AS 1684.2 Section 8 (Bracing)
Bracing is required to withstand the wind pressure on the timber framed structure.
The wind produces a lateral load, which must be transferred through the
structure to the foundation. The ceiling and floor form a horizontal diaphragm.
The wind force is transmitted through the ceiling diaphragm to the bracing
walls, which transmit them to the floor structure as indicated in the figure
below.
The ceiling and floor diaphragms play important roles in the transfer of
wind loads from the walls and roof to the braces. The ability of a ceiling
or floor diaphragm to effectively transfer the wind load depends on the
depth of the diaphragm. Narrow or long diaphragms will not transfer the
wind loads as effectively as a deeper diaphragm. The smaller the depth to
length ratio the more effective the diaphragm. For this reason the spacing
of bracing walls in limited as per Clause 8.3.6.7.
8.3 Wall and subfloor bracing
Bracing shall be designed and provided for each storey of the house and
for the subfloor, where required, in accordance with the following procedure:
(a) Determine the wind classification
(b) Determine the wind pressure
(c) Determine area of elevation
(d) Calculate racking force
The maximum design gust wind speed must be known to determine the forces
acting on the building. According to AS 4055 Table 2 the wind classifications
for Regions A and B is N1 to N6. The wind classification will
always be stated in the specifications.
8.3.4 Racking
force
The racking force of the building shall be determined by the method (A
area of elevation) or by the method (B simplified) given
in Appendix G
Method B (simplified)
Using Method B you do not require the area of elevation. The tables G1
to G4 for the various wind classifications differentiate between
a) gable hip & ends and b) long length of building
a) The gable or hip width and the roof pitch is required determine
the racking force. The figures in the
tables give you the force in kilonewton (kN).
b) To determine the racking force for the length of the building,
the width, roof slope and
the length need to be considered. The
figures in the tables gives you the wind force per unit
length in kN/m.
The width is required because the diaphragm depth is smaller
for the length of the building (refer to diagram above). These figures need
to be multiplied by the length of the building to obtain the racking force
for the length of the building.
Example 1, length of building 13.5 m, width = 8.5 m,
roof pitch = 26°, wind classification N3.
Use Table G3 (C) (single or upper storey) Wind Classification N3 (see below)
to determine the raking force for the Length (long side) of the building
Therefore the racking force is 13.5 m × 5 kN/m = 67.5 kN
Using interpolation 13.5 × 4.52 = 61.02 kN (reduction
of 9.04%)
(see Appendix D; Page 226)
If you are prepared to calculate the area of elevation then the racking
force will be smaller. Compare this with the figure below.
Method A (area of elevation)
Using this method you need to calculate the area of elevation to determine
the racking force. Table 8.1 to 8.5 include the pressure in kPa
(kN/m²) for various roof pitches.
Note 3 of Figures 8.2 (A, B & C) states
The area of elevation of the triangular portion of eaves overhang up
to
1000 mm wide may be ignored in the determination of area of elevation.
Area = 13.5 × 2.7/2 + (13.5 + 5)/2 × 2.07 =
37.37 m²
Example 2: Using the same measurement as in the previous example.
Given an area of elevation of 37.37 m², length of building 13.5 m,
width = 8.5 m, roof pitch = 26°, wind classification N3.
Refer to Table 8.2 below to determine the racking force
Therefore the racking force 
= 37.37 m² × 1.1 kN/m² 
* provision for 50% nominal bracing is not
considered
if you do, check that nominal bracing is achievable

= 41.11 kN (< 61.02 kN) 
The raking force is approximate 33% less in Example
2

8.3.6.3 Structural wall bracing
Structural wall bracing is purposefitted bracing, being either sheet or
crosstimber or steel bracing.
NOTE: Nominal bracing cannot contribute to bracing resistance
where it occurs in the same section of wall as structural bracing, such
as where plasterboard lining is fixed over a structural brace
Table 8.18 gives the specific capacity for each metre length of various structural
bracing types. 2700 mm high max. The table
caters for different types of wall bracing (timber and metal angle braces,
double diagonal tension or metal strap braces, plywood sheet bracing etc.)
The bracing capacity is indicated in kN/m. To find out how may metres of
bracing is required you need to divide the total racking force by the capacity
of each bracing type. According Table 8.18 the bracing capacity bracing
type (b) is 1.5 kN/m
Using the figure from Example 2
The running metres of bracing required 
= 41.11 kN / 1.5 kN/m

* provision for 50% nominal bracing is not
considered
if you do, check that nominal bracing is achievable

= *27.41 m 
There may not enough walls (external and internal) to accommodate 27.41
m; therefore another type of bracing should be selected. Plywood bracing
according to Table 8.18 (g) has a bracing capacity of 3.4 kN/m
The new running metres of bracing required 
= 41.11 kN / 3.4 kN/m

* provision for 50% nominal bracing
is not considered
if you do, check that nominal bracing is achievable 
= *12.09 m 
The bracing should be evenly distributed. Sheet bracing walls shall be a minimum
of 900 mm wide to satisfy the requirements of their nominated ratings
(Claus 8.3.6.3 & 8.3.6.5). The max. spacing between braced walls at
right angles to the building length or width shall not exceed 9 metres for
wind classification up to N2. For wind classification greater than N2 refer
to Table 8.20 & 8.21 (Clause 8.3.6.7).
8.3.6.2 Nominal wall bracing
Nominal wall bracing is wall framing lined with sheet materials such
as plywood, plasterboard, fibre cement or hardboard, or the like, with
the wall frames nominally fixed to the floor and the roof or ceiling frame.
All sheet materials have a potential bracing capacity.
Their capacity (strength) relies heavily on the fixing method used to
attach it to a wall frame.
As well as the relative strength of the sheet material itself, the size
(diameter and length of nails or screws), spacing and total number of
fixings plays an important role in the bracing strength given to a particular
sheet material.
Nominal braces generally consist of the same sheet product used for structural
braces but only have ‘nominal fixings’ to attach them to the wall frame.
These nominal fixings limit the strength of the brace. The most common
potential nominal bracing material used in houses is plasterboard wall
linings.
Plasterboard, fixed to the wall frame appropriately (to manufacturers
specification) is given ‘structural bracing’ status with a reasonable
strength rating. Fixed to the wall frame with nominal fixings, however,
its bracing strength is much lower.
Below is an example of bracing
distribution
Wind classification N2
Check even distribution and spacing
Check connection of bracing to roof/ceilings and floors
Use the same principle to indicate the bracing for Plan
E Wall Bracing  Structural.
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