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 18.104.22.168.
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 classificationThe 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 kilo-newton (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
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
22.214.171.124 Structural wall bracing
Structural wall bracing is purpose-fitted bracing, being either sheet or cross-timber 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
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 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 126.96.36.199 & 188.8.131.52). 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 184.108.40.206).
220.127.116.11 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.
Wind classification N2
Use the same principle to indicate the bracing for Plan E Wall Bracing - Structural.