This past weekend was all about snow and ice, or rather the removal of those two pesky forms of water. The last two winter storms have dropped a significant amount of snow here in the Boston area. Now, I am new to the area, hailing from England, and to be honest I didn’t pay much attention to it until the radio started reporting roof collapses and my neighbors got up on their roofs and started shoveling snow. So I did the same–I shoveled snow and removed ice. Although this was my first time doing this I had a sense of déjà vu. When had I done this before?
While taking a well earned rest (and having a nice cup of tea) it hit me. Ten years ago, during my first simulation class using COSMOS (which became SolidWorks Simulation), I had done an analysis of snow on a corrugated iron roof! With the roof cleared it was time to dig out my old notes. Being an engineer pack rat nothing is thrown away, and I was curious to see how much weight I had shifted off my roof.
According to my notes, snow is roughly 1/10th the density of water, at 100 kg per cubic meter (or 6.242 lbs per cubic ft). I cleared a good third of meter of snow off my roof, and with an area of about 150 square meters, that was a whooping 5,000 kg ( 5 .5 tons)!!!
No wonder the pretty white stuff can bring down a building.
Now I’m not an architect, but I wanted to see if SolidWorks could help me understand what’s going on. With SolidWorks CAD I can quickly rough up a rough roof truss–about 400 sq meters.
After jumping across to SolidWorks Simulation, I can hold down the the roof where it would attach to the frame, apply gravity and my snow load, and see what happens.
Looking at the stress result I can see that I have a safety factor of 4, which is a ratio of the maximum stress in the roof to the materials strength. But to me that seems a bit low. To improve the safety factor, I’ll add more roof trusses.
Then I recalculate my answer.
My factor of safety is now 6, which makes me feel better, but looking at the roof deflections, I see that some of the trusses are deflecting over 100mm (4 inches). That doesn’t exactly inspire confidence in a roof.
This looks better.
So one last change–two trusses, a couple of longitudinal braces and a change of wood.
Job done. Now, to clear my drive…