Last year, a former colleague at SolidWorks built himself an igloo, and my children deemed it ‘totally awesome’ and demanded one. I never managed to finish that particular project, but the thought of building an igloo has never really gone away, and with the cold weather we are (finally) having in New England right now, it’s got me thinking about it again. More specifically as I have two small children, it’s got me thinking of igloo structural safety. The last thing I want is an igloo I build collapsing on the neighborhood kids. So can SolidWorks help me design a safe igloo??
The classic igloo is a tessellated dome created from packed snow blocks, and it stays up because its internal forces are in balance with the external loads. If we keep things simple and ignore any wind loading, the only load is the snow blocks self-weight and the frictional force between the block resisting the blocks wanting to push apart and collapse the dome.
The geometry, with its 4 meter diameter and 20cm wall thickness, is relatively simple to analyze. But because friction plays such a big part in keeping the structure together, this will have to be a non-linear analysis. Capturing the material behavior of snow blocks is a bigger challenge, and a trawl of the Internet suggests that using a either Drucker Prager material model, or if the strain rates are low, a simple brittle failure model. As the igloo loading is static, we can use the simpler material model of a with the yield point at the stress at which dry packed snow will fail.
As I first want to get a safe dome design, I am not considering the entrance to the igloo initially. This means I can use the SolidWorks Simulation 2D simplification tool and run this problem as an axisymmetric analysis. The advantage of 2D simplification is that the problem solves very quickly with no loss of accuracy.
The initial results look promising, but there is a small region of possible failure between the first two blocks of the wall, so we need a design change. We need thicker walls, but if we uniformly increased the wall thickness, there would be greater weight bearing down, giving rise to greater load. So let’s consider a new design with a tapered wall thickness. A nice feature of 2D simplification is that the results can be viewed in 3D which makes it easy to explain the results
The new design has a maximum stress well below the snow block failure stress. Our basic dome structure is ‘safe,’ so let’s consider the entrance. Adding the entrance invalidates the assumptions behind the 2d simplification, so a 3D analysis has to be carried out using symmetry to reduce the computational load. The run time increases, reminding me how useful 2D simplification is for a lot of problems, but eventually we get our solution. Now there are two ‘hot spots’ where the stress looks to be too high, but I am not overly concerned about then because they occur at sharp intersections and they can easily be smoothed out by a judicious addition of some extra snow.
I now have a safe igloo design--all I need is the snow to build it.