CROSS Safety Report
Combination load cases in proprietary software cause concern
A reporter is concerned about a widely used software package that does not, in their opinion, generate load combinations in accordance with the Eurocodes being followed.
Key Learning Outcomes
For civil and structural design engineers:
- Software users should be sufficiently competent and experienced to recognise incorrect or unexpected situations and outputs
- It is good practice to carry out ‘sense checks’ and validate all analysis and design outputs
- If you are concerned with any outputs, raise this with the software companies technical support team and seek clarification
- When purchasing software, consider how outputs from the packages being considered will be validated
For software developers and suppliers:
- Evidence of validation against a wide range of published test cases is reassuring
- Ensure software updates and errors are notified to all users
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This report concerns a widely used software package that the reporter does not believe generates load combinations in accordance with the Eurocodes being followed.
The reporter’s experience of a proprietary package is that for roofs, the software combines imposed loads with snow loads and wind actions. This, the reporter says, contradicts the relevant Eurocode [clause 3.3.2(1) of EN 1991-1-1] which states that on roofs (particularly for category H roofs) imposed loads need not be applied with either snow loads and/or wind actions.
The reporter is concerned that this may lead to an overdesigned structure.
They are further concerned that for certain load combinations, the software utilises incorrect load factors for leading and accompanying actions.
The reporter believes that the algorithm for the automatic generation of load combinations is incorrect and furthermore, that the interface with the software does not readily allow for manual intervention. This makes it difficult for designers who rely on the software for the selection of load combinations and could lead to incorrect design outputs, such as unnecessarily conservative designs or unsafe designs.
The reporter has presented their concerns to the software supplier concerned.
Expert Panel Comments
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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.
The reporter deserves credit for doing enough validation to establish that the software appears to be combining loads incorrectly, and is right to have highlighted their concerns to the software supplier.
Where there is any concern with software outputs, the issue should be raised with the software technical support team and clarification sought. Raising awareness is the first step in the process of bringing about improvements to industry practices.
Software deficiencies are relatively rare but they do happen. CROSS Report 538 Failure to check designs produced by software, published in 2016, concerned an error in a design package that the software developer later confirmed had not been previously picked up. It is a pre-requisite for using software that the user must be able to recognise incorrect or unexpected situations and outputs. Simply put, software should only be relied upon by those who can anticipate the outputs, otherwise, they will not recognise errors in the software or more likely, errors in the use of the software. ‘Sense checking’ of all outputs, including load combinations, should be carried out as part of output validation.
Any concern with software outputs should be raised with the software technical support team and clarification sought
The reporter raises an important concern about selecting appropriate load cases and factors. The selection of combinations and factors should not neglect any possible circumstances, for example, where wind loading may cause uplifts on roofs, care must be taken when considering the partial factors. Under the Eurocode system, where an imposed load is favourable, as is likely in a wind uplift case, a suitable partial factor (normally zero) should be applied.
Software developer responsibilities
Software developers should validate that their software complies with code requirements such that users can trust the software when using it within clearly defined constraints. Evidence of validation against a wide range of published test cases is reassuring.
When errors in commercially available software are found, suppliers should be challenged to demonstrate both the validation and the calibration of their software. Where an error in marketed software is confirmed, it would seem reasonable to expect a software house to issue revised software to all licence holders. In addition, all previous users of the software could be notified of the error so that the implications upon earlier work can be assessed.
The reporter also makes a valid point regarding the ease of checking software outputs. When selecting software, designers should think through how the outputs presented by different packages will be validated. An offering with numerous intermediate outputs, and more transparent processes, may well enable effective validation to be more easily applied.
The designer should never forget however, that the responsibility and liability for all outputs rests with the designer and not the software supplier.
The designer should never forget however, that the responsibility and liability for all outputs rests with the designer and not the software supplier
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