by Anton D. Pugel, PhD
Since the introduction of plywood and glued-laminated timber (glulam) beams more than a century ago, engineered wood has continuously progressed to provide greater strength, versatility, consistency, and many other attributes. Today, the engineered wood category encompasses oriented strand board (OSB), strand and fiber siding, laminated strand lumber (LSL) I-joists, laminated veneer lumber (LVL), and mass timber products. These building materials allow longer clear spans, greater energy efficiency, and faster, more economical construction. In commercial construction, four- and five-story wood office buildings are common, with the tallest wood buildings now reaching 20 stories.
As customer tastes change and building requirements allow wider use of wood in building construction, engineered wood products find more applications in Type 1 (fire-resistive), Type 3 (ordinary), Type 4 (heavy timber), and Type 5 (wood-framed) buildings. Some of the latest trends in repurposing old warehouses and large retail stores into light manufacturing and office spaces incorporate many engineered wood products. These materials are often specified for schools, warehouses, restaurants, and hotels in features such as:
- panelized roofs;
- concrete formwork;
- diaphragm assemblies to resist wind and earthquake loads; and
- noise- and firer-rated assemblies.
Engineered wood products are manufactured by cutting, peeling, or stranding to make various shapes and sizes. These pieces of wood are then bonded back together using exterior adhesives to make panels or beams. This manufacturing process improves the products’ strength and stability while reducing variation and defects. In the course of this manufacturing process, increased durability, fire resistance, and radiant insulation can also be added to the products.
Engineered wood materials offer many advantages over traditional designs, including:
- exceptional strength and consistent dimensions, with much less shrinking, warping, and cupping than traditional lumber;
- better structural values providing longer spans or more efficient use of the material—typically, defect dispersion and removal reduces the natural variability of wood, while the selection and orientation of the engineered wood elements drive higher structural values;
- moisture contents representative of the interior conditions of finished structures, meaning more dimensional stability and less trapped moisture in walls and roofs;
- long lengths and wide widths to reduce the number of pieces purchased and handled in construction, with dimensions customizable in many cases to meet specific needs on the jobsite;
- straight lengths with defects removed or dispersed to reduce jobsite waste (a purchased unit should allow 100 percent utilization); and
- more sustainable and environmentally responsible qualities, as many engineered wood products are made from smaller, sustainably sourced trees rather than from large, old-growth trees, and the manufacturing process uses the entire log, including the bark and sawdust for energy. There may be more credits for wood-based materials when assessing compliance with green building codes.
Engineered wood joists, beams, and headers
Engineered wood plays a dominant role for numerous construction applications, beginning with concrete forming and structural framing. Wood components can speed up construction, due to ease of cutting and fastening. Additionally, wood structural components are often left exposed to create architectural or aesthetic features.
Laminated veneer lumber is composed of graded veneers bonded in the lengthwise direction. Used for beams, headers, rim boards, and I-joist flanges, LVL has far greater dimensional consistency and load-carrying capacity than traditional lumber. It is available in lengths greater than 15 m (50 ft) and up to 89 mm (3 ½ in.) thick, and resists warping, twisting, crowning, and bowing. LVL itself is often laminated together, forming thicker beams that enhance spans and increase load-carrying capacity.
Indeed, the material is engineered and manufactured for exceptional strength, straightness, and durability. To make it, ultrasonically graded veneers are arranged in specific patterns to maximize strength and stiffness, then bonded with exterior-grade adhesives under pressure and heat, resulting in low moisture content for dimensional stability.
Engineered wood I-joists use LVL for the flange and OSB for the web. They are lightweight, straighter, and more uniform in strength, stiffness, and size than traditional lumber. Holes are allowed through the interior of I-joists to allow access for plumbing and electrical runs. They also offer wide flanges for greater stability and easier nailing. Fire-retardant-treated (FRT) I-joists are also available.
Laminated strand lumber products are composed of long, thin wood strands from underutilized tree species. LSL uses a manufacturing process that provides superior performance with less shrinking, warping, and twisting. It is made from a mix of sustainably harvested aspen and maple hardwoods for greater strength, employing a steam injection process resulting in low moisture content (seven to 10 percent). Its ends and edges are sealed to reduce swelling.
LSL is used for studs, joists, rim board, beams, and headers. This wood product installs like traditional lumber, but surpasses it in both strength and consistency. Zone framing (e.g. hallways) and tall walls are especially suited to LSL’s lengths and uniform dimensions.