VanDusen Botanical Gardens Visitor Centre
Designed to captivate the imagination of passers-by, a vegetated green roof is supported by a curvaceous wood structure. Structurally, the roof has visual interest inside and out while maintaining a warmth that only wood can provide. The complex petal roof structure utilizes slightly curved glulam beams and straight lumber pieces with CNC-cut ends. A parametric 3D model was developed to rationalize the architecture and the complex geometry of the petal shapes, and then used to produce fabrication information to shape each beam. The beams are spanned with secondary joists suspending a lengthwise fir plywood slat exposed ceiling. The assembly is topped with two layers of plywood and conceals MEP services, all installed in our shop.
Visualizing the curved roof structure from the 2-D architectural drawings was very difficult. The modelling into a 3-D image using various software programs was not only necessary to envisage the final product, it was essential for completion of the design. Perkins+Will developed the initial roof geometry using Rhino® 3D and Autodesk® Revit® 3D modelling software. At that point, we (being responsible for the roof fabrication and installation), took the model and worked with Grasshopper®, a digital design add-in to the Rhino software, to develop the final geometry. Once all the parameters needed for the roof structure were understood and inputted into the Grasshopper program, the design team was able to change the form and optimize various aspects to keep the roof structure on budget.
The 3D model was used to digitally panelize the roof's series of overlapping petals and the central atrium area, or oculus. The geometry of each of the 70-plus panelized sections (panels) that made up the roof structure was unique and each described compound curves. The 3D modelling was used to play with the curvature of the glulam beams (located at the panel edges) in order to optimize the number and thickness of laminations and keep the costs down. By using larger radii for the curves, stock laminations could be used. The modelling was also used to identify appropriate locations for the low points in each panel segment for the most favourable placement of down pipes to drain the sprinkler system and for placement of the roof drains, as well as to ensure visual continuity of the ceiling.
It’s the dramatic free-form, organically-shaped roof structure that makes people take a second look - its form metaphorically represents petals of an orchid, drooping seamlessly into the surrounding landscape like an extension of the garden itself.
Each of the 71 different roof panel sizes and shapes were built to meet the dimensions of the flat deck truck that would transport them to VanDusen from our shop in Delta, B.C. The roof structure modules were typically trapezoidal shaped panels within a 3.6-metre-wide by 18-metre-long shipping size. These panels consist of doubly curved, glue-laminated edge beams that not only act as primary supporting components, but were also ingeniously used as “jig" members on adjustable shoring posts that defined spanning between the complex individual panel geometry during the prefabrication process. Conventional plywood sheathed 2x8 framing spans between the edge glulams, while a secondary layer of light framing forms the support structure for undulating wood slat ceiling finish. Particularly challenging was the framing of large, cantilevered “petal" tips and the central oculus with its steep side walls surrounding a skylight opening.
Engineers also developed a lateral system to support the heavier mass of the building's green roof, locating steel braces and curving concrete walls strategically, so both the functional layout and breathtaking views of the surrounding garden would remain unimpeded. In addition, the undulating twists and turns of the building's 50 foot atrium required the development of a universal panel-to-column connection to avoid unique connections at every support location.
Most of the roof's panelized sections are tapered with splayed glulam beams creating trapezoidal-shaped panel sections. Two 80 x 532mm deep glulam beams, each with their individual compound curves, set anywhere from 2.5 to 3.5 m apart, form the longitudinal edges of the panelized sections; the longest panels were 20m long and weighed nearly 5,500 kg.
The roof deck is constructed using 38 x 184 mm dimensional lumber roof joists to frame the space between the beams. They are installed at 600 mm centres using pressure blocks and sit flush with the tops of the beams. The joists are topped with two layers of plywood (first 12.5mm then 9.5mm). The top layer of plywood was left off for approximately 300mm along the longitudinal edges of the panels to facilitate assembly of the roof diaphragm on-site. A peel-and-stick roof membrane was shop-installed to the upper plywood layer, with enough extra to cover the joint between panels on-site. Since the roof installation took place during the rainy season, the membrane helped to keep the panels dry until the roof could be completed.
Once the top layer plywood was installed, the rainwater leaders for the roof drains were mounted by the mechanical contractor, the joist cavity was sprayed with closed-cell foam insulation and the plywood thermal barrier was installed to the underside of the joists. Next came the sprinkler lines which proved to be quite the challenge. Aside from having to undulate with the shape of the roof, the sprinkler mains had to be positioned such that they would be hidden in the walls. This decision had to be made before anything was even laid out on-site, since the concrete slabs were only poured after the roof segments went up. The sprinkler heads (which the architect wanted aligned with the ceiling slats) had to be positioned and everything had to eventually be connected on-site, from one panel segment to the other. The rough electrical wiring was then installed and work on the ceiling structure commenced.
The finished ceiling of double thickness shop-laminated plywood slats (6mm each) is supported by ceiling joists set near the lower edge of the glulam beams. The joists are custom profiled dimensional lumber (38x89mm or 38x140) set at 1.2 m on centre using pressure blocks. The varying elevations and profiles of the ceiling joists create the ceiling undulations which are independent of the roof's exterior undulations.
A black fabric faced mineral wool acoustic insulation was installed followed by 65mm wide, 7 to 10m long, ceiling slats with offset end joints, parallel to the beams. Since the slats are a constant width, their spacing varies to accommodate the trapezoidal space between the beams. The black fabric is left apparent between the slats. The finished ceiling surface is inset 25mm from the bottom of the glulams.
The larger roof panels were delivered singley, although the trucks could transport up to three of the smaller panels at one time. At the height of the construction efforts, about one and a half panels were installed per day. Each panel was lifted and set into place within 50mm of the adjacent panel and then the “stitching" process began. The insulation was blown into the space between the panels (150mm of closed-cell foam), and the vapour barrier, plywood and roof membrane were installed over the panel joints. Despite the overlapping roof membrane, there were still some challenges to keep water out of the panel joints during this process on the heaviest rain days.
Connecting the sprinkler pipes between the numerous panel sections was challenging. To maintain the structural integrity of the panels, meet the requirements of the sprinkler distribution system, and provide access to low-point drains, the installation of panel-end and wall bulkheads, as well as exterior parapets was foreseen. In certain areas where the tolerances were too tight, the decision was made to enlarge the bulkheads. Once these issues were settled, wiring connections were carried out and the panels were bolted together.
When the north end was completed, work moved to the south end of the building, once again advancing toward the oculus using the progressive stage approach. The oculus was constructed last, and proved to be the most challenging. Those panels curved upward almost 90 degrees with all petals arriving at the same elevation at the bottom of the oculus. There was no precedent for dealing with this complex roof geometry and many details had to be worked out on-site, including the installation of ceiling slats which could only be done after all panels were connected.
The prefabrication of roof panels off-site made it possible to optimize the construction process and allowed trades to meet their own deadlines. There was minimal waste generated on-site from the roof system installation as the panels were delivered completely finished on the lower surface (ceilings and soffits – with the exception of the oculus area), and ready to receive the roof installations on the upper surface.