Timber Structural Systems: Sawn Lumber, Glulam, and Mass Timber

Wood is the oldest structural material still in widespread use and, uniquely among the structural materials engineers work with regularly, it is renewable. A Douglas fir tree felled today is replaced by seedlings growing now; no carbon emissions from ore reduction or kiln firing are required to produce the material. This environmental profile has made timber the center of significant structural innovation in the past two decades, with engineered wood products enabling building forms and heights that would have been unimaginable with sawn lumber alone. Understanding the material properties and structural systems that govern timber design requires starting with what wood actually is and then working up through each product type.

Wood as a Structural Material

Wood is a cellular biological composite with distinctly different properties parallel and perpendicular to the grain direction. Parallel to grain, wood is strong in both tension and compression, with design values for Douglas fir-larch No. 2 lumber around 1,000 psi in bending (F_b = 900 psi for 2x4 repetitive member use) and 1,350 psi in compression (F_c parallel to grain). Perpendicular to grain, wood is much weaker: compression perpendicular to grain (F_c⊥) for Douglas fir-larch is around 625 psi, and tension perpendicular to grain is so low and variable that no design value is tabulated. This means that structural details must avoid configurations that put tension perpendicular to grain in bearing or connection zones.

Moisture content profoundly affects wood properties. Wood sold as dry lumber has been kiln-dried to a moisture content of 19 percent or less (KD19) or 15 percent or less (KD15). As wood gains moisture above the fiber saturation point (approximately 28 to 30 percent for most species), it loses strength. At the same time, wood shrinks when it dries below the fiber saturation point, with shrinkage occurring primarily perpendicular to grain (radial and tangential shrinkage) and almost negligibly parallel to grain (longitudinal shrinkage is typically only 0.1 to 0.2 percent from green to oven-dry). This differential shrinkage causes checking, splitting, and dimensional changes in constructed assemblies.

Fire resistance is counterintuitive in timber. Heavy timber members, with minimum dimensions of 6 inches (150 mm), char at a predictable rate of approximately 1.5 inches per hour for most softwood species. The char layer insulates the unburned core, and the heavy timber continues to carry load during the fire event with a reduced cross-section. This is why heavy timber construction is accepted as an alternative to non-combustible construction in the building code for many occupancies. Light-frame wood stud construction, by contrast, relies on gypsum board fire protection to delay ignition; once the gypsum fails, the small studs burn quickly.

Sawn Lumber Light-Frame Construction

Platform frame construction using dimension lumber (nominal 2x4, 2x6, 2x8, 2x10, or 2x12 members) is the dominant structural system for residential construction and low-rise commercial buildings in North America. Its efficiency comes from the repetitive member concept: closely spaced studs, joists, and rafters at 12, 16, or 24 inches on center create many parallel load paths. The allowable bending stress for a repetitive member (three or more members at 24 inches on center or less sharing load through sheathing) is increased by a repetitive member factor C_r = 1.15 from the tabulated design value.

Wood structural panel sheathing (plywood or oriented strand board) nailed to the framing acts as the diaphragm and shear wall system that resists lateral loads. Horizontal diaphragms at floor and roof levels collect wind and seismic forces and distribute them to vertical shear walls below. Shear wall capacity is expressed in pounds per linear foot of wall length; higher nail schedules and thicker panels increase capacity. The hold-down anchor at the end of each shear wall segment is often the critical connection: it must resist the net uplift at the tension end of the wall without allowing the wall to overturn.

Glued Laminated Timber (Glulam)

Glulam is manufactured by finger-jointing and face-gluing dimension lumber laminations (typically 1-3/8 or 1-1/2 inch thick for softwood) into larger sections. Because laminations are dried individually before gluing, the large finished member has lower moisture content variation than a sawn timber of the same size. Laminations are sorted and oriented so that higher-grade material is placed at the outer tension and compression zones where bending stress is highest, and lower-grade material fills the interior where stress is lower. This arrangement is called combination layup and is more structurally efficient than using uniform grade throughout.

Glulam beams are available in standard widths of 3-1/8, 3-1/2, 5-1/8, 5-1/2, 6-3/4, and 8-3/4 inches, with essentially unlimited depth in 1-1/2 inch increments. Spans of 40 to 100 feet (12 to 30 m) are practical, making glulam the material of choice for long-span roof structures such as gymnasiums, arenas, and church sanctuaries. Glulam can be curved during manufacture, allowing elegant arched forms that would require complex joinery in sawn timber. Curved glulam is described by its radius of curvature; the minimum radius depends on lamination thickness to avoid overstressing the laminations during bending in the press.

Mass Timber: CLT, LVL, and NLT

Cross-laminated timber (CLT) is a panel product made by gluing layers of dimension lumber in alternating perpendicular directions, analogous to plywood but at a much larger scale. Standard CLT panels are available in widths up to 60 inches (1.5 m) and lengths up to 60 feet (18 m), with thicknesses of 3-ply (3-1/8 inch), 5-ply (5-1/8 inch), 7-ply (7 inch), and up. The cross-laminated configuration gives CLT two-way bending capacity, enabling it to span in two directions as a floor plate without additional joists, similar in concept to a two-way concrete slab.

CLT floors carry loads by bending across the span in the direction of the face laminations. Out-of-plane shear in CLT is governed by the rolling shear capacity of the cross laminations, which is a failure mode specific to wood loaded in shear parallel to the glue lines. Rolling shear strength for CLT is approximately 215 to 350 psi depending on species and layup, which is substantially lower than the parallel-to-grain shear strength. Designers must check rolling shear at CLT panel supports, particularly at shorter spans where shear rather than bending governs the design.

Laminated veneer lumber (LVL) consists of thin wood veneers glued with all grain running parallel to the member length. The parallel grain orientation maximizes bending and tensile strength but eliminates the cross-grain stability of CLT. LVL is used as beams and headers where high bending strength in a single direction is needed, often substituting for multiple plies of sawn lumber in a single engineered product with more predictable properties.

Nail-laminated timber (NLT) and dowel-laminated timber (DLT) are older panel systems experiencing renewed interest. NLT consists of dimension lumber placed on edge and nailed face-to-face in a single direction, used as one-way floor decks. DLT achieves composite action between the laminations through hardwood dowels rather than nails or glue, permitting disassembly and reuse of components. Both products have lower bending strength than CLT of equivalent thickness because there is no cross-lamination to activate two-way behavior, but they offer simpler and sometimes less expensive manufacturing.

Mass Timber Building Systems and Connections

Tall wood buildings using CLT floors and glulam columns and beams have been constructed to eighteen stories in some jurisdictions, with the 2021 International Building Code permitting mass timber occupancy categories up to 18 stories under specific prescriptive requirements. The fire resistance of the exposed mass timber provides the required ratings when the char capacity of the section is analyzed for the required rating duration, eliminating the need to cover the wood with gypsum in many applications.

Connections are the most challenging design element in mass timber systems. Traditional timber joinery used mortise-and-tenon or dovetail joints cut in the wood itself; modern heavy timber and mass timber connections use steel knife plates, angles, and dowels concealed within routed slots. Hidden connections maintain the visual appearance of exposed wood while achieving the required load transfer. Seismic design of mass timber requires carefully detailed ductile connections because wood itself is brittle; the design philosophy places the inelastic behavior in the steel connection hardware rather than in the timber members.