WA Part 2.3 Changes to AS/NZS 1170.2:2021 and AS 4055:2021

This Part sets out the changes necessary for Western Australia to the application of AS/NZS 1170.2:2021 for wind Region B2 and AS 4055:2021 for site wind classification.

Delete Table 3.1(A) of AS/NZS 1170.2:2021 and replace with WA Table 2.3.2.

Delete 5.3.1 of AS/NZS 1170.2:2021 and replace with the following:

5.3.1 Internal pressure

5.3.1.1 General

Internal pressure is a function of the external pressures, and the leakage and openings in the external surfaces of the building or an isolated part of a larger building, and for some large buildings, the internal volume. The open area of a surface shall be calculated by adding areas of opening to areas of permeability or leakage on that surface of the building (e.g. vents and gaps in the building envelope).

The height at which the design wind speed is determined for calculation of internal pressures shall be the average roof height (h), as defined in Figure 2.1. However, for the cases of windward wall leakage or openings on a building greater than 25 m in height, the design wind speed at the height of the opening shall be used.

Pressure coefficients for internal pressure (Cp,i) shall be determined by either Clause 5.3.1.2 or 5.3.1.3.

NOTE 1 Damage inspections after wind storms, in Regions B2, C and D, have shown that large openings are very likely to occur accidentally due to failure of elements under direct wind pressure, or in the lower levels of a building envelope, by debris impact. Large openings can also occur in Regions A (0 to 5), B1 and NZ (1 to 4) under the same circumstances, although openings produced by debris impact are less likely.

NOTE 2 The equivalent free area of a ventilator (e.g. ridge or under-eave ventilators) can be determined from the product of discharge coefficient and throat area.

5.3.1.2 Internal pressure coefficients for all cases, except ultimate limit states for parts of buildings below 25 m in Regions B2, C and D

Clause 5.3.1.1 applies to buildings in all regions for serviceability limit states.

For ultimate limit states, it applies to all buildings in Regions A (0 to 5), B1 and NZ (1 to 4), and parts of buildings higher than 25 m above ground level in Regions B2, C and D.

Pressure coefficients for internal pressure (Cp,i) shall be determined from Tables 5.1(A) or 5.1(B). Table 5.1(A) shall be used for the design case where there are no potential openings in any surface with a combined area greater than 0.5 % of the total area of that surface, and the leakage in the walls lead to internal pressures. Table 5.1(B) shall be used for the design case where there are openings in any surface greater than 0.5 % of the total area of that surface, or they can be created accidentally.

5.3.1.3 Internal pressure coefficients for ultimate limit states for parts of buildings below 25 m in Regions B2, C and D

Pressure coefficients for internal pressure (Cp,i) for parts of a building in Regions B2, C and D below 25 m for ultimate limit states, shall be determined from Table 5.1(B) only.

The ratio of the sum of opening areas on one surface to total open area of other walls and roof surfaces as defined in Table 5.1(B) shall not be taken to be less than two unless —

(a) it can be demonstrated that an opening will not be created in the building envelope as a result of impact loading from the windborne debris defined in Clause 2.5.8; or

(b) a permanently-open roof ventilator, such as a ridge ventilator, has been installed with equivalent total area (see Clause 5.3.1.1 Note 2) of at least that of the largest areas of any potential accidental openings in the walls, considering the combined area of wall openings in each wall surface one at a time; or

(c) permanently-open, wall ventilators have been installed on at least two walls, with equivalent total area (see Clause 5.3.1.1 Note 2) of the ventilators on each wall at least that of the largest of any potential accidental openings in the walls, considering the combined area of wall openings in each wall surface one at a time.

NOTE 1 Low-rise buildings in Regions B2, C and D should be designed for the high internal pressures resulting from large openings, for ultimate limit states. Even in cases where the opening is small or there is no opening, Table 5.1(A) is not intended to be used for low-rise buildings in Regions B2, C and D for ultimate limit states.

NOTE 2 To date, the majority of windborne debris in Regions B2, C and D in Australia has not often impacted at heights on buildings above 25 m. This is not the case in other parts of the world and could change in the future with increasing numbers of high-rise buildings.

Delete 5.3.2 of AS/NZS 1170.2:2021 and replace with the following:

5.3.2 Openings

5.3.2.1 General

Openings shall be determined according to either Clause 5.3.2.2 (Regions A (0 to 5), B1 and NZ (1 to 4), and Regions B2, C, D at heights of 25 m or above) or Clause 5.3.2.3 (Regions B2, C, D below 25 m).

Subject to Clauses 5.3.2.2 and 5.3.2.3, combinations of openings and open area shall be assumed to give internal pressures, which, together with external pressures, give the most adverse wind actions.

NOTE Potential openings include doors or windows that are left open or may fail, vents that are normally open and holes in cladding caused by impacts by windborne debris during a major wind event. Openings can be doors (including balcony doors) or windows that are left open, open under pressure, or open due to the failure of latches or hinges. When determining internal pressures, consideration should be given to scenarios in which large openings may develop. Openings may also be generated by debris impacts, particularly in Regions B2, C and D (see Clause 2.5.8).

5.3.2.2 Openings in buildings in Regions A (0 to 5), B1 and NZ (1 to 4), and parts of buildings at heights of 25 m or above in Regions B2, C and D

The full area of doors, including large access doors (e.g. roller doors), and windows that are normally closed, shall be regarded as openings, unless they are demonstrated to be capable of resisting the applied wind pressures.

NOTE 1 When assessing internal pressures, designers should consider the principles of robustness, i.e. to avoid situations where the failure of a single component such as a door or window could lead to consequent and disproportionate failure of other elements, or even complete failure of the structure.

NOTE 2 The structural assessment of doors that are assumed to remain closed and intact should include elements such as supports, frames, jambs, roller door guides, wind locks, latches and hinges, and fixings, where the resistance of doors relies on those. This assessment of roller doors and their supporting structural elements should also account for any structural resistance to any catenary actions developed by the door under wind load.

5.3.2.3 Openings in buildings for ultimate limit states for parts of buildings below 25 m in Regions B2, C and D

Doors (including large access doors) and windows that are normally closed, and cladding elements, shall be regarded as openings with an area equal to the greater of—

(a) the full area of the element, where it has not been demonstrated that it can resist the applied wind pressures; or

(b) the area of opening that results from debris impact, where the debris impact loading criteria are defined in Clause 2.5.8.

Delete reference to ‘Table 2.2’ in clause 2.2 of AS 4055:2021 and insert WA Table 2.3.5 for determining site wind classification.