The building that partially collapsed in London in an incident that left two construction workers dead was built using cold-formed flat steel, an increasingly popular construction material for mid-rise buildings.

The collapse happened as concrete was being poured on one of the upper floors of the 41-unit, four-storey building at 555 Teeple Terrace. The accident is being investigated by Ontario's labour ministry and coroner. 

Five workers were rushed to hospital, where one was pronounced dead. A second worker was buried in the rubble. His body was removed late the next day. Others survived and have spoken about the experience. 

Unlike the forged steel girders used in skyscrapers, buildings like the one on Teeple Terrace use components made of flat steel built to the developer's specifications. Cold-formed products start out as flat metal sheets that are pressed into various shapes such as joists, wall studs and floor panels. The components are shaped at room temperature in a controlled environment away from the job site. 

The walls often arrive at the site as partially assembled panels, sometimes with an exterior finish already applied. The panels allow the building to come together quickly, something one engineer described as being similar to using Lego blocks.

Steve Fox is with the Canadian Sheet Building Institute, an industry organization that represents manufacturers of cold-steel construction products. 

Fox said when he started in the industry in the 1980s, the use of cold-formed steel in building construction was typically limited to non-load bearing components, such as interior-wall studs that hold up drywall, or external walls that have to face wind forces but don't hold up the building. 

He said as the manufacturing process improved, the flat-steel components could be used in other parts of the building.

"About 20 years ago, we started to see it used more for entire structures, what we would call axial bearing, where instead of just wind-bearing they carry the gravity loads as well," he said. "Once you do that, you can build the entire building out of cold-formed steel."

Fox said cold-formed steel has a strong strength-to-weight ratio but it's also a very cost effective way to build compared to wood, concrete or traditional steel. Because the sections are light, they often don't require heavy craning to put into position, speeding up construction time. 

Recent record-high lumber costs have made steel even more enticing to developers.

Fox said it's a construction method common for mid-rise buildings (typically no higher than six storeys). He said engineers have determined the material has the strength to support taller buildings but it would require so many vertical supports on the lower levels, it would create design challenges and lead to higher costs.

"Up to six storeys, you can be very competitive," he said. "Once you get higher, other issues come up that sway the decision toward other materials. You can certainly build taller using cold-formed steel, it just becomes less practical." 

The cold-formed steel components used at 555 Teeple Terrace were supplied by iSpan Systems in Princeton, about 20 kilometres east of Woodstock.

The company's components were used at 250 Oakland Ave., a six-storey apartment building off Highbury Avenue south of Dundas Street. The project gallery on iSpan's website shows its products have been used in buildings all over southern Ontario. 

Fox said concrete is often used for floors in cold-formed steel buildings, either as pre-formed slabs or wet concrete poured into a form. He said he didn't know of any other similar accidents where a similar building collapsed. 

He would only say the process of applying the concrete takes care and experience to ensure no section of the building gets overloaded. 

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