The Leaf at Canada’s Diversity Gardens

The Leaf features four indoor biomes: the Hartley and Heather Richardson Tropical Biome, which houses Canada’s tallest indoor waterfall; the Mediterranean Biome, filled with colorful and fragrant fruit-bearing plants; the Babs Asper Display House, which celebrates the art of horticulture; and the Shirley Richardson Butterfly Garden, where visitors can interact with various butterfly species. The biomimetic design of The Leaf complements the six surrounding gardens: the Indigenous Peoples Garden, Kitchen Garden, Sensory Garden, Performance Garden, Seasonal Garden, and The Grove. Teaching spaces, retail areas, and concessions provide visitors with a diverse array of color, texture, and fragrance throughout the year.

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ARCHITECTURAL VISION: THE STEEL DIAGRID AND TRANSLUCENT ROOF

Two of the most prominent features of the project are its ethylene-tetrafluoroethylene (ETFE) roof and its architecturally exposed structural steel (AESS) central tower.  

An ETFE system was selected for the roof to allow natural light to filter into the interior spaces, benefiting both visitors and plant life. To minimize shading from the roof support structure, a steel cable-net system was selected to support the ETFE air-filled pillows.

The cable system spirals and cascades down from the 108-foot (33-meter) tall central tower in a form reminiscent of the growth pattern of leaves around a plant stem, known as helical or spiral phyllotaxis. To provide an open and airy aesthetic, the highly pretensioned cables extend from the central tower to the building perimeter without the need for interior column supports.

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The transfer of forces through the cables is substantial, applying large tensile forces to the top of the central tower. The tower's structural framing was designed to transmit these significant vertical and horizontal loads to the foundations, where the resulting base shear and overturning moments are resolved. In addition to transferring considerable gravity and lateral loads, the central tower houses the building’s primary mechanical systems and environmental controls, which are critical to maintaining the sensitive environments within the biomes.

To transfer large structural forces, provide the necessary stiffness for the flexible cable-net system, and support the project’s aesthetic goals, a structural steel diagrid framework was selected for the central tower. The diagrid system was chosen for its high strength-to-weight and stiffness-to-weight ratios, as well as its ability to reinforce the biophilic design of the structural framing system, which is integral to the architectural character of The Leaf.

The unique central tower diagrid incorporates two parallel, continuous spirals that climb the height of the tower, echoing the spiral geometry of the roof. The diagrid geometry includes intermediate cross-bracing that connects to the primary chords, forming multi-axial, non-planar nodal joints.

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THE MANY REASONS CASTINGS WERE USED

Collaborating with CAST CONNEX, the design team explored how steel castings could support the central tower’s design. Leveraging the freeform capabilities of the cast steel manufacturing process, the external and internal geometry of the cast steel nodes was optimized to address the challenges of a conventionally fabricated connection while meeting additional architectural, structural, and constructability objectives.

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Through the collaborative design process, the project team was able to take advantage of the following steel casting benefits:

Inform the overall shape of the diagrid tower

The freeform geometry of the steel castings allowed the project team to create a unique, repeating diagrid pattern that also incorporated the curvature of the central tower. The curved steel castings enabled all connecting intermediate HSS members to be cut and welded as straight elements.

Enhance structural performance

The monolithic cast steel nodes are purposefully thickened internally to resolve the complex load paths within the central tower framing, particularly where the six members intersect.

Reduce overall tonnage

‍In typical HSS-to-HSS connection design (T-, Y-, and K-joints), local connection limit states often govern the design. Design teams traditionally increase HSS connection capacity by increasing the size and thickness of the HSS members and/or reinforcing the connection through the addition of stiffeners or other reinforcement methods. By locally thickening the internal geometry of the steel castings, the castings eliminated local connection limit states, enabling the use of lighter HSS member sizes and reducing the overall tonnage of the central tower.

Simplify steel fabrication and erection

‍The steel castings eliminated the need for complex fabrication of the HSS connections. Reinforcing the HSS intersections with grout or stiffeners would have added cost and complexity to the fabrication and installation of the diagrid structure. The use of steel castings controlled connection tolerances in all directions simultaneously. The castings also controlled erection tolerances and ensured proper fit-up of the multi-axial, non-planar geometries that would have been challenging to achieve using conventional fabrication methods.

