L

Fig. 5.7. Compressive strength of a member loaded at an angle a to the grain.

5.3.3 Members subjected to axial tension

These are members that are subjected to a tensile action acting parallel to the grain and through the centroidal x-x axis of the member as shown in Figure 5.8. Such members function as ties in pin jointed trusses and provide tensile resistance to overturning forces in stud walls.

Although the tensile strength, /t,0,k, of clear wood samples is greater than the compression strength, /c,0,k, because tension failure occurs in a brittle rather than a ductile mode and also because of its sensitivity to the effects of grain slope, knots and other defects, the tensile strength of structural timber is generally less than the compression strength. This is particularly the case at the lower strength classes.

With tension members buckling will not arise and so slenderness considerations are only relevant if there is a risk of a stress reversal condition, for example due to the effect of wind loading on members in truss frameworks or in wall studs.

A tension member should be checked at the weakest point and this will normally be at connections. As connection design will follow after the member has been selected, the design of tension members normally involves a trial and error process. Assumptions are made as to the type and size of connections to be used and the adequacy of the tension member is confirmed at the connection location allowing for a loss of area due to the assumed connection. After the connections have been designed, the member is rechecked taking into account the actual net cross-sectional area. It is to be noted that in EC5, 5.2 (3), the effect of loss of cross-sectional area may be ignored where nails and screws with a diameter of 6 mm or less driven without pre-drilling are used. However, as required by EC5, 5.2 (4), all holes within a distance of half the minimum fastener spacing measured parallel to the grain from a given cross-section should be taken as occurring at that cross-section.

The EC5 procedure for the design of axially loaded members is as follows:

The design tensile stress in the member must be less than the design tensile strength:

Fig. 5.8. Axial tension.

CTt,o,d is the design tensile stress parallel to the grain, and

^net where Nd is the design axial load and Anet is the net cross-sectional area after allowing for the effect of the connection. If the connections are different at each end of the member, the end with the minimum net cross-sectional area must be used.

/tj0jd is the design tensile strength parallel to grain, and

„ kmod ' ksys • kh ' /t,0,k , ft,0,d = - (5.18)

Km where kmod and ksys are as defined in equation (5.10), and kh is the size effect modification factor for members under tension, discussed in Chapter 2 and given in Table 2.11. The largest cross-sectional dimension of the member should be used to evaluate the factor. When dealing with LVL, the factor is defined as kt, and is associated with member length. /t 0 k is the characteristic tensile strength of the timber or wood-based product parallel to the grain. Strength information for timber and the commonly used wood-based structural products is given in Chapter 1.

See Example 5.7.2.

0 0

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