Snow Related Roof Collapses
The January 2013 issue of STRUCTURE magazine contains an interesting article, Snow Related Roof Collapse and Implications for Building Codes, regarding snow accumulations and roof collapses in the New England area for “the storm of the century” during the winter of 2010-2011. The article compared ground snow loads recorded at weather stations and roof snow loads recorded at various buildings throughout the New England area with the design snow loads calculated using the ASCE 7-10 (Minimum Design Loads for Buildings and Other Structures by the American Society of Civil Engineers).
The article states that the recorded ground snow loads varied from 45% to 92% of the calculated design snow loads of ASCE 7-10. The recorded roof snow loads were also less or only slightly greater than the calculated design snow loads of ASCE 7-10. Since the roofs were not overloaded by the weight of ice and snow, the writers concluded that other factors were responsible for the poor performance. They found that older roof structures (prior to 1970 in Connecticut) were not designed for the additional weight of drifting snow. They observed that some structures, such as agricultural buildings, are exempt from compliance with building code snow load requirements. They also noted that significant snow loading reveals hidden structural defects, such as: design deficiencies, construction defects, improper maintenance, the unanticipated subsequent addition of dead loads such as solar panels, improper construction modification, construction of a taller structure nearby without consideration for strengthening the existing lower structure for drifting, and material deterioration.
We have had similar observations in our experience with the evaluation of collapse under snow accumulations. However, we also note that we have evaluated many structures for partial or complete collapse under the weight of ice and snow, including those from the snow storms of November 1996 and December 2004 in the Cleveland, Ohio area, that had failed under the excessive weight of ice and snow. The review of weather records, an understanding of the principles of snow drifting, the effects of sheltering and exposure, the ability to analyze a roof structure, and an ability to identify possible shortcomings and defects that may have led to a collapse are valuable tools in the evaluation of a collapse under the weight of ice and snow.
From this article in STRUCTURE magazine I can again conclude, “If everyone complied with the requirements of the building codes, we would have a lot fewer failures to evaluate.”