The development at 400 Albert Street in Ottawa will produce two mixed-use towers in the downtown core combining rental, commercial and retail spaces within a complex three-storey podium joining the two towers at the base.
Entuitive is providing structural engineering services for the project, and Associate Robin Djuita recently shared his insights on some of the technical solutions that contributed to its design.
Robin, what can you tell us about the 400 Albert Street development?
On this project, we are providing structural engineering services for two purpose-built rental towers, 23- and 29-storeys each, which are connected at the base with a three-story podium. The podium will have designated amenity space for the rental units above and retail space on the ground floor. Construction of the three levels of underground parking is complete and slab placement is underway for the first level.
From the outset, I was drawn to the building design alone. The the curved corners and terracing on the North and South towers, with the transition into the commercial and retail space of the podium, would likely present a fun challenge.
What are some of the technical features of the design?
A change of program from residential to retail at the lower levels of the towers required greater space between columns. The change also forced us to think creatively about columns and wall placements to avoid the use of additional structural transfers, while accommodating the use of space intended.
At the podium levels where there is much more headroom, we were able to introduce transfer structures, and we proactively and creatively placed columns in an order that minimizes the number of transfers used. We didn’t want columns everywhere in the retail space, so in collaboration with our client and architects we reached a solution that is satisfactory for all parties.
The terracing (or step backs) at the east side of the North Tower also required careful coordination with various parties to ensure placement of vertical supports do not impact unit layouts negatively.
What was the most exciting challenge of the design?
From an architectural design perspective, it’s preferred to not impede the view provided by the curved corners. This means that there are no columns at the corners of the buildings, requiring the use of longer cantilevers. In addition to the architect’s desire to not impede the view, the unique curvature of the building corners introduced longer cantilevers, which we solved with the creative placement of columns and thicker slabs on some levels. Another challenge that we encountered is the design of the podium slab as a diaphragm adjoining two tall towers. Without an expansion joint in the podium, the podium slabs have to be designed to account for tower movement away and/or into each each, putting the podium slab in tension, compression and shear.
Which structural considerations were made to account for this condition?
We developed a multi-tower 3D lateral ETABS model and performed dynamic (response spectrum) earthquake analysis to see how the towers behave in connection to each other. The stresses in the podium slabs, based on different dynamic earthquake models, are then analyzed and podium slabs are reinforced accordingly (and in addition to the demand of gravity analysis). The technical solution was to use heavy 5-meter-deep steel transfer trusses, fabricated off-site, and to lift them into place over a short downtime period, off hours.
What other design considerations were made for this project?
We also made concentrated efforts to optimize the lateral design for this project. Victoria Sypher, the engineer responsible for the lateral design, developed and studied 3D lateral models with various framing options. The information from her study enabled us to strategically place structural elements that will be optimal for building performance in a higher seismic zone.
Thanks for taking us behind the project at Relevé, Robin! If you’d like to learn more about this project, reach out to Robin Djuita.