A Swooping Roof and Cantilevered Timber Columns

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As a showcase for innovative mass timber systems, the Presentation Centre at Fraser Mills uses community-focused architectural design and inventive structural engineering to create a welcoming space for the 96-acre residential development. The $10 million Centre is a key destination for those interested in Fraser Mills, encapsulating the essence of the area’s history and future through the use of timber and a swooping, free building form. Built on the former site of one of the area’s largest sawmills, the vision for this building was to pay homage to the rich tradition and craftmanship of the area. The decision to use structural timber was made by developer and owner Beedie Living at the start of the project. Patkau Architects acted as the design and executive architect with StructureCraft as the structural engineer of record and timber design-build subcontractor.

The purpose of the design by Patkau Architects was to weave the past, present, and future together by combining the traditional material—wood—with clean, modern lines and a unique geometry that requires innovative engineering and construction solutions.

The building is shaped like a trapezoid in plan view, but the unique geometry becomes even more evident in elevation as shown in Figure 1. The roof beams vary in slope, getting steeper towards the south portion of the building. The steeper slopes, combined with the wider plan dimension, result in a dramatic roof incline before slowly descending to the roof edge.

The front of the building features a cantilevered canopy that is 13 feet long and varies in slope along the length of the building, perpendicular to the main roof. When the main roof slope gets steeper, the canopy gets shallower and vice versa. This keeps the relative angle between main roof beams and canopy beams similar throughout the whole building.

The cross-laminated timber (CLT) back wall of the building leans outwards and is supported by splayed columns that weave the longer plan dimensions and taller roof elevation together naturally. As shown in Figure 2, numerous options were investigated. In order to avoid obstructions in the interior and to highlight the splayed columns in the back, the scheme on the bottom right without trusses and without webbing in the columns was chosen.

One of the most notable challenges is in the lateral force resisting system, resisting demands of 0.33 g in this high seismic zone. There are no shear walls that run perpendicular to the long direction of the building (i.e. in the plane of the sketches in Figure 2) and the front of the building needed to stay largely unobstructed to maximize the glazing area. The first intuition was to use the series of splayed columns at the back of the building to create a moment frame. However, the splayed columns connect into roof beams with spans so long towards the south that this load path was too flexible to resolve the lateral demand without engaging other building components.

The solution was to use the eight columns at the front of the building as cantilevered columns. Their height was constant throughout the building and their depth could be increased as needed without compromising the glazing area. There was only one caveat: The use of timber cantilevered columns as the main lateral force resisting system for this load magnitude has little precedence in Canada or elsewhere, especially as part of a mostly pre-assembled building system. This approach therefore required extensive studies and very careful detailing.
With the high seismic demands, StructureCraft had to design the columns for larger overturning moments. While the column itself had enough capacity, there was not enough space at the bottom to place conventional connection hardware.

Instead, the engineer designed a long, threaded rod that connects to the top of the column and anchors it down into the foundation directly, acting as a hold down connection. The asymmetrical shape of the building results in an overturning moment higher in the outward direction than it is in the inward direction, and thus the threaded rod was only required on the inside of the column. The inward moment with the uplift on the outside is resolved with smaller anchor rods glued in the timber. This allowed the column to serve another function: the downspouts of the roof drainage system are integrated into the column buildup. To accommodate the threaded rod, the downspout, and an easy fabrication and assembly process, the column is split in three components: two outer parts with an L-shaped notch at the top to support the transfer beam above and one inner blocking piece to set the distance between the outer pieces.

The cantilevered canopy and main roof beams create a trough in which sliding snow can pile up easily. Especially at the south end of the building, where the main roof gets long and steep, the amount of snow that can slide from the main roof onto the canopy is significant, causing a pile up in the roof valley of up to 6 feet. Therefore, StructureCraft had to design the moment connection of the canopy beams for high demands.

