Explained: Key changes introduced in AS/NZS 1170.2-2021

AS/NZS 1170.2-2021 introduces significant changes to load calculations which will have a profound impact on structural design in Australia and New Zealand for the next decade. This is not an exhaustive list of all changes and is intended to highlight the key differences from the 2011 version. You can download a copy here.

  • New wind region maps: Melbourne no longer has it’s own region, and central Australia is covered by the new A0 region. Linear interpolation is now allowed for Regions C and D based on the relative distance from the “smooth coastline”. New Zealand regions have been revised and have a new naming scheme.
  • Directional Multiplier (Md): Standalone poles and chimneys with circular or polygonal cross-sections to be designed using Md = 1.0. This is not applicable to structures with significant directional loading such as transmission lines.
  • Climate Change Multiplier (Mc): Effectively replaces FC and FD factors from AS/NZS 1170.2-2011, but now relevant to the new Region B2.
  • Terrain/Height Multiplier (Mz,cat): Terrain Category 1.5 for “open water surfaces subjected to shoaling waves” has been removed. The averaging distance is now MAX(500, 40z) instead of MAX(500, 40h) from AS/NZS 1170.2-2011. For buildings 25m, use z = h. For all other structures and buildings > 25 m, use z.
  • Shielding Multiplier (Ms): This section has been expanded to remove the ambiguity present in the previous versions regarding maximum slope requirements. Structures exceeding 25 m in height must now adopt a value of 1.0 for all values of z unless wind tunnel testing is carried out to prove otherwise. The number of upwind shielding buildings (ns) is now defined as the number of buildings with an average structure height (hs) exceeding the height of the structure (h) under consideration.
  • Topographic Multiplier (Mt): Sites in Region A0 now have their own equation, Mt = 0.5 + 0.5 x Mh. Topographic features less than 10 m in height can now be ignored. Further clarification is given that a topographic feature may be ignored “provided the crest is distant from the site of the structure by more than 10 times its crest elevation above sea level, and any intervening valley is more than 10 times the distance of the valley floor below the crest”.
  • New lee zone maps: Entirely new lee zone maps for New Zealand, including “lateral transition zones” to prevent the previous situation where Mlee could equal 1.35 in one location but 1.0 just 100 m away.
  • Internal Pressure Coefficients (Cp,i): Values now adjusted by Ka and Kl factors for situations with “dominant” openings. Cases where Kl > 1.0 must be considered in the calculation of Cp,i for all structural elements (both cladding and non-cladding).
  • Volume Factor (Kv): A new factor designed to adjust internal pressure coefficients (Cp,i) for buildings with dominant wall openings based on the internal volume exposed to that opening.
  • Area Reduction Factor (Ka): Can now be applied to windward and leeward walls for buildings with h < 25 m.
  • Dynamic Response Factor (Cdyn): A different equation is used for towers, poles and masts where the total projected area of attachments exceeds the total projected area of the structure.

The good news is that our CHECKPOLE, CHECKWIND and Hazard API software have all of these changes integrated to make your life as simple as possible.

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