The collapse of the roof at a mall in Elliot Lake, Ont. on Saturday has raised questions about how a building could suddenly fail.

A section of the rooftop parking lot at Algo Centre Mall caved in while people were shopping below. Although repair and maintenance work on the mall roof was done over the last year, it was not in the area of the collapse, according to Rhonda Bear, the Algo Centre Mall manager.

Paul Walters heads Walters Forensic Engineering, a Toronto-based firm specializing in structural failures. In an interview with CBC News, Walters pointed to water and salt rusting the reinforcing steel skeleton inside the garage as likely the primary culprit in its failure.

"What has happened here is the structure under the roof has exceeded what it can withstand."

Walters added that "it could be a compromised internal structure, like rusted reinforcing bar."

4 telltale signs

Here's how Walters breaks down what's involved in determining what caused a structural collapse:

  • Condition of the surface concrete

"You’re looking for cracks in the concrete surface, anything that could introduce a solution (water and salt) into the structure."

"When that happens, the bonding is compromised between the concrete and rebar and the structure’s integrity can be compromised."

  • Moisture barrier

Outdoor reinforced concrete slabs – like that found in the Algo Centre Mall rooftop parking lot – are built with a water impermeable "moisture barrier" to keep water from seeping through cracks and damaging the steel matrix within.

"Is the concrete horizontal, exposing the surface to the elements? Parking garages usually have a barrier, epoxy or rubber over the concrete to prevent water and salt from seeping through the concrete."

"You can imagine what happens. The rebar becomes exposed to rust." If it's present, "In an inspection, a suitable barrier will be obvious if there are large chunks of concrete visible."

  • Internal structure and supports

Concrete is often reinforced with an internal steel matrix or skeleton, typically in the form of I-beams, open web joists, or rebar.

At Algo Centre Mall "the structure under the roof has exceeded what it can withstand and it could be the result of a compromised internal structure, like rusted rebar."

  • Analysis

"A metallurgical analysis is done. A sample of the reinforcing bar is sent to the lab."

Algo Centre Mall "is a serious situation, whenever there is a life involved. Determining the basics of what has gone wrong is the first step" in a forensic analysis.

"With metallurgical examination, it could be a month and a half or two. Depending on the research, it can be as much as five months."

"You also have to know the history. The history is a key factor, especially knowing any changes that were made after the initial construction. That is often why you get wall collapses."

Fixing the damage

Walters says the likely solution will involve chipping down the concrete until the reinforcing bar is not rusted. That's a big job and an expensive one, he added.

roofinfogfx

View of how the Algo Centre mall may have collapsed