New ways to work with wood
New products like cross-laminated timber are spurring interest in wood as a structural material.
Although wood is commonly used as a finish material in nonresidential buildings, more and more Building Teams are specifying wood as a structural material to accompany steel or concrete—or sometimes to be used entirely on its own. Recent studies show that wood offers such benefits as speed of construction, cost effectiveness, durability, and sustainability (see box, page 53). Structural wood products such as cross-laminated timber (CLT) and parallel strand lumber (PSL) are attracting the attention of architects, structural engineers, and contractors.
Research by the National Science Foundation and other partners on the performance of wood buildings during seismic events—the so-called NEESWood Capstone tests—is yielding promising results. For example, Japanese researchers built a seven-story wood-frame residential structure on a shake table and subjected it to Kobe-level earthquake conditions—6.9 on the Richter scale (see http://www.apawood.org/level_b.cfm?content=srv_newsinfo_34). “There was virtually no damage to the building,” says Dwight Yochim, national director of WoodWorks, an alliance of North American wood associations, headquartered in Vancouver, B.C.
Yochim says that one of the concerns with building with concrete or steel is the temptation to overbuild to compensate for the mass of the material itself. “Wood has many redundant connections and it’s light,” he says, “so when an earthquake hits, it doesn’t have the same impact on the building” as it would with a steel or concrete structure.
Yochim says some glulam manufacturers are adding steel straps to their beams to help carry the load over long spans. The Richmond Olympic Oval, built for the 2010 Winter Olympics in Vancouver, B.C., has a wood roof with a clear span of more than 300 feet. Part of the roof is constructed of double Douglas-fir glulam beams reinforced with steel straps.
“Wood is a viable building product and a good alternative to conventional construction,” says Blakely C. Dunn, AIA, NCARB, principal of CADM Architecture, El Dorado, Ark. “It’s easy to erect, and it’s easy to correct something that’s incorrect.” He notes that wood usually has a shorter lead time for delivery compared to other structural materials: “You don’t have to wait weeks for it to show up at the job site.”
Applications for structural wood expand
- Architects and engineers are substituting wood framing for steel and concrete as a cost-saving measure.
- Manufacturers are adding steel straps to glulam beams to help carry loads over long spans.
- Cross-laminated timber is making its debut in the U.S. as an innovative framing material for buildings up to 10 stories in height.
- Heavy-timber construction showcases the versatility, strength, and beauty of wood and can help keep projects on budget.
CROSS-LAMINATED TIMBER CROSSES THE POND
Cross-laminated timber has been widely used in Europe since the 1980s but is not well known in North America. CLT panels are manufactured by stacking multiple layers of wood, each about 20 to 38 millimeters in thickness, at right angles and gluing them together in a press. Typical widths are 0.6, 1.2, and 2.95 meters (up to 4 meters), in lengths up to 24 meters.
The cross-lamination process minimizes swelling and shrinkage and increases resistance. CLT panels are used for floor, wall, and roof systems. Pre-assembled wall sections can be lifted into place with cranes and attached to each other with screws or steel brackets.
“CLT competes head-to-head with concrete buildings up to six stories,” says Yochim. So far, the tallest CLT structure built in Europe is nine stories. Researchers in Austria are testing something called the life cycle tower, a combination of glulam beams, CLT, and concrete slabs that could go much higher. “They’re all prefabricated assemblies,” says Yochim. “Once you’ve got your foundation down, the rest of the building just bolts together.”
The first nonresidential CLT building to be constructed in the U.S., a 78-foot church bell tower, was completed in December 2010 in Gastonia, N.C. The tower has a 12x12-foot base and wood panels of varying lengths, which provide the strength and stability of concrete but are much lighter, says Michael DeVere, principal of MDS10 Architects, Asheville, N.C. The foundation is three feet deep.
To better analyze the stresses inflicted by wind and seismic loading and swinging bells, Medlock & Associates Engineering, Asheville, N.C., modeled the tower using RISA-3D design software. This enabled the engineers to keep the project within its $450,000 budget. For ease of assembly, they used a panelized system and kept connection variations to a minimum.
