It’s unlikely that the exact cause of the Queen’s Park Arenex roof collapse will ever be known.
aDB Engineering was called to Queen’s Park on the evening of Dec. 19, 2016 to assess structural concerns about the Arenex roof and observed cracks in one of the main trusses of the roof structure. Hearing “very audible creaking and cracking” that was attributed to the growing cracks in the roof, the engineering firm deemed the building to be unsafe for entry and recommended it be evacuated.
“Further investigations were postponed until the structure could be stabilized,” said a letter from aDB Engineering to the city. “Approximately one hour later, the roof structure collapsed.”
Following the incident, the city hired the structural engineering firm to determine the possible causes and events that resulted the roof collapse of the 1938 building. In a July 5 letter to the city, David Rajendran of aDB Engineering stated the review couldn’t definitively identify one sole cause of the roof collapse, given the limited information available pertaining to the original roof design.
“Typically a structural failure is caused by a number of compounding factors and seldom can it be attributed to a well-defined singular item,” Rajendran wrote. “It should be noted that a detailed formal structural forensic investigation has not been completed for this collapse. As such, the information contained in this report is based on informal calculations, assumptions and opinions, and therefore should not be used for legal or insurance purposes.”
aDB Engineering determined a number of factors likely acted “in an unfortunate combination” that resulted in the collapse of the roof.
Not surprisingly, snow loads were cited as one of the factors contributing to the roof collapse as the region had seen a series of snow events and “significantly lower than normal” temperatures. The engineers estimated the total snow and ice weight on the roof at the time of the collapse would have been 22.5 pounds per square foot (psf).
According to aDB Engineering, there was no building code or municipal bylaws in existence when the Arenex was built so it didn’t have sufficient information to determine the snow load for the structure, but 1939 city bylaws stated roofs should be designed for 40 psf. The engineers also noted the Arenex would have previously been subject to snow loads greater than what existed at the time of the collapse, so it’s possible earlier snow events could have caused overstressing in the truss that wasn’t visible.
“In a weakened state, the truss may not have the capacity to carry the repeated snow loads from the series of snow storms leading up to the time of the collapse,” said the engineer’s letter.
At the time of the collapse, some city officials questioned whether a new insulated roof added to the structure in 2011 could have contributed to the collapse, as the original uninsulated roof would have allowed heat to escape from the roof and help melt any snow accumulating on top of the building.
“The heat from the building would certainly help melt the snow and thus possibly decrease the overall weight of the snow on the roof, however a detailed thermal study has not been completed so the rate of snow melt (and subsequent reduction in weight of snow) with the original roof construction is not known,” stated the letter. “It should be noted that building codes do not normally permit thermal melting of snow as a means of snow weight mitigation. Although the reduced weight of snow from thermal melting would have helped reduce the snow weight, based on the snow design loads, this means of snow mitigation should not have been necessary.”
The engineer’s report also noted that:
* The weight of the new roof is of a comparable weight to the original roof.
* No signs of water ingress into the building were observed prior to the collapse, so it doesn’t appear that rain water or snow melt were leaking under the roofing material.
* The weight of “hanging elements” from the roof, such as a basketball backboard and ropes used for gymnastics,” would have a “negligible” impact on the roof.
* No signs of building settlement to the Arenex foundation were determined.
* The city had a “comprehensive” maintenance schedule and the building showed no signs of neglect such as wood rot, user damage, etc.
“For the age of the building, we would consider the building to be in good condition and having been adequately maintained,” said the report.
aDB Engineering stated a number of potential factors could have led to the collapse of the roof structure, but a sole cause couldn’t be definitively identified given the limited information available pertaining to the original roof design. It concluded that a number of factors acted “in an unfortunate combination” that resulted in the overstressing in one of the primary roof trusses leading to the failure of the bottom tension member.
“Further testing will provide more information on the original capacity of the trusses, however this is unlikely to provide any further clarity on the overall causes of the collapse.”
