CROSS Safety Report
The misuse of standard details and notes on structural drawings
This report draws attention to the excessive and incorrect use of standard details and notes on structural drawings, as well as the assumption that the builder/contractor will somehow work out the designer's intention of these on site. Inadequate or conflicting design and documentation can potentially lead to failures.
Key Learning Outcomes
For structural and civil design engineers:
- Design in accordance with up to date versions of the correct Standards, and apply the same to the nomination of Standards in specifications
- Provide contract specific details and applicable selected standard details, sufficient to ensure there is adequate information to safely build the structure
- Delete all irrelevant and conflicting details, which create information overload and confusion
- Apply the same approach to drawing notes and to specifications
- Identify limitations, if any, on circumstances for the application of standard details
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The reporter has become concerned about the excessive and incorrect use of standard details and notes on structural drawings, as well as the assumption that the builder/contractor will somehow interpret the intention of these on site. Inadequate or conflicting design and documentation can potentially lead to failures.
This report follows the review of the structural drawings for a housing project, where the reporter encountered several issues. In this particular project, the notes referred to items that were not part of the project such as fabricated timber trusses, Y bar (out of date reinforcement rolled from about 1984 to 2001 in Australia), and covers to reinforcement in concrete footings not in accordance with AS3600. On checking the overall height of the project, the reviewer also noted the building was greater than 8.5 metres in height. This required it to be designed in accordance with the Australian Standard AS1170.4, an issue which may have been overlooked by the structural engineer. It appeared to the reporter that the structural drawings had not been checked or coordinated.
On another recent project, the reporter was advised that there were about ten sheets of standard details which were supposed to cover most of the sections for the project. However, there were no specific details and sections that related to the project itself. In addition, the reporter has often seen specifications referring to Standards that are out of date and, in some cases, incorrect Standards are specified.
Standard details and notes on structural drawings evolved many years ago and were intended to cover the general detailing that would occur on sites, such as lapping of reinforcement, arrangement of reinforcing bars and the like. They also cover standard details such as bolted connections for steelwork. The reporter has found that it is now not unusual to have two or more drawings of standard notes on any large project. However, having standard notes for materials that are not used on a project introduces unnecessary complications.
too much reliance on 'copy and paste' and not enough emphasis on 'real' engineering design
Standard details save on production time and labour costs, and have their place on projects, but must be used with caution, especially when they are not relevant or specific to the project. It is the reporter's opinion that there is too much reliance on 'copy and paste' and not enough emphasis on 'real' engineering design and drawing capability, to the detriment of the profession. The reporter's view is that structural engineers should provide sufficient information for the builder/contractor to understand how the structure is to be built.
The reporter believes that it is due to cost restraints and a lack of understanding that leads to structural engineers not drawing details and sections to show how the structure fits together. In the case of the drawings for the housing project reviewed by the reporter, it was fortunate that the building certifier had required the structural engineer to draw a section through the edge of the building. This identified that underpinning was required to the footing to the building on the adjacent property.
Expert Panel Comments
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The reporter has raised the concern that there is a tendency for standard details to be overused in structural documentation. It has been noted that, for small projects, the total number of standard notes and details can sometimes outnumber the project specific plans and details sheets. Whilst it is incumbent on the engineer to ensure drawings convey all the important information required to construct the building in a safe manner, the use of blanket standard details conveying superfluous and irrelevant information can be confusing, and will often result in a reduced focus on the critical information required for construction.
superfluous information ... will often result in a reduced focus on the critical information required for construction
Standard details that are not relevant, out of date and/or conflicting should be removed from documentation, and notes should be concise and relevant. Care should be taken to update standard notes for project specific locations (e.g. corrosion requirements, wind speeds, seismic accelerations, geotechnical conditions and the like), and to keep track with revisions to official construction codes and practices.
Where appropriate use of standard details is made, they should clearly specify to the designer and contractor the circumstances in which they are appropriate if there are limitations on their adoption. Examples, where this practice has not been followed, include:
- A typical construction joint detail in a post-tensioned (PT) slab appropriate for use at the slab quarter-span point where moments were close to zero. The detail was used away from the quarter-span point resulting in significant strength shortfalls in PT slabs which then required remedial action
- A standard multi-floor propping and back-propping diagram that assumed all floors were supported at columns. In this case, a construction joint at 3/4 span meant a different distribution of loads from that envisaged in the standard detail, contributing to the collapse of four levels of back-propping
- A contractor following a general note requiring a '6mm fillet weld all round unless otherwise noted' instead of applying the full penetration welds and commensurate NDT quality control specific to a particular connection detail
Third party audits and checking processes should include an assessment of the relevance and adequacy of standard details.
It is worth noting, with respect to the economic driver behind the overuse of standard details, that low fees are not a defensible reason for inadequate detailing, and fees should be structured accordingly.
As highlighted by the reporter, the above comments on details apply equally to the nomination of Australian and New Zealand Standards, with experience indicating that out of date, and at times even inappropriate, Standards are sometimes specified. This of course extends even further, to the need to ensure that the correct and up to date Standards are used in the design as well.
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