CROSS Safety Report
Design of internal partitions for horizontal loads
This report draws attention to the requirement for internal partitions to be designed for horizontal loads from wind and seismic effects in accordance with Section B of the Australian National Construction Code (NCC).
It discusses that structural testing of some typical panels demonstrated that they were inadequate to resist the anticipated internal pressure from wind.
Key Learning Outcomes
For designers, building owners and managers:
Be aware that so-called “non-structural” elements such as partitions, ceilings, and the like can be subjected to significant horizontal loads from wind and earthquake actions and require appropriate engineering design and installation by competent practitioners
For structural and civil engineers:
Raise the risk of non-compliant design of these so-called “non-structural” elements with other members of the design team
Keep informed with developments in codes and standards and ensure you are working to the latest edition
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The reporter is aware that on certain projects light gauge steel-framed fire-rated Inter-tenancy party walls are being specified and that these are being built without noggings. They are concerned that this design does not comply with the requirements of the NCC Section B. Whilst fire-testing of the walls without noggings appears to have been completed and compliance with Section C of the NCC may be attained, the reporter contends that the design does not address the structural requirements of Section B of the NCC.
Reporter contends that the design does not address the structural requirements of Section B of the NCC
NCC Section B compliance
For Section B compliance with the NCC, the Building Importance Level (IL) and design safety events for the building are determined in accordance with Tables B1.2a and B1.2b respectively. Typically, for fire-rated internal partitions the design safety events consist of both wind and seismic, and these will vary according to the Building IL. For wind actions, the NCC references AS/NZS1170.2: Structural Design Actions, and the wind actions on the internal partitions, whether fire-rated or not, are determined in accordance with Clause 5.3.4 of AS/NZS1170.2. Similarly, for seismic actions, the NCC references AS1170.4, and seismic actions on the internal partitions are determined in accordance with AS1170.4 Section 8. The individual actions should then be assessed for the worst-case scenario in accordance with AS/NZS1170.0 as nominated in Section B1.1 of the NCC.
The reporter was involved in an investigation of this type of framing that included structural testing with an independent engineer overseeing the testing and derivation of the design capacities. Four tests were conducted including (1) no noggings, (2) one nogging 100mm below the head track, (3) one row of mid-height noggings and finally (4) noggings at both the head track and mid-height. Two wall systems were tested, a traditional light gauge steel frame readily available in the market and the second a proprietary light gauge steel frame system.
The test results gave the ultimate design capacity for each wall system when subjected to horizontal load, and working back from these internal pressures the reporter could determine the equivalent external pressure and wind speed. This equated to a maximum building height of 10m and 30m for an Importance Level (IL) 2 building in Terrain Category 2 and 3, respectively. The building height would be further reduced for an IL3 building.
The projects where the reporter has seen such a specification being proposed are significantly higher than this and although they may comply with Section C of the NCC, the design will not comply with Section B. The reporter is concerned that the relevant building approval authorities do not have the necessary understanding of the NCC requirements for non-structural elements to ask pertinent questions in relation to the proposed framing, and this has allowed non-compliant designs such as these to proceed. In such cases, contractors should be wary of their responsibility and the potential consequences for constructing non-compliant designs.
The reporter is concerned that the relevant building approval authorities do not have the necessary understanding of the NCC requirements for non-structural elements
Expert Panel Comments
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This report highlights problems that arise with the use of the term “non-structural’’ when applied to certain elements of a building that may not be part of the main structure, such as partitions, ceilings, and external cladding, and yet still have to be designed to resist applied loads, including wind and seismic.
There was a time when internal partitions were mostly brick or block, and this was generally not an issue. However, with more common usage of lightweight systems, and as building size and height have increased, the effects of internal pressures from wind have become much more significant. As noted by the reporter, all partitions in a building are subject to internal pressures that should be determined in accordance with AS1170.2.
As building size and height have increased, the effects of internal pressures from wind have become much more significant
The requirement for these “non-structural” elements to be designed for seismic actions was not generally appreciated in Australia until relatively recently. The ABCB issued guidance in March 2019 on the requirement to design certain non-structural building elements (including walls and partitions, ceilings and services) to resist earthquake forces in accordance with AS1170.4 in its article Design of non-structural building elements for earthquake forces. CROSS-AUS drew attention to this article in its Newsletter 3 of February 2020.
The revised standard AS/NZS2785:2020 Suspended ceilings - Design and installation sets out the minimum requirements for the design, construction, installation, maintenance, and testing of suspended ceilings and brings the design requirements for wind and seismic into line with the requirements of AS1170 Parts 2 and 4.
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