When designing a structure, the greatest efficiency will be achieved by using structural sections that have a high stiffness and can carry the greatest load for minimum self-weight. The most common types of sections that come into this category are thin webbed beams and thin flanged beams, often referred to as composite I and composite box beams, respectively. These are very efficient sections and the design rules in EC5 [1] cover the two methods that are used for assembly, i.e.:

(a) composite sections formed using glued joints,

(b) composite sections formed using mechanical joints.

The profiles of some of the composite sections that are widely used in timber structures are shown in Figure 7.1. Examples of their use in timber structures are shown in Figure 7.2.

Glued composite sections are, as the name implies, assembled by gluing the elements of the section together to function as a single unit. With these sections, the design rules are formulated on the assumption that no slip will arise between the elements of the section at any of the joint positions and are addressed in EC5, Section 9. Mechanically jointed composite sections are assembled by securing the elements together using nails, screws or dowels etc., and with these sections slip will arise between the elements. The design rules in EC5, Annex B, take this into account.

It is normal practice for composite sections to be assembled by gluing, and this chapter only covers the design requirements of sections formed this way.

With these sections the webs and flanges are made from different materials; for example, the webs of composite I-beams are usually made from wood-based panel materials such as plywood, oriented strand board (OSB) or particleboard etc., and the flanges from structural timber, LVL or glued laminated timber. For glued composite sections, a high degree of quality control is required to ensure that sound jointing is achieved and for this reason they are normally factory produced.

In these sections, all of the elements are designed to work very efficiently and because of this only materials with relatively few and minor defects are used.

The general information in 4.3 is relevant to the content of this chapter.

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