Syk

d t (local z flange axis)

Fig. 8.8. Lattice columns.

The pack or gusset plates and their connections to the shafts must be designed for the effects of the shear force shown in Figure 8.7. The moment on the connection at the shaft is obtained by multiplying the shear force in the pack or gusset plate by the distance to the face of the shaft when packs are used or the distance to the centre line of the shaft when gusset plates are used.

See Example 8.8.2.

8.4.5 Built-up sections - latticed columns

A latticed column is a built-up column where there are two identical members separated and connected by N or V lattice members fixed to the members by glued or nailed joints. Examples of glued latticed columns with N and V lattice configurations are shown in Figure 8.8.

The conditions imposed by EC5 for the design of lattice columns are given in EC5, Clause C.4.1, Annex C, and are as follows:

• The structure must be symmetrical about the y-y and z-z axes.

• The lattice on each side of the lattice column may be staggered relative to each other by a length of i\/2, where t is the distance between adjacent nodes.

• There must be at least three bays of latticed column in the column, i.e. t = 3t i.

• Where the lattice members are nailed to the flanges, there must be at least four nails per shear plane in each diagonal at each nodal point connection.

• Each end of the lattice column structure must be braced, i.e. secured laterally in position.

• For an individual flange between adjacent node connections (i.e. length li in Figure 8.8), the slenderness ratio must not be greater than 60.

• Buckling of the flanges corresponding to the column length li will not occur.

• The number of nails in the connection between a vertical (of an N-truss - as shown in Figure 8.8b, position (3)), and the flange must be greater that n sin 0, where n is the number of nails in the adjacent diagonal connection (Figure 8.8b, position (4)) and 0 is the angle of inclination of the diagonal.

8.4.5.1 Design procedure for lattice columns

When the lattice column structures shown in Figure 8.8 buckle about the z-z axis, (i.e. deflection is in the y-axis direction), the flanges behave as individual elements and the strength of the column will be the summation of the strength of the flanges about the z-z axis.

When buckling occurs about the y-y axis (i.e. deflection is in the z-axis direction), the forces in the lattice members lead to additional lateral displacement in the flanges and the strength reduction caused by this is taken into account in the design procedure. As with spaced columns, this is achieved in EC5 by using an effective slenderness ratio, ¿ef, derived as follows:

where:

• A.tot is the slenderness ratio for a solid column having the same length as the latticed column, the same cross-sectional area (Atot = ^2n Af, where Af is the cross-sectional area of each flange, bd), and the same second moment of area Itot, Itot = (Af/2)((d2/3) + h2). On this basis:

For most practical column sections, d2/3 is much smaller than h2 and in EC5 it is ignored, giving

V is a factor that takes into account the stiffness of the connection: (a) For glued joints v will be as follows:

where e is the eccentricity of the bracing member at the nodes - as shown in Figure 8.8, Af is the area of each flange, If is the second moment of area of a flange about its own axis (w-w), t is the height of the latticed column, and h is the distance between the centre lines of the flanges.

(b) For nailed joints x will be as follows:

t2nKu sin 20

t2nKu sin20

where the symbols are as described above, and:

•n is the number of nails in a diagonal - if a diagonal consists of two or more pieces, n is to be taken as the number of nails, not the number of nails per shear plane;

• Emean is the mean value of modulus of elasticity of the timber;

The load-carrying capacity of the lattice column will be derived using the higher of the slenderness ratios of the column about the y-y and z-z axes, and for this condition it must be verified that equation (8.25) is satisfied. Also, to comply with the EC5 requirement that buckling of the flanges corresponding to a column length t1 will not occur, using the procedures described in 5.3.1 for an axially loaded column it must be shown that the strength of each flange between adjacent node connections will exceed 50% of the strength of the latticed column.

The bracing members and their connections to the shafts must be designed for the effect of the shear force Vd derived from the equations given in EC5, (C2.2), i.e.

120kc

Fc.d^ef

3600kc

Fc,d

60kC

for A.ef < 30 for 30 < A.ef < 60 for A.ef > 60

where the symbols are as previously defined, and Fc d is the design axial load acting through the centre of gravity of the lattice column, and A.ef is the effective slenderness ratio defined in equation (8.28).

The above shear force will be taken by the bracing members and their connections as shown in Figure 8.9. The forces in the bracing can be either tension or compression, f d 0 0