CROSS Safety Report
Snow loading in Scotland - ice load
This report is over 2 years old
A reporter’s practice who specialises in remediation have had new instructions on five roof collapses after a period of heavy snowfalls (January 2010).
Key Learning Outcomes
For civil and structural design engineers:
The fact that there are so many records of increased snow loading is a reminder that all loadings are statistical projections and there is always a chance that they will be exceeded
Careful consideration should be given to the risk of snow loads especially for areas prone to heavier snow falls
Consider what reasonably foreseeable loads could be applied beyond the code minimum values
For all built environment professionals:
- Designers and constructors of new buildings, particularly agricultural buildings, in areas potentially subject to heavy snow falls, should refer to:
- The SCOSS alert - Snow loads on agricultural and other building structures
- The Scottish government publication - Effect of severe weather on farming community
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
A reporter’s practice specialises in the investigation and remediation of civil and structural engineering failures. Over the past month or so (January 2010), they have had new instructions on five roof collapses. Three of these are agricultural buildings and of little consequence in terms of the point they want to raise. One of the other failures is to modern well built structure – a substantial portal frame shed in North East Scotland, completed in 2009.
The other is a very large domestic building in the south of Scotland with a flat roof carried on glulam beams (report 185). In both cases, the reporter observed a build-up of ice on the roofs ranging in depth from 75 to 150mm overlain by around 400mm of snow. Arguably, and very likely, the loadings were even higher when the failure occurred. The point is, therefore, that in each case, the ‘snow’ loadings were of the order of, according to the reporter, at least twice that which the current standards would indicate.
Notwithstanding that this winter has been quite exceptionally bad, the reporter thinks that there is an argument for increasing anticipated snow/ice loadings. Similar issues are raised in report 264 and also reports 183, 193, 195, & 248. You can search for safety information on snow loads and failures on the CROSS website.
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Expert Panel Comments
Expert Panels 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-UK Expert Panels page.
These are general comments on all of the above reports
The fact that there are so many records of increased snow loading is a reminder that all loadings are statistical projections and there is always a chance that they will be exceeded. Designers should be generally alert that safety is not prejudiced by any single assumption (in strength or loading). They should avoid situations where, should the assumption be in error, the result will be a significant safety hazard.
This might be the case if structures are very light weight and the only dominant loading is snow. Where there are significant amounts of dead and live loading, it is unlikely that both will be in error at the same time. Safety should not be sensitive to any one assumption. The example of the farmer burning straw to melt snow reflects the practice at some sporting events where there are marquees of having space heaters available to prevent snow from lying on the roofs.
Agricultural type buildings may be designed with lower safety levels for economy and because safety to humans is not likely to be prejudiced. Designers (and their clients) should also consider carefully the commercial consequences of failure. It might be worth accepting a higher risk of failure if the structure is just protecting stored fodder, but if the protected contents are valuable livestock, it might be thought prudent to look for a more robust structure. The marginal cost increase of a safer more robust design may well be a sensible investment. It is not known however what design loads were used in any of the reported cases.
Designers (and their clients) should also consider carefully the commercial consequences of failure.
Adapting to changes
The industry should also beware of changing practice and inadvertently rendering inherent assumptions invalid. Thus, the demand in many structures for increased roof insulation renders it less likely that snow will quickly melt or slide. A number of portal frames in these reports failed under direct overload which might be tolerable in terms of safety if the form of failure were a standard plastic collapse mechanism (i.e. excessive displacement short of unstable failure), but from the pictures this appears not to be so.
An investigation of forms of failure might show less risk of catastrophic damage simply by boosting connection capacity or paying greater attention to avoiding failure by instability: benefits that might be achieved at minimum cost. These examples and other evidence has been collated by the Scottish Government and used to issue guidance to the agricultural community and designers, and a report has been sent to BSI so that the data, along with other information, can be considered with reference to Eurocodes and their UK National Annexes.
Recent SCOSS alerts
Alerts were also issued by the Standing Committee on Structural Safety (SCOSS) in 2010 and 2011. The situation emphasises that design is a risk management process; it needs thought, and some argument perhaps, to decide if the relaxations currently permitted for agricultural buildings are as acceptable as once thought, when weighed against the cost of failure – albeit mostly economic in these cases.
For other buildings, it may also be that designing for increased snow loads and making additional effort to avoid build-up/drifting opportunity may be a worthwhile investment when set against business disruption should they present a high risk of failure.
However, many of the collapses were probably of buildings which were designed to superseded Codes and it is possible that some buildings may not have been built as designed and that there were cases of construction defects and of maintenance issues
Designers and constructors of new buildings, particularly agricultural buildings, in areas potentially subject to heavy snow falls, should refer to:
The SCOSS alert - Snow loads on agricultural and other building structures
The Scottish government publication - Effect of severe weather on farming community
The winters of 2009/2010 and 2010/2011 were very severe in other countries and in the USA, for example, there were numerous reports of failure due to snow loads and it will be interesting to find out whether these are being regarded as abnormal. Of course, snow falls may be related to climate change, but the science, and work to support it, is not sufficiently advanced to provide an answer.