COP v3.0:structure; asazs-117022011

3.3 AS/AZS 1170.2:2011 

Roof and wall cladding must structurally comply with the requirements of the NZBC Clause B1 Structure. Strength demand may be calculated in accordance with AS/NZS 1170:2 (which is called the “Loadings Code” in this Code of Practice) or NZS 3604.

Designers should know about changes in requirements of the current Loadings Code and amendments to the code. Manufacturers' printed technical literature, using different criteria or test values and a previous Loading Code's design can cause confusion when it is compared to the latest requirements.

The Loadings Code identifies four load categories relevant to metal roof and wall cladding.

  • Wind actions:
    Wind loads are the result of local changes in wind speed as the wind flows over and around the building. High positive forces (pressure) apply where the wind is slowed, high negative pressures (suction) apply where the wind accelerates.  Wind force varies with the shape and position of the building. It also increases with height because the influence of groundsurface drag decreases.
  • Permanent action:
    Dead load is the permanent weight of the roof structure and the permanent part of an imposed load, such as an air conditioning unit.
  • Imposed action:
    Live loads are variable loads imposed on the building by its occupants and contents, such as a person standing on the roof (point load).
  • Induced actions:
    Loads such as wind, snow or ice, and ponding rainwater.

When a structure or part of it, fails to fulfil its expected basic functions, it is said to have reached a limit state. There are two limit states—Serviceability and Ultimate. 

3.3.1 Modes of Failure 

Serviceability limit is a state when a building, or any part of it, becomes unfit for its intended use due to deformation or deflection.

Ultimate limit is a state associated with collapse or failure, or when a building or any part of it becomes unstable or unsafe.

These limit states are not limited to the metal roof and wall cladding, but are intended to be applied to the entire building structure.

Because the prime function of metal roof and wall cladding is to exclude water from the structure, irreversible failure at the serviceability limit state, for example permanent distortion around the fastener head, is generally the governing limit state for pierce fastened roof and wall cladding. This Code of Practice treats serviceability as the criterion of failure for pierce fastened roofs, as these failure levels are far lower than those at which ultimate limit state failure is experienced.