Written by The popular winter-time ice arena in Bellevue averted a near tragedy yesterday, when employees and firefighters quickly rescued 10 people trapped under the failed roof tent. The snow load exceeded the capacity of the roof structure, and the whole thing failed in the blink of an eye. While a fabric tent collapsing may not sound super dangerous, it’s easy to underestimate the tonnage of snow involved, the hazard caused by the metal support stuctures, and the electrical hazzard resulting from the lights and sound system plunging to the ice.
Fortuanatly, there were only minor injuries. I’m sure there we be an analysis to determine whether the tent supplier included proper instructions for snow clearing or not, and whether the users paid attention to the growing hazard. It seems obvious, at least in hindsight, that there should be some engineering which gives the user an idea of when to clean the tent roof or clear out if the tent was really designed for winter use.
I’m really glad there were not any serious injuries.
This is a great reminder about the hazards caused by overburdened roofs. Back in engineering school, we were given a great deal of education about how roof loads can quickly accelerate in bad weather due to accumulations of snow and ice, frozen drainage systems, and general “ponding” of water. This “ponding” phenomena remains one of the most common and most dangerous roof failure modes. It occurs on flat or nearly flat roofs, when obstructions like leaves or ice reduce the normal drain paths for water. The weight of the accumulated water on the roof will then “deflect” the structure, causing support timbers to bow under the weight, and creating more opportunity for ponding. The more the pond grows, the more the boards deflect, and the more water the roof will contain. Many people have seen this mechanism where they have tried to string a tarp up over a picnic table while camping, only to see the whole thing collect water. “Ponding” represents an engineering instability, in that the more it occurs, the more likely it is to get worse. The only protection is the keep the drains open, inspect the roofs frequently, and be extremely aware of how things are holding up when there is any kind of load on the roof. So, if we get more snow, and when the inevitable rain comes, be very aware of the conditions of your low-pitch roofs. Even if conditions are too dangerous for you to clear your roof, you can still make sure loved ones are not sleeping or spending large amounts of time under anything suspect.
If you are concerned that your roof is not holding up, and you need advice, the city building department can give you some recommendations.
Here is the story about the Bellevue Ice Rink roof tent collapse
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