The Sky’s The Limit: Designing airports with wood

For the Cranbrook airport, light-wood frame construction was chosen for its economy, flexibility, lightness, and compatibility with the original structure. The new facility is mostly made of glass and wood to ensure the stunning landscape can be viewed from all areas of the facility. Photos © Nic Lehoux
For the Cranbrook airport, light-wood frame construction was chosen for its economy, flexibility, lightness, and compatibility with the original structure. The new facility is mostly made of glass and wood to ensure the stunning landscape can be viewed from all areas of the facility.
Photos © Nic Lehoux

Conceptually, the structural and architectural approach focused on careful detailing rather than elaborate expression. The simple post-and-beam glulam frames provide a visual connection to both the original and new structural steel framing in the central corridor. The new timber on the left and steel on the right alternate as one walks down the corridor, paying homage to the building’s history. This alternating play of materials acts as a visual link, both logistically and architecturally, joining the new and old structures.

The polyurethane-glue glulam, notably pristine for its lack of black glue lines, was created using five-axis CNC equipment. The beauty of the premium glulam material is emphasized through the use of discrete stainless steel tight-fit pin and hidden knife plate connections. Sand-cast ductaline steel castings are employed to lift the roof plane above the beams and achieve a floating appearance. The double-glazed, mullion-free structural glass airside façade is supported by the same castings, which are mounted on the elliptically shaped glulam columns. The architect also used the castings for benches along the departure lounge airside glazing.

The $12.6-million project received numerous architectural and wood industry awards for the simplicity in the design and engineering details.

The mixed use of concrete, wood, and large windows provides an esthetically pleasing structural building for the Prince George Airport (also pictured on the cover). Wood brings warmth to the interior space and welcomes passengers arriving or departing. Photo courtesy McFarlane Green Architecture & Design
The mixed use of concrete, wood, and large windows provides an esthetically pleasing structural building for the Prince George Airport (also pictured on the cover). Wood brings warmth to the interior space and welcomes passengers arriving or departing.
Photo courtesy McFarlane Green Architecture & Design

Raleigh-Durham International Airport
North Carolina’s Raleigh-Durham International Airport is a major international hub servicing the southeastern United States. The three-phase, three-storey Terminal C expansion of the airport included:

  • a new 122 x 137-m (402 x 450-ft) terminal;
  • a 27 x 229-m (90 x 750-ft) concourse to the north providing new gates and connecting to other terminals; and
  • a 27 x 244-m (90 x 800-ft) south concourse providing additional departure gates.

In all, the Fentress Bradburn Architects project added almost 93,000 m2 (more than 1 million sf) to the existing airport complex.

While the lower structure consists primarily of structural steel construction, the roof consists of undulating king-post glulam and steel trusses with spans of 27 and 48 m (90 and 156 ft)—constituting the largest timber and steel hybrid roof structure in North America at approximately 29,730 m2 (320,000 sf). The terminal and concourse roofs step up and down to form clerestories, adding complexity to the truss structure.

The project’s engineers of record, Ove Arup (New York) and Stewart Engineering (Raleigh), originally tendered the massive composite steel and glulam king-post truss structure with glued-in rod connections, based on Ove Arup’s successful use of the system in Australia. However, the timber package was over budget and was
re-tendered under a design-build format.

This view of the Raleigh-Durham concourse showcases the long glulam spans used in the airport as the structural building material.
This view of the Raleigh-Durham concourse showcases the long glulam spans used in the airport as the structural building material.

The core of the value-engineering recommendations consisted of suggestions to replace the glued-in rod connections with a proprietary German connection system, first implemented in North America by Equilibrium Consulting in 2002. (Equilibrium worked with Lysagh Engineering on this airport project.) The system consists of a cast steel insert, secured by intersecting tight-fit steel pins, which are grouted into place with a high-flow, high-performance cement slurry. The resulting connections are concealed, and therefore fire-resistant, tight-fit, efficient, and reliable.

Using the German connections simplified the CNC fabrication requirements from an estimated 17 months to 11, while providing the streamlined connection details needed by the architect. The system’s established track record also helped reduce the requirements for full-sized connection testing. These were the two most important factors in bringing the timber structure package on budget.

All king-post truss top-chord elements were designed with moment fixity to the steel sections at the king post and support locations. Dealing for the shrinkage of the 0.8- and 1.4-m (2 ½- and 4 ½-ft) deep glulam sections, rigidly connected to the non-shrinking steel elements presented a significant detailing challenge.

At Prince George, the double-glazed mullion-free structural glass airside façade is supported by sand-cast ductaline steel castings, which are mounted on the elliptically shaped glulam columns. Photo courtesy McFarlane Green Architecture & Design
At Prince George, the double-glazed mullion-free structural glass airside façade is supported by sand-cast ductaline steel castings, which are mounted on the elliptically shaped glulam columns.
Photo courtesy McFarlane Green Architecture & Design

Shrinkage at the king-post location was dealt with by allowing the upper portion of the connection to slide freely over the king-post stub, while allowing the out-of-plane moment to be transferred directly to the roof purlins, as the 6.1-m (20-ft) deep trusses are free of bridging. Shrinkage at the truss tails was dealt with by reinforcing the glulam sections with long self-tapping lag screws and waxing the end grain of the arch to reduce friction stresses, forcing the wood and German connections (in slotted holes) to slide against the steel bearing plate despite the high dead-load reactions.

Conclusion
Airports are gateways to communities, regions, and countries. They are usually impressive, but a key factor to these public spaces should also be welcoming. Whether they are small regional facilities or large international hubs, airports, like many other public buildings, are successfully being built with high-quality wood products. Wood in airports is used for the main structure as well as for finishes, achieving modern design yet warm and beautiful facilities.

Eric Karsh, M.Eng., P.Eng., Struct.Eng., MIStructE, Ing., co-founded the structural firm, Equilibrium Consulting, which has worked on award-winning projects such as the Raleigh-Durham Airport expansion and the Art Gallery of Ontario (AGO) Galleria Italia by Architect Frank Gehry. Originally from Québec, Karsh began his structural consulting career in Ottawa in 1987 and moved to Vancouver in 1993, where he was first introduced to timber engineering. Karsh has engineered numerous innovative projects using various forms of solid-wood construction, including Dowling Residence—the first all-cross-laminated timber (CLT) building in North America, and the University of British Columbia’s (UBC’s) Earth System Sciences Building. He can be contacted via e-mail at info@eqcanada.com.

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