This past weekend was all about snow and ice, or rather the removal of those two pesky forms of water. Winter storm Nemo dropped a significant amount of snow here in the Boston area and although I am not a New England native, I know the pain and strain of clearing snow from my drive and roof. But have you ever stopped to wonder how snow can bring down a building?
Every time I am on snow clearing duty it reminds me of when I carried out an analysis of snow on a roof while learning the art of structural analysis many years ago. Snow is roughly 1/10th the density of water, at 6.242 lbs per cubic ft and this weekend I cleared a good 20 inches of snow off my roof, which means that I moved a whooping 5.5 tons! No wonder the pretty white stuff can bring down a building.
Now I’m an engineer and I cannot help but try to understand what a roof is doing under these kinds of loads. With today’s technology like Computer Aided Design (CAD) and Finite Element Analysis (FEA), you can virtually test the design of your roof to ensure it is strong enough to hold tons of snow. Using my CAD tool, SolidWorks, I quickly mocked up a simple wooden roof and snow pack.
One of the nice things about SolidWorks is that it has an integrated structural analysis package allowing me to ask the question “what happens to a roof covered in snow”. It was a snap to apply the snow loads and hold the roof as if it was attached to some supporting walls. I had the answer to my questions faster than it would have taken me to put on my jacket to go outside and find out.
If I build the roof that I designed and it had been in the path of Nemo, the roof panels would have bent by a maximum of 0.45 inch. Which doesn’t sound like a lot but how safe would it have been?
The structural safety or Factor of Safety is measured by ratio of the local calculated load to the maximum load the material can take. We can see that the roof is at least five times as strong as it needs to be. This information allows you to decide whether you need to put more roof trusses in or are you happy with your design.
This type of analysis is not limited to wooden roofs it can also be used on structural steel roofs. The next time you’re walking around the mall, look up. See how all those beams are joined together, some are welded, some are bolted but all have to perform to maintain the buildings structural integrity. Simulation can look at problems at a large scale, the whole building, or at a small scale, how a group of beams are bolted together.
The computer analysis predicts the roof is strong enough to hold almost 65 inches of snow but in the real world, it would probably fail well before that. As soon as the supporting wall started to bow or if a truss joint wasn’t perfectly done, the roof would cave in. That is why we have factors of safety.
So the next time you think of carrying out a little building project, do yourself a favor and make sure you use the tools that can tell you how safe your building is, whether it is to prevent issues with snow, wind or earthquakes. Guessing how a building will withstand Mother Nature has proved to be costly and potentially life threatening.
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