Plywood wind bracing – standard solutions

The standard plywood bracing solution provides a simple yet safe method of designing wind bracing to suit a large proportion of low rise timber framed structures in Australia. This solution is the method used in Australian Standard AS1684 National Timber Framing Code.

DESIGN LIMITATIONS

Wind Speeds

The three wind speeds employed in the standard solution are 28m/sec (W28), 33m/sec (W33) and 41m/sec (W41). These design gust wind speeds for permissible stress design can be derived using AS1170.2 - Wind Loads. Alternatively, the wind classification of building sites is more easily found using AS4055 Wind Loads for Housing, a rationalised interpretation of the Code.

Building Geometry

This solution is applicable to conventional timber framed housing with a shape in plan view of a rectangle or a simple combination of rectangles. The solution is limited to buildings with a maximum ceiling height of 2700mm and a maximum roof pitch of 35 degrees.

Bracing Units

The two standard structural plywood bracing units are 900mm wide and are designated Type A or Type B as illustrated in Figure 1. The minimum plywood thicknesses and fastener spacings are detailed in Tables 2 and 3. The Type B bracing unit has a nominal bracing capacity double that of Type A, and would normally be used in the lower storey of double storey construction, or in single or upper storeys of large buildings sited in the higher wind speed zones. Type A bracing units are normally used in single or upper stories.

Sheathed sections greater than 900mm wide may be used as bracing on the basis that the number of bracing units is directly proportional to the overall length; e.g. a 1200mm wide panel is equivalent to 1200/900 or 1.33 bracing units. Sheathed Type A bracing units 600mm wide may be used on the basis of being equivalent to 0.33 Type A bracing units, or if the additional M10 coach screws are used (see Figure 2), the panel will be equivalent to 0.67 Type A bracing units. Note that Type B plywood bracing units must be installed in panels of minimum width 900mm.

NUMBER AND DISTRIBUTION OF BRACING UNITS

The minimum number of bracing units at right angles to the building length or width is given in Table 4. However, there must be at least 4 bracing units at right angles to each of the length and width i.e. a minimum of 8 bracing units in each level of the building. A minimum of two bracing units shall be installed in each length of each external wall. It is recommended that bracing units be installed at external wall junctions to achieve corner fixity. Internal and external bracing units shall be evenly distributed throughout the building framework. The maximum distance between braced walls at right angles to the building length or width shall be 9m where conventional flat or raked ceiling linings are securely fixed (nailed or screwed) to the roof or ceiling framing. Refer to Figure 3.

AREA OF ELEVATION

As wind can blow from any direction, the area of elevation used when evaluating Table 4 shall be that for the worst direction, i.e. an elevation containing flat walls, gable or skillion ends or roof slopes to 5° will produce the greatest number of bracing units required. The elevation of a building may be broken into smaller sections and the number of bracing units required for each section determined. The total number of bracing units required for each level of the building is then the sum of all the units for each section. Figure 4 illustrates how to calculate the area of elevation.

BOTTOM PLATE FIXINGS

The plywood braced wall sections must be connected to the subfloor using one of the methods detailed in the article "Additional Installation Requirements – Plywood bracing". Type A bottom plate fixing method is required for Type A bracing units, while the Type B method is for Type B bracing units. The top plate of braced sections must be fixed to the ceiling or roof framing in such a manner as to transfer the lateral forces to the bracing panel.

DESIGN EXAMPLE - STANDARD SOLUTION

This design example uses the Standard Solution to determine the bracing requirements for a 15m x 9m double storey gable roofed dwelling with roof pitch of 20° and ceiling height of 2.7m as shown in Figure 5. Assume a design wind speed of 33m/sec.

Step 1.

Establish the area of elevation for the upper and lower storeys in both wind directions using the method detailed in Figure 4. The gable end can be treated separately as a triangle. The area calculations and results are detailed in Table 5.

Step 2.

Select the number and type of bracing units for each wind direction and storey by reference to the 33m/sec columns in Table 4. For wind direction A use the 6° - 35° roof slope column to select the bracing units required at right angles to the 15m building length, and the flat walls and 5 degree roof slope column to select the bracing units required at right angles to the gable end.

Step 3

Figure 6 illustrates the distribution of bracing units for the upper and lower floors.

Reference: Plywood Association of Australia brochure: "Structural plywood wall bracing"

NOTES:

> Fasteners with equivalent dimension i.e. head size and shape, shank diameter and length, to those in Table 1 are deemed acceptable.
> All fasteners are to be galvanised or suitably coated.
> If smaller diameter hand driven nails are used, the spacings of nails can be reduced in the ratio of the basic lateral loads per nail for J4 or JD4 joint group given in Table 4.1 of AS1720.1 Timber Structures Code. This is for the lower nail diameter relative to the tabulated load for a 2.8mm diameter nail.

NOTE: All plywood must comply with AS/NZS2269 and be branded with the " PAA Product Certification" mark.

NOTES

> Refer to Table 1 for approved fasteners
> No nogging required except at horizontal plywood butt joints
> Nails to be a minimum of 7mm from panel edges
> Panel edges shall be supported by studs, noggings or plates
>The fastener spacings in Table 3 specified for staples also apply to the Duo Fast fastener C/25/50.