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The House in the Ravine

Toronto, ON June 2020 – by Karl Doucas

It’s the summer of 2018 and I’m representing our firm at a truly inspiring and historic residential job site. I didn’t know much about the history of the home but I knew if Architect Peter Clewes was the Architect, and he was; or if Bruce Clemmensen was the builder, and he was; then something special was being orchestrated. Sharing the lineage of the house would surely give away its address, so it shall remain private for now.

Driving down from my office, there was anticipation and curiosity about the property and residence I was visiting for the first time. Not for the dense forest, but the elevation of the street made for an unexpected visual. As I looked outward and down, I noticed a green flat roof and a modern geometric silhouette as the sun was setting in the distance. “Could this be the place?” Understated and barely there from the exterior, I was sure a more intimate introduction would prove to be a truer vision. So, I made my way down the steep driveway to get a closer look.

There, standing on the partially built Ipe and stone deck, gazing across the mature ravine below, I saw and heard both the imagery of a big city, and the beauty of an all immersive natural habitat setting. Nestled quaintly on the side of a wooded embankment, on a settled side street in an inner suburb of Toronto, I contemplated the challenges I was about to face, and the exceptional opportunity this unique project would bring to me personally, and our firm PICCO Engineering.

The house was under renovations and Clemmensen Builders was seeking engineering solutions for a special stone feature stair within the home. The original design suggested constructing stone “plank” treads cantilevered from an adjacent foundation wall. After all similar concepts have been done many times successfully and this approach was feasible for this project as well, except for some influencing factors. Firstly, the foundation support wall was made of rubble stone and substantially thick. Reinforcing the structure to deliver the cantilevered treads would require a disruptive, invasive design and costly surgical precision and coordination. Maintaining the historic construction was also debatable and not a palatable path to take. After consideration, a different opportunity proved worthy. A bold and creative proposal by PICCO, to design and construct a “floating”, post-tensioned stair of solid stone was envisioned to be a better way to go. With all stakeholders in agreement, we started the process.

Inspired by elegant post tension stair examples by Webb Yates (London), we leveraged our relationship and partnered with them through the project design and engineering phase. The stair was defined by 19 total treads with a shallow slope of approximately 3 rise, 5 run. Its width was 1.02 m (41”) and the contemporary stepped profile included a minimal nosing. Aesthetically, Architect Peter Clewes was particular and we appreciated the need to stay true to his design without compromise.

Our collaboration with Scott Boote (Webb Yates of London) and his team created a spirit of innovation and passion. Scott noted that this was most likely the longest straight run post-tensioned stair they had worked on, and it was our first (shhhh!). Ideas, strategies, and approaches were free flowing and tested. Scott developed engineered models, while Xin Gao (PICCO) reviewed and directed requirements of the design. Xin’s engineering calculations for tension cables, pad foundation design, and beam anchorage were integrated with stone shop drawings, construction documents, and stone fabrication tickets.

The stone used for the stair was to match the specified flooring installed throughout the house. Chinese Black Basalt G684 was sourced from limited reserves direct from the Chinese quarry in Fujian. The dense material presented favourable properties for our application with each tread weighing in excess of 250 kg. (550 lbs). Engineering dictated a special sheer key design facilitating the nesting of one tread to the next. A custom base plate with 12 – M16 concrete wedge anchors would serve as the main tension cable stay transferring load to the 4ft x 4ft x 28” thick concrete footing. The top of the stair had a less robust connection plate that bolted directly to an existing steel cross beam.

With a sound design logic and calculations to back it up, fabrication and stair installation preparations commenced. Active coordination with our Chinese fabricator required guidance, dry laying, and stone mock-ups. The precision of drilled holes within tight tolerances through consecutive treads proved more difficult than we expected. Even subtle misalignment of these holes would have presented risks of cable “kinks” and increased cable stresses. QC measures at the fabricator premises confirmed both quality fabrication and craftsmanship with near perfect hole alignment. Logistics planning and “just-in-time” delivery were also essential expectations.