It was a dark and stormy night. The Greenhouse stood as a sentinel, protecting its precious contents against the glowering snows outside. A lone attendant made his way through the greenhouse, adjusting the heat and doing whatever else needed to be done. He looked up to a scary sight — the roof over the far end of the greenhouse was rapidly falling. He dashed for the exit, which he reached just before the huge structure came crashing to the ground. All that remained was a sorry pile of twisted and broken aluminum and shattered glass, along with many, many roses that would never grace a florist's shop or be a Valentine's day gift to a sweetheart.
The Scene of the Crime
There were actually five greenhouses, side-by-side, with individual gabled roofs, but sharing common walls. The entire structure was flattened. The property loss was probably in the millions of 2007 dollars. The insurer of the greenhouse was understandably upset by the payout and sought to find someone else to pay for the mess.
The Investigation
The insurer provided me with a box of wreckage from the greenhouse along with the usual request, “please tell us what caused this mess.” The framing was wrought (probably extruded) aluminum angle pieces that were bolted to aluminum alloy castings.
My study included chemical analysis of the castings and angles and metallographic study of the castings. The figure (magnified to about 100 diameters) shows a polished section of a casting. The big crack is a fracture that occurred during the collapse. The black spots are from porosity. (The online version of this story, http://rbi.ims.ca/5389-610, also shows a fractured casting with a piece of twisted angle).
Hydrogen is the bane of aluminum casting. Molten aluminum reacts with water in the air to form aluminum oxide while the hydrogen is dissolved in the melt. Hydrogen is almost insoluble in solid aluminum and is rejected into the liquid during solidification. The result is bubbles, as shown in the figure above. The strength of the casting is severely compromised. Chemical analysis showed the castings to have about 100 times the allowable amount of hydrogen.
There are several accepted ways to remove hydrogen from molten aluminum. The foundry had clearly not availed itself of any.
The Smoking Gun
The foundry screwed up! Not necessarily. Foundry and customer must work hand-in-hand to ensure a suitable product at an acceptable price. If the castings were ornamental, as for porch light brackets, the added cost of hydrogen removal might not have been justified. I am not sure just how much communication there was. In addition, the greenhouse manufacturer had never done business in the north. Their other buildings had to withstand tornados and other southern threats, but not snow loads. The engineer likes to count on compensating errors. In this case, weak gassy castings and an unexpected snow load reinforced to give collapse. This case is similar to the “Collapsing Coliseum,” where a combination of snow load, iffy welds and iffier design led to roof collapse. There was a huge financial loss, but almost by miracle no loss of life or limb.
In the words of the prison captain in the movie Cool Hand Luke, “What we have here is a failure to communicate.” It was a very expensive failure.
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 Fractured gutter support casting |
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