Fig 3 Fasteners for table 3

Fig 4 Fasteners for table 4

Fig 5 Fasteners for table 5

Fig 6 Fasteners for table 6

Fig 7 Fasteners for table 7

Fig 8 Fasteners for table 8

Plywood wind bracing – engineered solutions

The Engineered Solution provides building designers, architects, building technologists and engineers with a method of designing optimal plywood bracing for a timber framed building. The design method is applicable to all low-rise domestic and similar timber framed buildings. It provides an engineered solution to bracing buildings that are outside the scope of the Standard Solution. The Engineered Solution provides design freedom by allowing a range of walls less than 0.9m high to be utilised as structural bracing walls.

WIND FORCES

The tables of wind forces generated for a design wind speed of 33m/sec have been extracted from AS4055 and included as Tables 2a, 2b and 2c. These tables are limited to buildings where the height from the ground level to the underside of the eaves is a maximum of 6m, the highest point of the roof is a maximum of 8.5m from ground level and the floor to ceiling height at external walls is a maximum of 2.7m. As wind forces are proportional to wind velocity squared, wind forces due to other wind speeds can be factored to suit the particular design wind speed, e.g. for 41m/sec wind speed the values in Tables 2a, 2b and 2c are factored by 41²/33² or 1.54

DESIGN LIMITATIONS

Building geometry is not a design limitation using the Engineered Solution. Wind speeds and forces must be established in accordance with Australian Standard AS1170.2-1989 SAA Loading Code - Wind Loads or AS4055 Wind Loads for Housing.

DESIGN METHODOLOGY

The Engineered Solution to plywood bracing requires a three stage design approach:

1. Establish the wind forces parallel to the wind flow in the two primary building dimensions i.e. normal to the building length and width (kN).
2. Select a plywood bracing system from Tables and Diagrams 3 to 7 to suit the building design, local availability and required racking resistance (kN/m).
3. Compute the total length of plywood braced shear wall in each direction to resist the design wind force by dividing Stage 1 by Stage 2 i.e. kN divided by kN/m Þ m.

DESIGN DATA - RACKING RESISTANCES

For the system with the plywood thicknesses and stud spacings given in Table 3 and fastener spacings given in Diagram 3, no rods, straps, bracing or nogging, the allowable design racking load is: 2.25kN/m.

For the system with the plywood thicknesses and stud spacings given in Table 4 and fastener spacing given in Diagram 4, no rods, straps or other bracing and one row of nogging, the allowable design racking load is: 2.25kN/m.

For the system with the plywood thicknesses and stud spacings given in Table 5 and fastener spacings given in Diagram 5, rodded but with no straps, nogging or other bracing, the allowable design racking load is: 4.0kN/m.

For the system with the plywood thicknesses and stud spacings given in Table 6 and fastener spacings given in Diagram 6, no rods, straps, bracing or nogging, the allowable design racking load is: 4.0kN/m.

For the system with the plywood thicknesses and stud spacings given in Table 7 and fastener spacings in Diagram 7, no rods, straps, bracing or nogging, the allowable design load is: 2.0kN/m.

DESIGN DATA - NOTES

1. The above design racking resistances can only be applied to plywood sheathed sections of minimum width 0.9m. The design racking resistances of braced wall sections of widths less than 0.9m for the systems detailed in Tables and Diagrams 3 to 5, must be modified using the factors specified in Table 8.
2. Thicknesses of structural plywood for intermediate stress grades may be obtained by reducing the thickness in proportion to the stiffness (EI) of the nearest stress grade above. Minimum values in the Tables must be met. Extrapolation is not allowed. Values of Modulus of Elasticity (E) and Moment of Inertia (I) are given in AS/JAS2269 Plywood - Structural.
3. The allowable design racking resistances given for the systems detailed in Diagrams 3 to 7 are applicable to frames sheathed one side only. The resistances may be doubled for frames sheathed on two sides, however the hold down requirements of the bottom plate must also be doubled. Bottom plate sizes should be checked under these circumstances to ensure safe moment capacity.
4. The allowable design resistances are appropriate to all species, joint strength groups to J4 or JD4 and stress grades of wall framing timbers. This covers most Australian hardwoods and exotic pines. If species of JD5 joint strengths are used then additional testing has shown that the design values should be reduced by 12.5%. Imported unidentified softwood may fall into this category.
5. The allowable racking resistances apply to wall frames to a height of 2.7m. For wall frame heights above 2.7m the allowable racking resistances must be reduced by the ratio of 2.7 to the new frame height e.g. for a 3m high wall frame the allowable resistances should be reduced in the ratio 2.7/3.0.
6. If the power driven staples or DuoFast nails specified in Table 1 are used to fasten the plywood bracing to the frame the fastener spacings specified in Diagrams 8 to 12 must be multiplied by 0.66.

REDUCTION FACTORS FOR SHORT WALLS

For sheathed sections less than 0.9m wide fixed as per Tables and Diagrams 3 to 5 the following reduction factors of Table 8 apply to the bracing resistances. The systems detailed in Tables and Diagrams 6 and 7 cannot be used in sections less than 0.9m wide. A 600mm or wider section of the 4kN/m system detailed in Table and Diagram 5 with the M10 rods fitted does not require the fitting of the M10 coach screws to achieve the reduction factor of 1.0.

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

TABLE 1: Fastener Specification

NOTES:

1. Fasteners with equivalent dimension i.e. head size and shape, shank diameter and length, to those in Table 1 are deemed acceptable.
2. All fasteners are to be galvanised or suitably coated.
3. 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.

TABLE 2a - Wind Forces for 33m/sec Wind Normal to Ridge;
(Permissible) Lateral Forces per metre Length of House (kN/m) Hip or Gable End ; H

TABLE 2b: Wind Forces for 33m/sec Wind Parallel to Ridge - Gable Roof
Total (Permissible) Forces on House (kN)

TABLE 2c - Wind Forces for 33m/sec Wind Parallel to Ridge - Hip Roof
Total (Permissible) Forces on House (kN)

TABLE 3 - Minimum Sheathing Thickness Vs Stud Spacing to give 2.25 kN/m Resistance

TABLE 4 - Minimum Sheathing Thickness of Plywood (mm) for Alternate System to give 2.25kN/m Resistance.

TABLE 5 - Minimum Sheathing Thickness Vs Stud Spacing to give 4.0kN/m Resistance

TABLE 6 - Minimum Sheathing Thickness of Plywood (mm) Alternative System to give a Bracing Resistance of 4.0kN/m.

TABLE 7 - Min. Sheathing Thickness Vs Stud Spacing – Plywood Bracing to give 2.0kN/m Resistance

TABLE 8: Racking Resistance Reduction Factors

| New Search |

| Projects | Technical Data | Specification | Environment | Education | Research | Glossary |

Timber Research Unit
Department of Architecture
University of Tasmania