A new amphitheater in Vancouver, British Columbia offers the potential for mass timber to move beyond schools, offices and institutional buildings to large public assembly structures traditionally dominated by steel and concrete systems.
The approximately $183 million Freedom Mobile Arch at Hastings Park opened on June 5 and will host events for the FIFA World Cup, which begins this week. It has a ceiling of 105 m free light supported by only three primary points.
The structure combines 60 laminated timber arches with three heavy-duty steel royal arches arranged in six intersecting barrel vaults, creating a star-shaped roof over the 10,000-seat venue. The project was developed by the National Pacific Exhibition and the city of Vancouver.
“The majority of long-span timber arch structures worldwide are exhibition halls, arenas or football facilities with spans between 80 and 90m,” Fast + Epp, the venue’s structural engineer, said in a project statement.
The firm’s Richmond Olympic Oval roof, completed for the 2010 Winter Olympics, spanned some 95m and was believed to be among the longest wooden roofs in the world at the time.
For Fast + Epp director Robert Jackson, the significance of the Freedom Mobile Arch lies not only in its size, but in what it suggests about the future of wood in large civic spaces.
“This is what we believe is one of the longest wooden arches in the world,” Jackson told ENR in an interview hours before the venue opened. “The big innovation is pushing wood into long-lasting conditions and showing that it can be a good material to use in stadiums and civic centers.”
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Three-point structural solution
Designed by Revery Architecture, the roof was conceived to meet an architectural brief that called for unobstructed views of Vancouver’s North Shore Mountains while creating a covered venue that extends the city’s outdoor concert season beyond the summer months.
Crane mast towers and temporary hydraulic jacks support the roof of the Freedom Mobile Arch during construction in Vancouver. Engineers used 13 temporary support towers while assembling the hybrid steel and timber structure before executing a carefully sequenced de-clamping operation once the roof diaphragm and arch system were complete.
Courtesy of the City of Vancouver/X
The resulting structure rests on three massive concrete supports located at the corners of an equilateral triangle, an unusual geometry that prompted the engineering solution. As the wooden arches extend outward toward the center of the roof, they generate significant thrust forces. These forces are transferred to the steel king arches in the roof valleys before passing to the concrete buttresses.
“King’s bows were made of steel because they took tremendous force from that kick,” says Jackson. “We tried making the king’s bows and keys out of wood, but we felt it wasn’t really the right material in the right place.”
The shape evolved from three intersecting barrel vaults. Engineers then lowered the valleys between the vaults, creating a doubly curved roof surface that allowed the structure to rest on three primary supports instead of four.
“What he gave us was only three points instead of four, and it worked better for the site,” says Jackson, adding that the project was inspired by the CNIT showroom in Paris’ La Défense district, a postwar concrete structure associated with engineer Pier Luigi Nervi.
Fast + Epp saw the project as an opportunity to revisit a proven compression-based form using modern mass wood fabrication, robotics-assisted machining, and advanced structural modeling.
“We said very quickly, OK, Nervi has done this,” says Jackson. “Let’s see if we can do it with new materials, solid wood, steel, new technology for all the modeling of the connections and the search for form and all the machining.”
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construction challenge
The glulam members were manufactured by Nordic Structures in Quebec and shipped across the country by rail. Individual segments were limited to about 70 feet in length and had to be connected on site. Engineers also performed extensive global buckling analysis, designed complex timber connections and commissioned a full wind tunnel study to model the forces acting on the structure in the open air.
The CNIT showroom in Paris’ La Défense district helped inspire the structural concept of Vancouver’s movable arch of liberty. Fast + Epp principal Robert Jackson said the design team looked at the post-war shell structure and asked, “Let’s see if we can do it with new materials, solid wood, steel, new technology for all the modeling of the connections and the finding of forms and all the machining.”
Image/Wikipedia
Construction was based on 13 temporary crane mast towers equipped with hydraulic jacks to support the roof while crews assembled the steel arches, timber frame and cross-laminated timber diaphragm. Once complete, the engineers executed a carefully sequenced de-bracketing operation. Jackson notes that Maffeis Engineering, based in Italy, performed engineering analysis and construction phase demolition for the project.
Erection effort developed under intense programmed pressure.
Jackson says that while the city plans to use the site as a fan festival site during the World Cup, project stakeholders looked further ahead to create an indoor facility that would extend Vancouver’s outdoor concert season beyond its traditional summer window.
He calls the project one of the strongest examples of collaboration he’s seen between engineers, contractors, fabricators and trades.
Fabricator Walters Group supplied approximately 800 tons of structural steel and installed both the steel and wood arch systems, including about 900 tons of wood arches. According to Ontario-based contractor EllisDon, crews pre-assembled and spliced the steel king arches into a custom frame before lifting them into position, with each arch weighing more than 16,000 kg.
“The canopy is incredibly challenging and the installation will be just as rewarding,” Brendon Vining, EllisDon’s senior project manager, said in a statement.
Asked when he knew the ambitious design would work, Jackson dryly replies, “When the dams dropped. It was a big moment for the team.”
Jackson says the lessons learned from the project could extend far beyond a single site: “The big innovation is pushing wood into long-lasting conditions and showing that it can be a good material to use for stadiums and civic centers.”
