NCC 2022 Volume One - Building Code of Australia Class 2 to 9 buildings

4

Specification 4 Design of buildings in cyclonic areas

Specification 4 Design of buildings in cyclonic areas

(1) This specification contains requirements for the design of buildings in cyclonic areas in addition to the requirements of AS/NZS 1170.2.

(2) For the purposes of Specification 4, cyclonic areas are those determined as being located in wind regions C and D in accordance with AS/NZS 1170.2.

WA S4C1 Scope2019: Spec B1.2: 1

Delete subclause S4C1(2) and insert S4C1(2) as follows:

(2) For the purposes of Specification 4, cyclonic areas are those determined as being located in wind Regions B2, C and D in accordance with AS/NZS 1170.2.

To clarify that Specification 4 contains requirements for the design of metal roofing cladding assemblies in cyclonic areas.

This specification must be read in conjunction with the provisions of AS/NZS 1170.2. The ABCB commissioned research to establish a national consistent testing regime for metal roof cladding assemblies in cyclonic areas. The results of this research are reflected in Specification 4.

Test for strength: Metal roof cladding, its connections and immediate supporting members must be capable of remaining in position notwithstanding any permanent distortion, fracture or damage that might occur in the sheet or fastenings under the pressure sequences A to G defined in Table S4C2.

Table S4C2 Low-high-low pressure sequence
Sequence Number of cycles Load
A 4500 0 to 0.45 Pt
B 600 0 to 0.6 Pt
C 80 0 to 0.8 Pt
D 1 0 to 1.0 Pt
E 80 0 to 0.8 Pt
F 600 0 to 0.6 Pt
G 4500 0 to 0.45 Pt
Table Notes
  1. Pt is the ultimate limit state wind pressure on internal and external surfaces as determined in accordance with AS/NZS 1170.2, modified by an appropriate factor for variability, as determined in accordance with Table B1 of AS/NZS 1170.0.
  2. The rate of load cycling must be less than 3 Hz.
  3. The single load cycle (sequence D) must be held for a minimum of 10 seconds.

To define the expected performance of roof cladding assemblies in cyclonic areas when subjected to the specified test regime (Table S4C2).

Strength performance of roofing system

Low cycle fatigue cracking of metal roof cladding elements during tropical cyclones is a complex process where small changes in load, geometry or material properties can significantly affect the fatigue performance of the cladding system (includes immediate supports, fixings and cladding). The consequences of failure of an element can quickly lead to more elements progressively failing. These failed elements may become wind driven debris and so pose impact threat to people and other structures.

Table S4C2

The fatigue loading sequence prescribed in Table S4C2 is to simulate the wind load induced by a cyclonic event. In order to have a repeatable standard test that can be performed by different testing laboratories within a reasonable time frame on different types of test equipment, the loading sequences a simplification of the dynamic wind loading environment. In the formulation of the fatigue loading sequence, assumptions such as cyclone counts, load range, cyclone duration, wind direction change, building orientation and building geometry have been made.

If a system does not successfully resist the fatigue loading sequence in Table S4C2 it does not comply.

The test section is to consist of cladding elements, fastenings and immediate supporting members assembled together in a manner identical to those parts of the particular roof which the test section is intended to replicate.

SequenceNumber of cyclesLoad
A45000 to 0.45 Pt
B6000 to 0.6 Pt
C800 to 0.8 Pt
D10 to 1.0 Pt
E800 to 0.8 Pt
F6000 to 0.6 Pt
G45000 to 0.45 Pt

Table Notes:

  1. Pt is the ultimate limit state wind pressure on internal and external surfaces as determined in accordance with AS/NZS 1170.2, modified by an appropriate factor for variability, as determined in accordance with Table B1 of AS/NZS 1170.0.
  2. The rate of load cycling must be less than 3 Hz.
  3. The single load cycle (sequence D) must be held for a minimum of 10 seconds.
NCC Blurbs

(1) Where a residential building of Class 2, 3, 9a or 9c, in Region C as defined by AS/NZS 1170.2, is designed to be used by the Aged or Infirm it shall incorporate a “strengthened area” for use as shelter during cyclonic conditions and must comply with the following criteria:

  1. The floor area of the “strengthened area” is to be calculated at the rate of 1.2 m2 per person normally accommodated within the building.
  2. The design wind pressure for the overall “strengthened area” is for an Importance Level 3 building.
  3. The minimum standard of debris protection to walls, floors and ceilings (or roof) bounding the "strengthened area" shall be that which resists (without complete penetration) the impact loading specified in AS/NZS 1170.2, using a regional wind speed associated with an Importance Level 3 building.
  4. All doors, windows, vents and the like in walls bounding the “strengthened area” are to be protected against windborne debris with permanently installed screens in accordance with (c).
  5. Consideration must be given to the selection of materials and fittings to ensure doors, windows and vents can withstand the required design wind pressures.
  6. All doors serving as required entries/exits to a “strengthened area” are to be inward opening with locking devices suitably noted for use in a cyclone emergency.

(2) The term “strengthened area” is defined as the strengthening of an area to increase its potential to facilitate debris protection.

NCC Title
Strengthened area
NCC State
NT
NCC Variation Type
Insertion
NCC SPTC Current
Strengthened area