Support aesthetics

If conventional fabrication methods had been used, the larger HSS member sizes required to satisfy connection limit states would have increased the overall tonnage of the central tower and diminished the desired aesthetic of a lightweight diagrid form. The exterior and interior surfaces of the custom-designed castings were sculpted to provide a clean, natural shape, creating visual interest in this significant AESS feature.

Provide economy of scale

The steel casting manufacturing process enabled the use of a repetitive nodal geometry throughout the height of the tower. The castings provided a consistent and cost-effective solution for what would otherwise have been a costly and complex detail using conventional fabrication methods.

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ADDING ECCENTRICITY FOR ECONOMY

Originally, the centerline geometry of the diagrid tower was fully concentric. The narrow interior angles between the diagonal members resulted in vertically elongated, heavy cast nodes that were not the most efficient use of cast steel material.

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CAST CONNEX suggested that a more efficient and economical design could be achieved by introducing eccentricity into the centerline geometry. Although the eccentricity increased the shear and moment forces within the node, the ability to locally thicken the interior of the steel casting in areas of increased demand resolved both the complex geometry and force flow while precluding local connection limit states. This optimization of material usage resulted in project cost savings and created a more efficient casting to manufacture.

CAST CONNEX performed a parametric three-dimensional modeling study to determine the eccentricity that would optimize the node’s height and weight. A 7.9-inch (200-millimeter) eccentricity reduced the node height by approximately 40%. The design team adopted the optimized eccentric centerline geometry in its structural analysis model. CAST CONNEX then completed a finite element analysis (FEA) using the design team’s structural model to confirm the structural performance of the modified casting geometry.

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COLLABORATIVE NODE STUDY

After the design team embraced the introduction of eccentricity into the casting geometry, CAST CONNEX collaborated with the team to explore the various nodal forms that could be used to shape the diagrid. Three node configurations were evaluated using both three-dimensional geometric models and 3D-printed prototypes to assess their aesthetic influence on the overall appearance of the diagrid.

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• Nodal Shape 1 accentuated the chord members and emphasized the spiral articulation of the diagrid.

• Nodal Shape 2 favored the vertical elements and enhanced the vertical articulation of the diagrid.

• Nodal Shape 3 blended the external profiles of both the vertical elements and spiral chords, creating no directional preference in the articulation and giving the tower a concentric appearance despite the eccentric centerline geometry.

After careful consideration, the design team selected Nodal Shape 2 as it accentuated the vertical articulation of the diagrid, drawing the eye upward toward the impressive roof system and the visible sky beyond.

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ONE PATTERN, TWO CORES: REDUCING TONNAGE‍

When considering custom-designed steel castings for a project, design teams are encouraged to rationalize the casting geometry to leverage the economies of scale inherent in the casting manufacturing process. The Leaf design team rationalized a single casting geometry to achieve the project's aesthetic and structural goals. Having established this strong starting point, CAST CONNEX was able to explore additional manufacturing optimizations and efficiencies.

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A few critical locations initially governed the design of all 72 casting nodes. The higher stress profiles and thicker connecting HSS members required at these locations meant that the majority of the castings would have been conservatively designed and heavier than necessary.

Given the size and quantity of the castings, CAST CONNEX developed a casting strategy that utilized one external pattern and two internal core patterns. This approach provided a consistent external geometry for all 72 casting locations while allowing the internal geometry to be optimized for critical locations (thicker-walled nodes) and non-critical locations (thinner-walled nodes).

Although this approach required the creation of an additional internal core pattern during the casting manufacturing process, the resulting overall cost savings in cast steel tonnage more than offset the added tooling cost.

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THE WATERFALL PATH

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CAST CONNEX standardized Architectural Tapers and Universal Pin Connectors (ART + UPC) were also used at the ends of the HSS struts supporting an elevated visitor walkway surrounding the central tower. The walkway extends from the diagrid tower and guides visitors around an interior waterfall that cascades from the top of the tower.

The pre-engineered ART + UPC system provides a sleek, minimalist connection detail for these AESS elements, directing attention to the central tower and Canada's tallest indoor waterfall.

The Leaf is an architectural landmark in Canada and a testament to the innovative and collaborative possibilities achieved through the integration of custom steel castings. Working closely with the project team, CAST CONNEX helped deliver steel casting solutions that advanced the project's architectural, structural, and constructability goals while contributing to the creation of a culturally significant and enduring public destination.