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Design Cited in Fatal Mall Cave-in

Thursday, March 21, 2013

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Engineers investigating the deadly collapse at Algo Centre Mall in Ontario say it’s surprising that the 32-year-old structure did not fail sooner.

A sobering forensic study by the global engineering firm NORR found that the Elliot Lake structure faced years of unmitigated leakage, resulting in continuous corrosion of the steel frame at rates typical to that seen in a marine-type environment.

“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 rooftop 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 others.

A 142-page portion of the 700-page engineers’ 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.

One Car, One Second

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.

“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.

Marine-Like Corrosion Detected

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

corrosion
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.

Most of the shopping mall’s beams’ top flanges, bolts, welds and angles had a severe to very severe corrosion level. According to the report, this state of corrosion 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.

NORR noted that the severity of corrosion was generally underestimated by numerous inspections, including some completed by structural engineers, and that some of the most critical corrosion examples in its report were found “in plain sight.”

For example, a pedestrian walkway showed steel had been allowed to corrode to the point that some of the steel plates were completely depleted, NORR said.

NORR also said that “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.”

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

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

HollowCoreSlabs
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.”

 

   

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

Comment from Tom Schwerdt, (3/21/2013, 11:43 AM)

The final photo shows how poor the sealing really was - from other reports, they only dribbled sealant along the seams instead of using an actual sealing membrane across the whole deck. There was apparently a remediation recommendation ~20 years ago for a full sealing membrane across the whole deck, which the owners rejected. (Again, from another report)


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