September 1, 2012
By Eric Karsh, M.Eng., P.Eng., Struct.Eng., MIStructE, Ing.
Before the 1850s, wood was commonly used as a primary structural building material in myriad types of non-residential construction around the world. Many of these timber-built structures remain standing and are still in use today, including factories, warehouses, schools, temples, and churches—some dating as far back as the seventh century (e.g. the 32.5-m [107-ft] high Horyu-ji Temple in Nara, Japan), demonstrating the durability and strength of building with wood.
However, the construction of landmark projects such as the Eiffel Tower in 1887 and the Empire State Building in 1929, in addition to technical steel and concrete advances during the industrial revolution, quickly resulted in those materials becoming the default structural choices for large public building construction. Timber engineering has experienced rapid and transformative advances over the past two or three decades, and continues to quickly evolve, making it ready for a comeback.
Many architects and engineers evaluate wood as a viable alternative for the construction of large-scale, non-residential structures. Technical advancements—including new engineered wood products, computer-numerical-controlled (CNC) fabrication, versatile high-efficiency timber connectors, and progress in fire engineering—have contributed significantly to timber’s rise to the forefront of the industry as a construction material. The cost advantages of wood construction are also changing as a result. Further, as the world’s population continues to grow, there is renewed architectural interest in taller buildings made of wood, now made possible by the development of solid wood construction.
Several recent global studies indicate timber structures exceeding 15 storeys (possibly reaching 30) could soon become reality. This trend has also been spurred in part by the need to use sustainable materials in construction; wood is the only major building material that grows naturally and is renewable. Lifecycle assessment (LCA) studies also consistently show wood products can offer clear environmental advantages over concrete and steel in terms of embodied energy, air/water pollution, and greenhouse gas (GHG) emissions. (For more information on wood and environmental attributes, see the article, “A Deeper Shade of Green: Using Wood as a Sustainable Structural Material,” by Michelle Kam-Biron, SE, and Lisa Podesto, PE, in the November 2010 issue of Construction Canada. Visit www.constructioncanada.net and select “Archives.”)
As buildings that introduce visitors to a community and welcome residents home, airport facilities’ architectural design is critically important. The examples in this article showcase wood as a primary structural building material in significantly large spaces, demonstrating its versatility, cost-effectiveness, ability to meet code, and potential to bring warmth and a connection with the natural world through design.
Cranbrook/Canadian Rockies International Airport
Cranbrook is a small community located near the south end of the Alberta–British Columbia border. In 2006, a significant terminal building expansion of the existing 1970s facility was commissioned to accommodate larger aircrafts and more passengers. These changes included an extension to the existing runway and an expansion of the terminal building, along with a series of upgrades to the fuelling, baggage handling, and related operational areas. Of significant interest is the Canada Customs and Immigrations facility, upgraded to ensure timely processing of international passengers. Completed in 2008, the new terminal building expansion has an area of 2044 m2 (22,000 sf).
The existing terminal, considered small by airport standards, was one of a few B.C. airport facilities built in the 1970s with light wood-frame construction. The original structure consists of an open-steel-web engineered wood truss roof on load-bearing masonry walls and piers. The main floor structure, framed over a partial basement and crawl space, also consists of light wood framing.
The airport had to remain operational throughout construction of the expansion, adding considerable complexity to the work. The $6.3-million project was rolled out in a three-phase timeline:
The design team chose light wood-frame construction for several reasons, including the material’s economy, flexibility, lightness, and compatibility with the original structure. Several architecturally expressed structural steel elements were introduced to replace the masonry walls and piers, opening up the air-side façade. The new facility is mostly made of glass and wood to ensure the stunning landscape of the Rocky Mountains is prominently positioned for viewing.
Prince George Airport
Prince George is a mid-sized, forestry-based community located in central British Columbia. Its airport serves as a hub to the Canadian Northwest and handles a significant amount of regional and international commercial traffic, linking the eastern provinces to Asia. The city also attracts the tourism industry and welcomes numerous flights of outdoor enthusiasts from North America and Europe.
Built in 1970, the original airport structure consists of load-bearing masonry walls and piers, along with a structural steel main floor and roof structure. Given the local connection to forestry, the airport authority wanted to introduce timber as the primary structural material in the two-phase expansion. Using wood in the airport was a way to represent the community and showcase Prince George’s identity and values.
The first phase of the McFarlane Green expansion included adding a departure lounge and baggage handling facility on the airside, north of the original facility; it consists of Douglas fir glued-laminated (glulam) timber post-and-beam construction. The second phase, to the west of the original terminal, includes the addition of an international arrival area on the airside, also constructed with glulam post and beam. For structural and architectural reasons, steel was used for the customs office to the south.
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.
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:
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.
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.
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.
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 email@example.com.
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