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CROSS Safety Report

Snow loads on agricultural buildings in Scotland

Report ID: 248 Published: 1 July 2011 Region: CROSS-UK

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Overview

The reporter raises concerns that the snow loading situation with regards to agricultural buildings (in Scotland) and perhaps all buildings may require review.

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:

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The reporter here says that the snow loading situation with regards to agricultural buildings (in Scotland) and perhaps all buildings may require review. In some areas of Scotland, in the first week of January 2010, there were unprecedented accumulations of snow on the roofs of buildings in lower and up lying areas.  The snow fell for over a month in windless conditions which meant the BS6399 x 0.8 factor to allow for wind blowing snow off roofs to reduce load did not occur in practice.  

Class II buildings

Agricultural buildings are generally designed Class II to BS5502. The Class II classification gives a reduction in snow loading of x 0.78 and a further reduction of x 0.925 on the resulting characteristic loading. The result of these reductions is generally a snow loading which is at or about the Class II imposed loading of 0.5 kN/m2. In certain up lying area the snow loading will be higher. The classification system allows lower loading on buildings infrequently occupied by people. There were reported accumulations of 1,100 - 1,200mm in places depending on elevation.

Snow densities

These snow depths, says the reporter, would give imposed roof loads of 2.4 kN/m2 and 2.2 kN/m2 based on the BS6399 Appendix snow density of 2 kN/m3.  The snow in some areas was described by many farmers as ‘very heavy’ meaning that the Appendix snow density may be an underestimate of the actual density. Snow does vary in density from about 1 kN/m3 to 8.5 kN/m3 as it transforms from light powder snow to ice.

Given that the load factor at ULS is 1.6 for imposed loads and that the above loads are up to 4 x the design imposed loads it is predictable that some buildings would have collapsed under the snow loading experienced in early January. Class II farm buildings, if steel portal frames, are also allowed a reduction in dead load factor, in BS5502, which we usually ignore.

Buckling of rafters

A collapsed steel portal framed agricultural building examined by the reporter, which had about 1100mm of snow on the roof failed, in all probability, due to tensile failure in the top pair of eaves connection bolts due to excessive snow loading. There was also evidence of lateral torsional buckling instability of rafters between rafter restraints due to overload. 

Instability of the rafter in the apex haunch area suggests that it would be good detailing practice to fit rafter stays at each purlin position immediately either side of the rafter and at the two penultimate purlin positions either side of the apex as these are in the vicinity of plastic hinge positions which are very likely to form should the building be overloaded by snow loading.

Applied snow loadings

To reduce the potential for collapse, suggests the reporter, might buildings be designed Class 1 to BS5502:Part22:2003?  Applied roof snow loadings, as opposed to ground snow loadings, could be as follows according to the reporter:

 0-100m        Above mean sea level            0.75 kN/m2

 100-200m     Above mean sea level            1.00 kN/m2

Above 200m   Above mean sea level           1.50 kN/m         

But in no case less than the BS6399 snow loading.    

This approach means that snow loading will be the critical imposed load in all cases as the BS roof imposed loading is a maximum 0.6 kN/m2.  The above figures are loosely, but conservatively, based on a BRE small buildings snow loading guide for Scotland.
What is not known is how the return period of the recent snow event relates to the 0.02 and 0.05 probability of exceedance for a Class 1 and Class 2 buildings respectively. What is apparent is that farmer’s in general want their buildings designed for real loads, which have been experienced recently, and are not comfortable with the rational for what is portrayed as a 1 in 20 year design life for a Class II building.

What is apparent is that farmer’s in general want their buildings designed for real loads

During the week of the deepest snow accumulations many farmers were space heating their buildings to reduce snow loading which was generally successful in saving buildings. Some stocked cattle buildings continued to be self clearing of snow due to rising heat from stock. It appears to be the case that farmers consider that snow slid better off single skin steel roofing sheets than off the thicker fibre cement sheets which are very common in livestock buildings for animal husbandry reasons.

Expert Panel Comments

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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 buildings

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.

Industry guidance

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 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.

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