a member with fixed ends (no rotation).

Brackets can be tested to determine the relationship between an applied moment and the resulting rotation of the member, and also to assess the behaviour of the brackets under shear load. The stiffness can then be taken into account in the design of the connected members.  In the absence of such test data, curtain walling assembly and fixing connections may be assumed to have negligible rotational stiffness. This gives a safer design as ‘real’ structural joints have some rotational stiffness, which will increase the bending stiffness of the connected member.

It is normal practice to neglect the additional stiffness imposed by such elements as infill panels, pressure plates and cover caps when designing the frame sections because their stiffening effect is not easily quantified.

Both of these assumptions are safe but result in a loss of efficiency in the mullion and transom design.

Stresses and deflections vary along the length of the framing members. In design it is normal to calculate stresses and deflections where they are greatest and ensure that these values are acceptable. Assuming the cross section of the element is constant along its length, all other sections will also be adequate. The points of maximum bending stress, shear stress and deflection may not be coincident and it is necessary to establish the critical section for each criterion.
 

Testing
Tests may be carried out to establish the load carrying capacity of curtain walling. Tests may either be carried out on full size sections of the complete curtain wall or on individual components. If tests are carried out on components, care is necessary to ensure that the test conditions accurately simulate the conditions in the completed structure and that all parts of the system are tested.

Test criteria are given in the CWCT Standard and procedures are given in CWCT Test Methods for Curtain Walling. Tests are expensive to carry out but may give more economical design as strength of test assemblies may include effects ignored in calculation methods.
 

Design charts
To simplify the design process, most manufacturers of curtain walling systems produce design charts. These charts may have been derived from calculations in accordance with codes of practice or from load tests. Charts generally show the relationship between allowable mullion and transom spacing for different levels of wind load as illustrated here.
 

Factors of safety
In structural design it is normal to apply a factor of safety to allow for the possibility of weaknesses in the material or loads being greater than expected. In some cases a single global factor may be used but it is more common to apply partial factors to both the loads and the material strengths. Different factors may be used for dead and live loads and when considering safety and serviceability.

It is important that the factors of safety are related to the design method as different design procedures may be based on different assumptions affecting safety. BS 8118 and BS 5950 give values for partial factors of safety for use in the design of aluminium and steel members respectively. The CWCT Standard gives safety factors for use in conjunction with load tests.