Wind assessment standards for Australian residential projects (Pt. 2)

Scope and relationship between AS 4055 and AS 1684.2

This webinar, the second in the Australian residential wind series, covers AS 4055 and Part 2 of AS 1684, both updated to the 2021 version that became mandatory in Australia when NCC 2022 came into effect. AS 4055 is a simplified standard applicable only to Class 1 and Class 10a buildings. It uses the same underlying environmental factors as AS/NZS 1170.2 (shielding, topography, terrain) but converts site wind speed into a discrete wind classification through table lookups, making it faster to apply for routine residential work. AS 1684.2 is the timber framing code for non-cyclonic areas; it uses those wind classifications as direct inputs to its prescriptive bracing tables.

Qiming noted that AS 4055 is intentionally conservative and that its simplified assumptions are the trade-off for speed. For engineers who want a more precise result, AS/NZS 1170.2 can still be used for wind classification provided the ultimate limit state design wind speed is no more than five percent greater than the values in AS 1684.2 Table 1.1. Both standards share the same geometry limits: building width no greater than 60 metres, total height no greater than 8.5 metres, eaves height no greater than 6 metres, and roof pitch no greater than 35 degrees.

Wind classification and racking force demand

AS 4055 derives the wind classification from four site-specific factors: the wind region (A, B, C, or D read from the map), the terrain category (1 through 2.5 or 3), the topography classification (T0 to T5), and the shielding classification. Combining these four factors produces a wind class from N1 to N6 for non-cyclonic sites. Once the class is established, wind pressure is read from the lookup tables in AS 4055 or AS 1684.2; the two sets of tables use the same method but produce slightly different values, with AS 1684.2 tending toward the conservative side.

Racking force is calculated as the area of elevation multiplied by the lateral wind pressure. The area of elevation must be computed for both sides of the building, and the critical direction governs. The pressure value depends on the surface type: vertical walls use a single table entry keyed to wind class, while sloped roofs require the building width and roof pitch as additional inputs. Qiming demonstrated this in the Calcs.com wind bracing calculator using a worked example with a 16.2 m by 7.2 m building, wall height 3 metres, wind class N2, and roof pitch 20 degrees. The long-side area of elevation came to 40.8 square metres; the short-side area came to 15.52 square metres.

Nominal bracing and structural wall bracing design

Before sizing structural wall bracings, AS 1684.2 allows a portion of the racking demand to be assigned to nominal bracing: internal linings such as plasterboard or villaboard that already exist in the building. The code permits up to 50 percent of the total racking force to be attributed to nominal bracing, at a capacity of 0.45 kN/m for one side of sheeting or 0.75 kN/m for both sides, provided the linings are evenly distributed and each length is at least 450 mm. The remaining demand must be resisted by structural wall bracings.

AS 1684.2 Table 8.18 lists 14 structural bracing types. Diagonal cross bracings (Types A to D) are cheap and easy to install but have capacities below 3 kN/m and are limited to walls no taller than 2.7 metres. Plywood sheet bracings (Types G to K) achieve higher capacity and are the most commonly used structural option in residential housing; their capacity depends on plywood grade, thickness, stud spacing, nail size, and nail spacing. The 2021 version reduced plywood bracing capacities by 12.5 percent compared with the 2010 version because the benchmark framing group changed from JD4 to JD5. Engineers using JD4 framing may apply a capacity increase as permitted by the code. For walls taller than 2.7 metres, all tabulated capacities must be reduced by multiplying by 2.7 divided by the actual wall height.

Spacing, distribution, and connection requirements

Distributing bracing evenly matters as much as sizing it correctly. AS 1684.2 Clause A3.66 requires structural wall bracings to be placed in both directions, positioned preferentially on external walls and close to building corners. For N1 and N2 wind classes, the maximum spacing between bracings is 9 metres on a single or upper storey; on the lower storey of a two-storey building, 14 metres is permitted where the ceiling height exceeds 4.8 metres. For N3 and N4, the permitted spacing also depends on roof pitch, and closer spacing applies. Qiming summarized these rules in a simplified table during the presentation, but noted that the full requirements in Tables 8.20 and 8.21 must be consulted for design.

Connections at both the top and bottom of each bracing wall must be designed to transfer the bracing reaction. For bracing walls with capacities up to 1.5 kN/m, nominal fixing is sufficient. For higher-capacity walls, Table 8.22 specifies the required fixing at the top of the wall and Tables 8.23 and 8.24 cover the bottom plate connections. The Calcs.com wind bracing calculator automates the racking force calculation and total bracing length output for each side of the building. Spacing checks, connection design, and bracing layout documentation were not covered in the first version of the calculator and remain the engineer's responsibility for building approval submissions.