How DST Consulting Engineers cleanly and safely demolished a building along one of Toronto’s busiest downtown intersections
How do you safely demolish a seven-storey government building in downtown Toronto with minimal impacts to a busy traffic and pedestrian corridor, while recycling 95 per cent of the construction materials?
As DST Consulting Engineers has figured out, it’s less a demolition than a deconstruction—dismantling the building piece by piece, floor by floor—and recycling the materials into something new. And, it involved the improvisation of some very novel (and effective) health and safety techniques and equipment to keep everyone safe.
Engineering Dimensions talks with George Thomas, P.Eng., DST Consulting Engineers’ director of infrastructure client group, on how the firm safely demolished the 70-year-old building at 880 Bay Street.
Engineering Dimensions: Can you provide a brief overview of the project?
George Thomas: DST Consulting Engineers was the prime demolition consultant responsible for demolition planning, design and abatement of all designated substances for the retired Ontario government building located at 880 Bay Street, in downtown Toronto. The seven-storey structure was constructed in 1947 and had a basement totaling approximately 170,000 square feet.
Due to the multi-disciplinary nature of the project and the number of sub-consultants involved, highly effective contract management, stakeholder management and sub-consultant management was required. The project involved a high level of health and safety oversight given the very busy urban location of the building. There were also major environmental considerations, with a 90 per cent waste diversion target for the project.
ED: How did this project differ from other building demolition projects?
GT: This was the largest demolition project ever completed by the client—Infrastructure Ontario (IO).
Given the proximity to public sidewalks and a very busy intersection at Bay and Grosvenor streets, IO considered this to be a high-risk project and public safety was paramount. However, requesting to shut down a sidewalk or lane of traffic on Bay Street was completely out of the question and we were basically told not to think about it. We had to complete the demolition within the confines of the site, which was right at the edge of the sidewalk.
Public safety was given added urgency because of an incident a year earlier during a building demolition at Bathurst Street and Eglinton Avenue in Toronto. The facade of the building collapsed and crushed the hoarding, or safety structures, that had been erected to protect the public around the site. Fortunately no one was killed, but two parents and their child were trapped inside the hoarding after the collapse and had to be rescued. That incident definitely heightened everyone’s awareness around public safety for the 880 Bay Street project.
In demolition, you can knock down a building and not worry about debris falling if you’re in an open area—the risk is much less. But in this circumstance, we were deconstructing a seven-storey building, plus penthouse, with a brick veneer facade, at an extremely busy downtown intersection. Even one brick dropped in the middle of Bay Street could cause disaster.
Before we started the actual deconstruction, we did some test trial runs on the wall facing the parking lot. We tested our dismantling procedure on the non-public area first to determine what the risks were and then modified the procedures accordingly for work on the public side.
ED: Many of DST’s engineering innovations for this project related to the extreme public safety concerns involved in demolishing a building in such a high-traffic area. Can you describe the safety innovations you devised for this project?
GT: There were a couple of safety innovations that were unique to this job: shipping container hoarding and outrigger safety nets. Firstly, the hoarding (the safety structures shielding the public around the perimeter of the site) was made of shipping containers—the rectangular steel boxes used to transport goods on ocean-going container ships.
We basically took standard shipping containers and modified them to create steel rectangular boxes that could be laid seamlessly, end- to-end, around the public-facing perimeters of the building. We had them all manufactured offsite: removing both ends, reinforcing them, and installing LED lighting inside. We also cut windows on the street-facing side so people could see out as they passed through.
It was the first time this type of hoarding had ever been used in Ontario. There are many advantages to this type of system: it’s easy to install, it’s solid steel and there are no openings. If you walk through a standard hoarding made of scaffolding and plywood, you’ll see openings. All it takes is something small to come through to hurt someone. But shipping containers are 100 per cent enclosed between the public and the site.
The second safety innovation was an outrigger-design safety net to capture any debris that might fall outside the building footprint and hit the public area. The outrigger was a steel structure that fitted two to three metres beyond the building facade and captured any debris that might fall from the upper levels. Basically, it’s a heavy duty, structural steel net angled at 45 degrees toward the building so falling debris would hit the outrigger and slide back to the building at the level below the one we were working on.
ED: Considering the very limited space with which to work, including little staging or storage areas to remove materials and debris, how did DST work around these constraints?
GT: Yes, because the building site was right in downtown Toronto, there was very little room to work with—especially considering we weren’t allowed to use the adjacent parking to store deconstructed building materials like concrete and brick. If you look at the building, the only extra real estate we had was an alley at the back of the building where you could barely drive a truck through. There was very little for a staging area.
To get around this, we created vertical shafts within the building—all the way up from level eight down to the basement—and dropped the deconstructed materials down the chute to the basement. We then filled up the basement cavity with the materials. The shafts had to be reinforced to handle major chunks of concrete and rebar being dropped down through it.
In the basement area, we had machinery in place to move the deconstructed materials around because as the space got filled, the chute would get blocked up.
ED: Part of the project requirements was a 90 per cent waste diversion target. How did you go about recycling a seven-storey building?
GT: To allow for optimal dismantling and segregation of materials on this project, DST utilized the 3R (reduce, reuse, recycle) approach and implemented stringent waste management reporting and monitoring to ensure the waste diversion goal of 90 per cent was not simply met, but exceeded. In the end, we achieved 95 per cent waste diversion.
We brought in a mobile crusher and placed it in the basement to minimize dust and noise. All the concrete from the building was processed through the crusher, removing the steel rebar and crushing the concrete to create a granular material—granular B aggregate—that we used to backfill the basement cavity. And before we started work on the deconstruction, we scraped off all the adhesives from the concrete and removed any foreign materials so in the end we were crushing pure, clean concrete. We also recycled all the steel and copper piping and crushed all the glass for reuse in other purposes.
The remaining five per cent was mostly painted wood finishes. You can recycle wood into chips, but if it’s painted or treated you can’t.
Finally, we recovered all the Queenston limestone slab that covered the ground-level facade. It’s all limestone from the Queenston formation from Niagara Region, and you see a lot of it in old buildings around Queen’s Park. We recovered all those pieces and restored them onsite for future reuse.
IO was proud enough to create a video of the process, which you can view here: www.youtube.com/watch?v=8eJtudDXStk.