CROSS Safety Report
Safety concern about the design of thin concrete walls
This report is over 2 years old
The reporter raises their concern about the design of thin concrete load-bearing walls, sometimes supporting 10 or more storeys.
This report highlights the need to keep up-to-date with design standards, and to understand the real behaviour of structures rather than rely on standards and codes of practice to achieve safe and reliable structures.
Key Learning Outcomes
For structural and civil engineers:
- Need to keep up-to-date with changes in design codes, in this case AS3600
- Possible remedial works required to buildings with thin concrete walls designed before AS3600-2018
For building owners and managers:
Awareness that buildings with thin concrete walls built before AS3600-2018 may need to be assessed by a competent structural engineer for their likely performance in extreme conditions
Consider structural assessment of buildings containing thin concrete walls and potential remedial works
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The reporter refers to the Standing Committee on Structural Safety (SCOSS) Alert of November 2018 about the effects of scale on design.
About 4 years ago the reporter was involved with a group of structural engineers who were concerned about the design of thin concrete walls (usually precast), often only 150mm thick and reinforced centrally with a single layer of mesh. These were load-bearing walls being used in multi-storey buildings and sometimes supporting 10 or more storeys.
This concern was as a result of the Steel Reinforcement Institute of Australia (SRIA) writing a Guide to Seismic Design and Detailing of Reinforced Concrete Buildings in Australia and a seminar series around Australia on the topic at that time.
Danger of collapse under extreme loading
Also, the Christchurch earthquake of 2011 showed how poorly reinforced concrete walls performed, and that was another reason why this group were so concerned.
Change to AS3600
The group lobbied the main committee for AS3600 (Concrete Structures) and as a result a sub-committee was formed that substantially changed the design of concrete walls in the AS3600-2018 edition.
The changes have a significant impact on the design of load-bearing concrete walls for high-rise buildings, and although this problem has been known for some time, the changes did not come into the National Construction Code (NCC) until May 2019.
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AS3600-18 was published in June 2018 and designers should have been using the revised standard from that time. Standards will generally lag behind practice and this raises the question that when an issue arises, such as this, how do we ensure that knowledge is disseminated? The SCOSS Alert referenced above is one such means.
A further issue is that designers do not always appreciate that any standard sets out minimum requirements and too often designers are not looking behind the standard to understand fully the issues and background to the design of the particular element, in this case the design of load-bearing walls.
More generally, there are risks when using superseded codes and standards and another example is given in report 624 published in CROSS Newsletter 54 of April 2019.