The connection also needed to be easily adjustable to allow for the changing beam slopes throughout the building while allowing for easy installation and future disassembly. The solution here was two separate bent plate assemblies, one attached to the end of the main roof beam and another to the canopy beam. At the top, they are connected to the timber with glued in rods to develop the tension capacity for the cantilever moment. These assemblies were preinstalled to each of the beams ahead of time, allowing the main roof beam to land on the transfer beam before the installation of the canopy beam. Linking the two beams were connector plates with through-bolts. To complete the push-pull couple resolving the cantilever moment, four dowels were driven through the transfer beam, allowing the compression from the canopy beam to be passed through the transfer beam to the main roof beam without crushing the wood fibres perpendicular to grain.

At the ends of the building, the transfer beam had to stop at the inside face of the roof beams to protect its end grain. This meant that the typical connection was not applicable here, and StructureCraft designed a unique solution that connected the roof beams and transferred their combined vertical reaction to the top of the transfer beam through bearing. The eccentricity was resolved by screws at the top of the transfer beam, bearing against its end grain at the bottom. The strong advantage of this connection was that the two roof beams could be fully assembled ahead of time, allowing for a simple drop-in installation on site with only four screws to install.

On top of the roof beam, 3x4 Douglas Fir dimensional lumber and ½-inch plywood comprises the roof decking that spans 8 feet between each beam. The challenge here was to find an arrangement of the straight purlins that followed the curved roof without creating a faceted appearance.

To design the curved roof using straight elements, a genetic algorithm solver was employed, optimizing purlin arrangement and enhancing material efficiency. The optimization process focused on three key aspects: minimizing unique types by reducing variations in purlin lengths and cut angles, warp reduction by analyzing curvature and adjusting parameters to flatten highly warped plywood areas, and material efficiency by optimizing plywood panel layouts to minimize waste during the unrolling process. By leveraging this computational approach, StructureCraft rationalized the design to maintain structural integrity and aesthetic intent while streamlining fabrication. This method significantly reduced the number of unique purlin and plywood assemblies, improving constructability and cost-effectiveness.

By definition, the presentation centre will only serve its original purpose for a limited time, so it was critical to design the structure and its connections to be easily dismantlable. While the long-term plan is not finalized yet, the developer wanted the option to relocate the structure elsewhere in the development and repurpose it as a community center.

To make that process as smooth as possible, most of the connections use threaded fasteners that can easily be removed. The long rods that anchor the cantilevered columns down to the foundation are equipped with a coupler just above the top of concrete that separates the portion of the rod that is integrated into the column from the portion that is cast into the concrete. The canopy beam connection can be undone by simply loosening four bolts. A process that is estimated to take less than 10 minutes per beam.

All timber components were CNCed and pre-assembled in StructureCraft’s shop in Abbotsford, BC—close to the project site. This was especially advantageous for the roof beam connection that required high precision and a controlled assembly environment for the epoxied rods as well as the plywood and purlin panels that came in many different geometries. This approach offered the highest quality finish and sped up the on-site installation time so efficiently that the whole timber superstructure was erected in just 5 weeks.

To guarantee that the exposed connections properly fit together, the glulam frames of roof beams and splayed columns were first assembled flat on the ground, allowing for adjustment and alignment of the members. Once all connections within the frame were completed, the whole assembly was lifted and slowly rotated in its vertical orientation using chain hoists as part of the crane rigging.

This unique project presented many opportunities to push the limits and explore new solutions in the world of timber engineering. The use of buildup column assemblies as the lateral system solved the absence of interior shear walls without compromising the architectural intent of large, unobstructed glazing, and ultimately, the satisfaction of the client.

Careful detailing and prefabrication as well as close collaboration between architect, engineer, and owner allowed all structural connections to be fully exposed while still achieving a pleasant appearance. This is a key consideration for mass timber structures, where covered up or clad structural components are undesirable.

Furthermore, most of the connections that were installed on site are simply bolted or screwed and therefore easily reversable when the presentation centre is no longer needed for this development and is relocated for a second life.

The delivery of the prefabricated structural components and their install was made possible by StructureCraft's unique, vertical integration of structural engineering and timber construction along with state-of-the-art computational analysis and design tools. ■