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CROSS Safety Report

Poor quality of structural design on high-rise buildings

Report ID: 823 Published: 1 July 2019 Region: CROSS-AUS

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Overview

The correspondent has been very concerned about the quality of structural engineering on some projects in recent years, particularly for certain high-rise buildings.

Over-reliance on standards and codes may produce potentially inadequate structural members; designs must be undertaken by competent people with an understanding of how the construction is physically to be made and the issues that need to be addressed in construction.

Key Learning Outcomes

For structural and civil engineers:

  • Standards and codes give information about minimum requirements for adequate construction

  • Designs of structural elements (e.g., precast floors) should only be made by competent structural engineers

  • Some codes and standards (e.g., AS3600) may give potentially misleading information; designs must be checked using basic structural engineering principles to avoid dangerous situations

  • Lateral loading (including seismic loads) needs careful consideration

  • Checking and coordination of drawings can be highly beneficial in producing successful construction work

For policy makers:

  • Ensure that standards and codes do not produce designs which may be potentially unsafe

For clients:

  • Allocate sufficient time and fees for checking of design, and accurately recording as-built construction

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The correspondent has been very concerned about the quality of structural engineering on some projects in recent years, particularly for certain high-rise buildings. According to the correspondent, there are engineers who regard the requirements of the Concrete Structures Standard (AS3600) to be generally conservative and are the maximum requirement, rather than the minimum requirement.

The correspondent has experienced the following issues:

Post-tensioned floors and thin concrete walls

  • When the design of post-tensioned floors is given to a specialist sub-contractor, the design may not be coordinated nor indeed checked by the structural engineer. Hopefully the changes in AS3600-2018 in the design of diaphragms and the tying together of elements will overcome this.

  • When the design of precast concrete is given to the precast manufacturer, they may have little understanding of structural design unless they employ structural engineers themselves. Again, there can be a lack of coordination between the in-situ concrete design and the precast concrete design.

  • It is possible that clause 11.5 of AS3600-2009 may be misinterpreted using the simplified method to design concrete walls that may be supporting 20 to 30 storeys. This may result in a 150mm thick wall with a single layer of mesh in the middle. However, in the view of the reporter, the clause was never intended to be used in this way when it was originally included in AS3600-1988. This problem should be resolved by the revisions to AS3600-2018 which have introduced stricter requirements for the design of load-bearing concrete walls.

Need for better understanding of lateral loads

  • Lateral loads and in particular seismic loads are sometimes not well considered and in some cases are ignored. Again, changes in AS3600-2018 together with a minimum earthquake hazard factor (Z) of 0.08 should bring this to the fore.

  • Many young engineers lack site experience and although they may have good structural analysis skills, the practicalities of construction and buildability issues may not be considered.

Lack of checking and coordination of drawings

  • Checking and coordination of drawings is often lacking. While there is no such thing as a perfect set of design documents, a good set of documents goes a long way to making sure that everything works. When changes occur during construction, structural engineers are reluctant to amend the drawings and mark-ups are done on shop drawings instead.

  • Private certification may, in some cases, have contributed to the problem. There should be independent building surveyors and certifiers who are appointed by somebody other than the parties directly involved in the project.

There is much focus currently on the need for everybody to be registered, but that's not the answer in this correspondent’s opinion. What is needed are quality people and quality time to do the job properly.

Expert Panel Comments

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This report raises several issues of concern, not all of which are new, and some that have been raised in other reports such as design of RC walls (AUS-3) and managing changes during construction (AUS-7). It also highlights several of the issues raised in the Shergold & Weir and Opal Tower reports around competency, co-ordination of drawings, the correct application of Australian Standards, site inspections, quality control, private certification and registration.

Attitude to Australian Standards and design software

The issue of attitude to the Australian Standards raised in the report, and also in the accompanying AUS-3 report, is a serious one which has arisen from the changing nature of Standards and Codes over the last 50-60 years. The present Australian Standards have developed from Codes of Practice written to represent conservative good practice. Increasingly they seem to have become regarded as being accurate reproductions of theory that can be used without a detailed understanding of their background or the limitations of their use. When incorporated into design software their use becomes even more remote from their theoretical backing, and the outcomes are certified as meeting the code because the software is apparently based on the code. Ensuring that the relationship between theory, codes, and practice is properly understood is the responsibility of the profession.

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