The winter of 2010-2011 saw a record number of structural roof failures in the Northeast of the U.S.. According to a recent article in Structure Magazine, nearly 400 roofs in that region suffered a partial or complete structural collapse. That’s a big number, and engineers and code officials from the area are questioning (with good reason!) the adequacy of the existing building codes relative to the structural design of roofs subjected to snow loading. So Michael O’Rourke, Ph.D., P.E., long time chair of the American Society of Civil Engineers (ASCE) ASCE 7 Rain and Snow Load subcommittee and Professor of Civil Engineering at Rensselaer Polytechnic Institute (R.P.I.) undertook research to answer the question;
Were the roof failures due to structural building elements not as strong as the building code anticipated, or did the current building code underestimate the roof snow loads for the Northeast?
O’Rourke and his team analyzed weather data for the period, and found that ground snow loads “approached but did not exceed” snow loads dictated by the building code. But the ground snow load is just one piece of the puzzle when developing structural snow loads to a roof, so the researchers dug deeper into the weather data to assess whether snow drift loads might be a common factor in the roof failures. Again, the researches found that the roof snow drift loads predicted by the building code were not exceeded.
So what happened?
Professor O’Rourke concluded that the fault lay with the construction of the buildings. Construction defects, design defects, older buildings designed to meet older codes (that may not have included snow drift provisions), improper structural building modifications that weakened buildings, or deferred maintenance leading to damaged structural elements all likely contributed to the structural failures.
I would have liked to seen a mention at least of historic snow loads versus roof failures. Was the snow loading of the winter of 2010-2011, though less than the loads dictated by the building code, much greater than prior years? How many roof failed in past years? In Connecticut, a state wide building code was not adopted until 1971. 61% of the roof failures in Connecticut occurred in buildings constructed prior to 1970. That would suggest that winter of 2010-1011 saw the largest snow loads in the past 40 years. I’ve asked Professor O’Rourke these questions, and will update this post when he responds.
***UPDATE by Michael Curry June 22, 2013*** Professor O’Rourke responded to my email inquiry back in January.
MJC: Was the snow loading of the winter of 2010-2011, though less than the loads dictated by the building code, much greater than prior years?
O’Rourke: Don’t know, but suspect so.
MJC: How many roof failed in past years?
O’Rourke: Don’t know.
MJC: In Connecticut, a state wide building code was not adopted until 1971. 61% of the roof failures in Connecticut occurred in buildings constructed prior to 1970. That would suggest that winter of 2010-1011 saw the largest snow loads in the past 40 years. Is that true?
O’Rourke: Again, do not know – we were interested in the 2010-11 winter and comparisons to ASCE7.
I thank Professor O’Rourke for his response, and apologize for the long delay in posting it here! – Michael Curry