NCC 2019 Volume One Amendment 1
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Specification B1.2 Design of buildings in cyclonic areas

Specification B1.2 Design of buildings in cyclonic areas

1. Scope

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

For the purposes of Specification B1.2, cyclonic areas are those determined as being located in wind regions C and D in accordance with AS/NZS 1170.2.

Intent

To clarify that Specification B1.2 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 contained in Specification B1.2.

2. Roof Cladding

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 1.

Table 1 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

Notes to Table 1:

Notes to Table 1:
  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.

NT Specification B1.2 Clause NT3 — NT4.

Intent

To define the expected performance of roof cladding assemblies in cyclonic areas when subjected to the specified test regime (Table 1).

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 become wind driven debris and so pose a threat to people and other structures as potential missiles.

Table 1

The fatigue loading sequence defined in Table 1 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 sequence is 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 1, it does not comply.

The test section consists 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.