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Mall Study: Extreme Corrosion Unnoticed

Monday, March 25, 2013

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The only surprise in last summer's fatal cave-in at an Ontario shopping mall is that it didn't happen a lot sooner.

That is the sobering conclusion of a 700-page forensic study by the global engineering firm NORR, which found failures and shortcomings at every stage of the complex: from design, to materials selection, to construction practices, to maintenance during more than 30 decades of ruinous leaks.

From its first day to its last, water intrusion at the Algo Centre Mall was so pervasive and severe that corrosion of the steel support structures was akin to that seen in a marine environment, the engineers' report found. Indeed, corrosion of steel beams was noted in some inspections even before the complex opened in 1980.

“It is, in fact, somewhat surprising that failure did not happen earlier,” the engineers concluded in the report conducted for the Ontario Provincial Police.

Algo Collapse
NORR report / Ontario Provincial Police

The Algo Centre Mall collapse in Ontario last year killed two women and injured two dozen people.   A new engineering report has become a key piece of evidence in the investigation into the accident.

The parking deck situated on top of the shopping mall caved in June 23, 2012, killing Lucie Aylwin, 37, and Doloris Perizzoli, 74, and injuring two dozen people.

A 142-page excerpt of the report was released Tuesday (March 19) as part of the Elliot Lake Inquiry, a public hearing into the collapse.

The report has become a key piece of evidence in the probe, which began March 4.

Advanced Corrosion 'Unnoticed'

The engineering firm reported that corrosion levels observed after the collapse were at levels typical of marine environments.

Most of the shopping mall’s beams’ top flanges, bolts, welds and angles had severe to very severe corrosion, the report found. Such advanced degradation would normally trigger rehabilitation or replacement if the extent affected the overall performance of the member.

“It is not entirely clear how this situation went unnoticed for such an extended period of time,” the report said.

Structural Inspections Faulted

NORR said numerous inspections, including some by structural engineers, had generally underestimated the severity of the corrosion. Some of the most advanced corrosion examples noted in the report were found “in plain sight," the authors said.

NORR report

A number of locations in the steel frame showed signs of very severe corrosion. Here, severe corrosion is seen in the angle and weld of the connection at gridline intersection G-12 and the beam connected to the West flange of the column.

For example, steel on a pedestrian walkway had corroded to the point where some plates were completely depleted, NORR said.

"One is hard pressed to find a similar example where carbon-steel framed building in North America or Europe continued to corrode to the point of failure, when no extreme loading is present,” NORR said.

“Occupied steel buildings are assumed to be kept in dry conditions which are not conductive to corrosion,” according to the engineering firm. Thus, inspecting buildings for corrosion is not typically called for, it said.

On the other hand, the leakage reported at the building would have raised flags for the owner and those engineers inspecting the structure over the years. But none of the engineers’ reports raised an alarm regarding the corroding steel frame or suggested monitoring the situation, the report said.

Waterproofing, Core Slabs

Water leaked at the mall from the day it opened until the day it collapsed, including the period right after a limited waterproofing system was installed.

“Its ramifications potentially could have been avoided if a proper waterproofing system with a continuous membrane was installed at the outset,” NORR said.

Elliot Lake Commission

The report indicated that the chronic leaks could have been solved after construction if not for “a deficiency in the capacity of the hollow core slabs.”

Previous testimony in the Elliot Lake Inquiry detailed the original waterproofing system supplied by Harry S. Peterson Co. The report concluded that the system selected and used did not work—and neither did the years of attempted repairs and remedies.

The report also indicated that the leakage problem could have been solved after construction had it not been for “a deficiency in the capacity of the hollow core slabs.”

The report alleges that Coreslab, the pre-cast concrete manufacturer based in Burlington, Ontario, and structural engineer John Kadlec misled the mall’s owner about the capabilities of the system in an “aggressive effort to win a competitive tender.”

An eight-inch slab to take 120 psf superimposed load was specified by the design.

“According to Coreslab’s tables this slab is not achievable for the 30’ spans of the mall even with the highest level of prestressing,” the report said. “Coreslab could have advised the owner accordingly, as did their competitor Precon, and risk losing the tender but they did not.”

Failed Weld

The combination of road salt and water had a lethal effect on the steel structure, according to the report. 

With corrosion levels so severe, it took just one car driving over an unstable welded connection between a support column and beam to cause the cave-in. The collapse was short, lasting for one second, the engineers’ report said.

The welded connection had been so heavily depleted by years of corrosion that a passing SUV was the “last straw the connections could take,” according to the report. The weld was found to have lost more than 85 percent of its as-built nominal capacity by the time of failure.

video stills
NORR report

Surveillance footage of the connection failure shows the moment of the roof deck's collapse. The SUV cleared the failed section just as it gave way.

When the connection was lost, the beam couldn’t support the roof parking deck, and the concrete panels fell onto the Upper Mall Level and caused a similar collapse, the report noted.

The report also found that concrete slab movement had been reported at the location of the collapse in 2009.

Building Envelope: 'Poor Practice'

NORR also criticized the design, construction and materials used in the building envelope, saying that the parties had not exercised their required duty of care.

“The choice of placing a parking deck on top of a public mall was an unfortunate one,” the firm concluded. The design of the structure has previously been called into question during the inquiry.

Relying on a “waterproofing sealer material to maintain the integrity of a building envelope is considered poor practice,” the firm said.

JohnKadlec RodCaughill

Architect John Kadlec (left) testified that he had approved the project, despite evidence of construction problems. Rod Caughill, representing the original owner, said the firm rejected an engineer's waterproofing recommendations.

“Let alone having the material as a horizontal layer guaranteed to be covered in water and snow, and subjected to vehicular traffic.

“The lack of roof drain detailing is troubling and does not speak well of the level of care given to the design of the roof.”

While indicating that having a parking deck act as a roof over occupied spaces is challenging from a construction detailing angle, the report said it was not an “insurmountable challenge.”

But, in this case, material choices and assembly details failed to rise to the level of “sophistication and durability required.”

“Simply adding a layer of insulation and a fragile layer of waterproofing to either side of a parking deck does not transform it into a roof that should be placed over the public’s head,” the report concluded.

The forensic report involved field inspection, in-situ testing, laboratory testing, analysis of test results and modeling of failed structural components and the review of documents.


Tagged categories: Accidents; Architecture; Building design; Construction; Corrosion; Corrosion protection; Engineers; Fatalities; Steel

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