Recent Experience on Building Collapses under Snow Loads

Stewart Macartney – blyth+blyth

Background• A significant number of buildings collapsed in

January 2010 and late 2010/early 2011• Majority were agricultural buildings although some

unheated industrial buildings also failed• Share with you some of the issues identified in

building failures we have looked into• Form of structures varied – portal frames, trussed

roofs, and concrete buildings.• I would stress that these are my views and not

necessarily those of SER

Some Questions to Consider

• What was the weight of snow and ice on the roofs at the time they collapsed

• Were the buildings designed and constructed properly for the site, altitude and location

• Were the materials used correctly specified• Were there workmanship issues during

construction• Were there maintenance issues• Was this an exceptional event??

What is the Density of Snow?

• Much research on snow density and how it changes with age has been done in the study of avalanches.

• Typically new snow has a density of between 70 – 120 kg/m3.

• Wind driven snow can reach a density of between 300 – 400 kg/m3.

• Old snow, which has gone through a number of freeze/thaw cycles can reach densities as high as 600kg/m3.

What is the Density of Snow?• BS 6399:Pt 3 Appendix B gives an assumed

loading of 2 kN/m3 in drift conditions• EC1 is better and Annex E which is permitted to

be used by the National annexe gives a range of values depending on the type of snow:

Measured Density

• • Measured densities on site ranged from 110 kg/m3 for fresh snow to 550kg/m3 for old crystalline snow

• Density varied across the site both with location and height

Agricultural Building Classifications to BS5502

BS5502 guidance on snow loads

Comparison of Design Snow LoadsDesign Code

Calculated Snow Loading based on a site altitude of 180m above

sea level

Load Factor, ϒf (Factor of Safety)

that would apply for design of structure

Design Loading Comments

BS6399: Part 3: 1988 0.75 kN/m2 1.6 1.2 kN/m2

BS5502 – Class 1 Building 0.75 kN/m2 1.6 1.2 kN/m2 Min 50 yr design lifeBS5502 – Class 2 Building 0.58 kN/m2 1.6 0.94 kN/m2 Min 20 yr design lifeBS5502 – Class 3 Building 0.46 kN/m2 1.6 0.73 kN/m2 Min 10 yr design life

BS5502 – Class 4 Building 0.16 kN/m2 1.60.26 kN/m2 Min 2 yr design life

BS EN1991-1-3:2003, Eurocode 1 — Actions on structures — Part 1-3: General actions — Snow loads and NA to BS EN1991-1-3

0.6 kN/m2 1.5 0.9 kN/m2

1.2 kN/m2 1.0 1.2 kN/m2

Exceptional Snow Loading, where the Cesi factor is taken as 2 (Cl 4.3) and is treated as an accidental load caseexceptional snow load is the snow layer on the ground resulting from a snow fall which has anexceptionally infrequent likelihood of occurring.

Design Loads v Actual Snow

• Estimated Snow Load on Structures at Time of Collapse was1.51 kN/m2 based on snow structure inspected

• Met Office snow records give depths at various weather stations. However it is my opinion that loadings calculated from these would not necessarily recognise any ice build up or snow aging

BS v EC

• A comparison of the snow maps between BS6399 and EC1 NA shows that for some areas of Scotland, the loads in Zone 4 may be underestimated compared to the contour map given previously

Other Findings

• One of the buildings we looked at was a “second hand” structure that had been rebuilt on a new site. The new location was higher than the altitude assumed in the original design.

Other Findings

• One of the buildings had been extended by adding another portal frame creating a valley drift condition.

Conclusions• When considering the reduced design loadings

permitted in BS5502 we can see why agricultural buildings were more prone to failure

• The climatic conditions which resulted in repeated snow falls and freeze-thaw cycle led to gradual incremental increase in loading to failure

• The density of the snow accumulations are likely to have been higher than the codes state

Conclusions

• I believe that some of the snow falls were exceptional events - i.e. Greater than the 1:50 yr probability given in the codes

• Low temperatures may also have impacted on material performance – i.e. Temps as low as -22oC were recorded.

Some Questions?

• Are two consecutive winters of near identical climatic conditions two exceptional events?!

• Given that we have had two consecutive winters where considerable loss of property was experienced - do we need to look at revising the design codes?

• Are we at risk of under estimating snow loads using the zoning given on snow map in EC1?

A Parting Thought

• With the recent changes in Building Regulations in relation to insulation; which will lead to better insulated buildings - Do we need to review snow loading guidelines?

• This thought is prompted by the collapses of buildings that were unheated ....... A condition we are effectively creating by limiting heat losses through the roofs