where the symbols are defined in equation (8.10) and (E7)ef y is the effective bending stiffness about the y-y axis.

Where fasteners are used the fastener design must comply with the requirements of equations (8.13) and (8.14) (equations (B.10) and (C.5) in EC5).

(c) Where, in addition to axial load, small moments due to self-weight also arise. The relationship given in Clause 6.3.2(3) of EC5 must be satisfied for each element.

See Example 8.8.1.

8.4.4 Built-up sections - spaced columns

A spaced column is a built-up column where there are two or more identical members (referred to as shafts) separated and connected by spacer packs or gusset plates that are either glued or fixed by mechanical fasteners. They are often used in architectural applications, in trusses as compression chords and in frame construction.

Due to the geometry of its cross-section, a spaced column will have a higher load-carrying capacity than a single solid timber member of equivalent volume, and, apart

Fig. 8.5. Beam and post construction using beams inserted between spaced column shafts (photo courtesy of Constructional Timber Limited, a member of the Glued Laminated Timber Association).

from being economical, they provide suitable construction through which other members such as bracing, beams or trusses can conveniently be inserted and connected, as shown in Figure 8.5.

Where glue is used there will be no slip or relative rotation at the connections between the spacer packs (or gussets) and the shafts, and full composite action will exist. When mechanical fasteners are used, slip will occur and the effect of this on the composite action behaviour must be taken into account. Examples of spaced columns are shown in Figure 8.6.

The conditions imposed by EC5 for the design of spaced columns with packs or gussets fixed by glue or mechanical fasteners are given in Annex C, Clause C.3.1, and summarised as follows:

• The columns are subjected to axial loading acting through the centre of gravity of the spaced column.

• The shafts must be parallel to each other.

• The cross-section of the spaced column must be composed of two, three or four identical shafts.

• The cross-sections of the spaced column must be symmetrical about the y-y and the z-z axes.

• In the height of the column there must be at least three unrestrained bays. To satisfy this the shafts must, at a minimum, be connected at the ends and at the third points.

• a < 3h if spacer packs are used and a < 6h if gusset plates are used, where a is the free distance between the shafts and h is the thickness of the shaft.

• The joints, packs and gusset plates are designed in accordance with the requirements of Clause C.2.2 in EC5 (i.e. equations (8.13) and (8.14)).

• The pack length, l2, must equal or be greater than 1.5a.


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