The racking strength Fi VjRd of wall i is the lateral withstand capability of the wall and can be considered to be equivalent to the maximum resistance force the wall will be capable of sustaining at its top as shown in Figure 9.11. The risk of the wall sheets buckling under load can be ignored provided equation (9.33) in EC5 is complied with, i.e.:

bnet

where bnet is the clear distance between the vertical members of the timber framing of the wall, and t is the thickness of the sheathing material.

For a wall assembly made up from n walls, the design racking resistance of the assembly will be:

The design condition to be met is:

Fv,Rd > Fv,Ed where Fv Ed is the horizontal design force on the wall assembly.

The design racking strength, Fi v Rd of wall i is obtained from equation (9.25) in EC5 as follows:

where:

if Rd is the design capacity of an individual fastener in lateral shear. bi is the length of the wall (in metres).

s0 is the fastener spacing as calculated by equation (9.26) in EC5 and:

Pk where, for dimensional accuracy, the diameter of the fastener, d, must be in metres, the characteristic density of the wall framing timber pk is in kg/m3 so that the spacing s0 will be in metres. This ensures that in function Ff Rdbi /s0 the same units are used for bi and s0.

kd is the dimension factor of the wall and is a variable function of its aspect ratio. It is obtained from equation (9.27) in EC5 as follows:

bi u bi

where h is the height of the wall in metres.

• ki>q is called the uniformly distributed load factor and is obtained from EC5, equation (9.28). It is a function of the vertical loading on the wall, increasing the wall strength as the loading increases and equals unity when there is no vertical loading. It is derived from:

kiqq = 1 + (0.083qi - 0.0008q,2) i — I (EC5, equation (9.28))

where qi is the equivalent uniformly distributed vertical loading acting on the wall (in kN/m) with qi > 0 and is obtained using only permanent actions and any net wind effects together with any equivalent actions arising from concentrated forces, including anchorages, acting on the wall. Concentrated forces are converted to equivalent uniformly distributed loading by assuming that the wall is rigid, in which case, for the concentrated load F,,vert,Ed shown as acting on the wall in Figure 9.11, the equivalent uniformly distributed load, qi (as given in EC5, equation (9.31)), will be:

2aFi vert FH

• ks is the fastener spacing factor and is a function of the spacing used around the perimeter of the sheets and the basic fastener spacing, s0. It is obtained from equation (9.29) in EC5 as follows:

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