General matters

In the Eurocodes, the procedure for displaying a decimal point is to use a comma, e.g. 4,5, in accordance with the requirements of the ISO. The United Kingdom, however, uses a full stop for the decimal point, i.e. 4.5, and the UK practice has been used in the book.

The traditional practice in the United Kingdom is to show the z-z axis as the longitudinal axis of a member and the x-x and y-y axes to denote the respective major and minor axes of its cross-section. In the Eurocode suite the longitudinal axis is referred to as the x-x axis and the y-y and z-z axes are the respective major and minor axes of the cross-section. The Eurocode convention, which is shown in Figure 2.5, is used in the book.

Symbol denoting that the grain direction is parallel to the x-x axis

Fig. 2.6. Deformation.

Where it is relevant to show the direction of the grain of the timber it is defined by the symbol used in Figure 2.5.

2.3.2 Serviceability limit states (EC5,2.2.3)

In EC5 the deformation of a member or structure is required at two stages:

(i) When the loading is immediately applied; this is called the instantaneous deformation: Minst.

(ii) After all time-dependent displacement (i.e. creep deformation, ucreep) has taken place; this is called the final deformation: ugn.

These deformations are shown diagrammatically in Figure 2.6 in relation to a simply supported beam without any pre-camber.

Deformation is calculated in two different ways, depending on the creep behaviour of the structure:

(a) Structures comprising members, components and connections having the same creep behaviour

Creep behaviour in timber and wood-related products is a function of several factors, and to simplify the design process the assumption is made in EC5 that when subjected to a permanent load over the lifetime of a building, the instantaneous deflection (uinst) and the creep deflection (ucreep) are related as follows, ucreap — kdefu inst (2.27)

where kdef is a deformation factor whose value is dependent on the type of material being stressed as well as its moisture content. Values for the factor have been derived for timber and wood-based materials at defined environmental conditions when subjected to constant loading at the SLS over the design life, and are given in EC5, Table 3.2. The environmental conditions are referred to as service class 1, 2 or 3 (discussed in 2.2.20) and values for kdef for timber and some wood-related products at these conditions are given in Table 2.10.

For structures or members complying with the above conditions the final deformation, ufin, can then be written as ufin — uinst + u creep — uinst(1 + kdef) (2.28)

Table 2.10 Values of kdef at service class 1, 2 or 3*

Material — standard

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