The panels were prefabricated in Austria, reducing the amount of on-site labor and virtually eliminating job-site waste. Tim Richards, vice president of general contractor M-Y Construction of Tryon, N.C., says a comparable steel structure would have taken three to four weeks to complete. With CLT, it took five-and-a-half days.
DeVere points out that CLT is also a green material, accounting for significantly less greenhouse gas emissions than concrete or steel. He also likes its creative nature. “It can free you from many of the constrictions of conventional construction,” he says. “Depending on the design, you can eliminate lintels and headers as well as columns and deep horizontal framing members.” Exterior wall panels distribute the bearing load evenly across the entire length, so most point loads can be dispersed, avoiding piers and pad footings and reducing the amount of concrete in the foundation.
“Some have described CLT as ‘Legos on steroids,’” says DeVere. “We see it as a game changer for the construction industry.” He and his business partner, Crawford Murphy, hope to open a CLT manufacturing facility in the U.S.
HEAVY TIMBER PLAYS LEAD ROLE IN THEATER
In British Columbia, the use of heavy timber in nonresidential projects is commonplace. But the concept raised eyebrows when first proposed for Arena Stage at the Mead Center for American Theater, in Washington, D.C.
“The local building authorities were skeptical at first about the use of timber for large institutional assembly buildings,” says Michael Heeney, MAIBC, FRAIC, LEED AP, principal of Bing Thom Architects, Vancouver, B.C. “They were concerned about flammability.” The firm and its fire engineers, LMDG, presented a fire report and char analysis which showed that the effects of a fire on the structure would be minimal. In fact, charring on the outside of the wood columns would actually protect the interior of the wood.
One of the project goals was to double the space of Arena Stage and the adjacent Kreeger Theater. There was no money in the budget for finishes, yet the structure had to be beautiful, so wood made perfect sense as both a structural and finish material. The architects wrapped the two theaters with an insulated glass wall, providing acoustic separation from nearby Reagan National Airport and highway traffic.
StructureCraft Builders, a specialty timber-frame design/builder based in Delta, B.C., crafted 18 giant columns out of parallel strand lumber (PSL) for the perimeter of the Arena Stage façade. The columns are unreinforced, solid engineered wood that use no internal steel support. Bing Thom Architects designed the kinds of connections used in a steel-frame building so that local steelworkers could install them.
“Wood is a very versatile material, but you need to spend time making the connections economical by encouraging as much repetition as possible,” says Heeney. “The PSL columns at Arena Stage connect to specially designed iron castings that would have been prohibitively expensive had we made only one.”
WOOD FRAMING SAVES $2.7 MILLION FOR SCHOOL
When the El Dorado (Ark.) School District needed a new high school for 1,600 students, the Building Team compared the cost of structural steel, precast concrete, and wood as a framing system.
Blake Dunn of CADM Architecture says the school’s construction budget was $134.78/sf. Had it been built with steel and masonry, the cost would have been $50/sf too high. Wood framing saved $2.7 million.
The original design intent—to use wood for exposed areas inside the building—was extended to concealed areas such as columns, beams, demising walls, office partitions, exterior walls, floors and roof systems. The structural components are predominately Southern yellow pine. Interior doors are maple; the paneling and trim are red oak. “The auditorium has large acoustical deflectors on the side walls that are made out of maple plywood,” he says. “They’re angled in such a way as to tune the space.” +
MORE ON THE BENEFITS OF STRUCTURAL WOOD
“http://fs.fed.us/news/2011/releases/09/green- building-report.pdf">Science Supporting the Economic and Environmental Benefits of Using Wood and Wood Products in Green Building Construction,” Michael A. Ritter, Kenneth Skog, and Richard Bergman, USDA Forest Service.
“Wood Products Used on the Construction of Low-Rise Nonresidential Buildings in the United States, 2008,” David B. McKeever, USDA Forest Service.
“Maximizing Forest Contributions to Carbon Mitigation” (CORRIM Fact Sheet, March 2009).
“Product and Process Environmental Improvement Analysis for Buildings (Carbon Life Cycle Assessment)” (CORRIM Fact Sheet, December 2009).