CROSS Safety Report
Incorrect use of software for wind loads on solar panels
The adoption of software for structural analysis is not a substitute for the designer having an understanding of structural behaviour.
Key Learning Outcomes
For civil and structural engineers:
- Always check the limitations of any software used for analysis and/or design, and that any assumptions in the software and/or input data are appropriate for its intended use
- Do not use software where there is any doubt about its limitations/assumptions or whether it is fit-for-purpose for the intended analysis/design task
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The reporter's concern is the tendency for designers to rely completely on the output of software for structural analysis when they do not fully understand the requirements for data input.
The reporter is aware of a commercially available item of software, for the determination of wind actions on buildings, which is used by structural engineers and non-structural engineers working in the design and installation of solar panels at roof-top level.
The software is relatively straightforward to use. The user enters the building parameters, orientation, and location which the software uses to determine the terrain category, shielding and topography. It then determines the site wind speeds for the eight compass points. The software calculates the design wind actions on the walls and roof for each face of the building in accordance with AS1170.2, as well as quantifying the wind actions on the RC1, RA1, and RA2 local roof zones.
The reporter is concerned that default values within the software for wind direction factor Md in Regions B2, C, and D (0.9), and values for the action combination factors Kc,e and Kc,i (0.9) assume the software is being used for overall building design and not for cladding, its immediate supports, or roof-top solar panels. Thus, there is a risk that fixings for mounting solar panels may be overloaded and, in such a case, the solar panels may become detached from the roof.
In the reporter's opinion it is assumed, even amongst engineers, that software in general is set up to provide 'the answer' automatically. Significant trust is placed in the software output. However, it is incumbent upon the user to have sufficient knowledge to understand the input values appropriate for each relevant variable and the use of software is not a suitable substitute for an understanding of structural behaviour.
Expert Panel Comments
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The use of computer software for the analysis of structures and assessment of structural design loads has been commonplace for decades. As with the use of all tools, appropriate levels of supervision and checking, including self-checking, are essential. In the case of engineering analysis, it is good practice to validate design software independently before it is adopted for wholesale use.
In this particular case, the reporter has raised a situation where the software has been adopted for the assessment of loads for which it was not meant to evaluate. This demonstrates a significant, and worrying, lack of understanding by the designers of the overall requirements of the design codes, and the software inputs and/or limitations. It is an example of the uninformed use of software, undertaken without checks to ensure the software is fit-for-purpose for its intended use.
With respect to the assessment of design wind loads on solar panels, it is noted that the latest version of AS1170.2 (2021) includes the latest research with respect to wind effects on solar panel arrays. This has effectively been introduced into Australian law since the introduction of the National Construction Code (NCC) 2022 in September 2022.
Circumstances in which the misuse of engineering software could lead to unsafe structures have been noted in several previous CROSS reports. Typing "software" in the search function on the Safety Information page on the CROSS-AUS website produces 44 such reports, with similar comments, such as:
- Users without adequate structural engineering knowledge or training may carry out structural analysis
- There may be communication gaps between the design initiator, the computer program developer, and the user
- A program may be used out of context
- The checking process may not be sufficiently rigorous
- The limitations of the program may not be sufficiently apparent to the user
- For unusual structures, even experienced engineers may not have the ability to spot weaknesses in programs for analysis and detailing.
This report highlights the need for designers to have an appropriate understanding of structural behaviour and requirements, of the applicability and limitations of software packages used in design, and the need for them to check software is appropriate for its intended use.
In one sense, computer software can be compared with a modern motor car. Both can be complex but require a good understanding of principles and rules before being used safely. No sensible person would try to drive a car without knowing what the pedals and switches do, how fast it might travel, how quickly it might stop, or what the flashing lights mean, especially the blue ones on top of the car behind!
In short, if there is doubt about any aspect of using software where structural safety is involved then another engineer or specialist should be consulted.
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