Remember the plastic ball and stick molecule models from high school chemistry class? Teachers were particularly fond of showing how carbon forms two different minerals—diamond or graphite—depending on the alignment and bonding of the individual atoms.
Along the same lines, one material—trees—can become two different building materials—sawn lumber and engineered wood products (EWPs)—depending on the manufacturing process.
Most people know how solid sawn lumber is produced, but may not be as familiar with how EWPs are made. For EWPs, manufacturers peel logs into veneer or shave them into strands, which are coated with durable adhesives and aligned or pressed to create a structural building product that is uniform, predictable and consistent.
While contractors frequently use EWPs in single-family homes, the products’ performance characteristics also are well-suited to solving construction challenges in multifamily structures, motels, dormitories and assisted living facilities. EWPs have desirable environmental attributes and provide a cost-effective alternative to steel and concrete for tall walls (walls taller than 10 feet), long-span ceilings and floors, and lateral bracing against earthquakes and high winds.
In the past year, more construction firms have used wood framing, including EWPs, to reduce costs and building cycle time while meeting demanding structural needs. For example, developers of a five-story apartment complex in Arlington, Va., switched from concrete to Type III wood construction for an estimated 40 percent savings on total construction costs. In London, builders recently completed the world’s tallest mixed-use wood structure, which cost approximately 15 percent less per square foot than concrete. The wood design eliminated the need for a tower crane and reduced the construction schedule by more than five months.
Many Shapes and Sizes
Manufacturers typically make I-joists with EWP flanges and an oriented strand board (OSB) web between the flanges.
These prefabricated wood I-joists often are installed in floors and in place of rafters for sloped and flat roofs.
The engineered materials provide exceptional dimensional stability, which helps I-joists resist warping, twisting and shrinking. The “I” shape also is structurally efficient and can do more work while using less material than a typical rectangular joist.
Laminated strand lumber (LSL) is made from long, thin wood strands that are dried and bound with resin. Manufacturers align the strands to create a more efficient configuration that capitalizes on the wood’s natural strength. Common LSL framing members include beams, headers, rim board, stair stringers and wall studs.
An economical choice for many light construction applications, LSL beams create smooth and straight walls for studs and provide a solid base for the installation of cabinets, countertops and lighting.
Parallel strand lumber (PSL) is manufactured from long, narrow veneer strips bonded together with adhesives using a press and a microwave heating process. The result is one of the strongest EWPs available.
Ideal for applications requiring long spans or heavy loads, the wide-section, one-piece beams eliminate field assembly. Due to their large cross-sections, long lengths and weight, PSL beams typically require a crane to be placed safely.
Structural Applications
EWPs provide solutions for the challenging structural requirements common in wood-framed multifamily and light commercial buildings. These challenges include tall walls, open floor plans with long-span ceilings and floors, and heavier code-mandated design loads.
Walls taller than 10 feet—with numerous windows—are common in motels and apartments, and ancillary facilities such as clubhouses, conference centers and recreation rooms.
In many markets, tall wall designs use platform framing (i.e., stacked walls) because conventional studs are not readily available in lengths longer than 20 feet. In foyers, great rooms and other tall wall applications that are not braced by a floor system, a hinge point is created, which can cause drywall cracks and leaks around windows.
Contractors can use LSL studs to incorporate single-length framing members in walls up to 30 feet high. These continuous pieces avoid the hinge point and are straight along their entire length. The result is clean wall lines and minimal culling at the jobsite.
Many light commercial buildings require open floor plans with uninterrupted spaces. Absent massive timbers, sawn lumber beams often are not up to the task due to insufficient length or girth.
PSL solves these challenges with single-piece, high-strength, long-length beams. LSL beams, which cost less, also are a desirable choice for long-span floors or roofs when the demands of the application do not require the higher strength of PSL.
Wood I-joists also come in long lengths, which help create open floor plans with fewer intervening walls or columns.
Another structural frame issue in many buildings is providing adequate lateral-force resistance against earthquakes and high wind loads without compromising aesthetic design or the functional layout.
In wood-framed buildings, contractors typically rely on site-built shear walls using OSB or plywood sheathing. This works well for wall segments 4-feet wide or greater. However, for narrower wall segments created by numerous window and door openings, prefabricated wood bracing panels made of LSL are an optimal solution. In some cases, a 12-inch-wide prefabricated panel can be substituted for a conventional 4-foot by 8-foot OSB or plywood shear wall section.
Some prefabricated wood bracing panels can be used in a stacked configuration for multi-story applications or are available in single-piece heights up to 20 feet.
A Return to Wood
In the past, building professionals adopted materials other than wood to meet the structural demands imposed by larger buildings. Using a life cycle analysis, wood products have shown better environmental performance than steel or concrete frames. (For more information, refer to the Consortium for Research on Renewable Industrial Materials at
www.corrim.org.)
Through technology, EWPs maintain cost effectiveness, ease of use and availability while providing strength and long lengths comparable to steel and concrete. In an economy in which tight margins demand faster construction and lower material costs, wood framing with EWPs can help make the difference in profitability.