9.4 Wood Systems: Light-Frame, Heavy Timber & Mass Timber (CLT)

9.4 Wood Systems: Light-Frame, Heavy Timber & Mass Timber (CLT)

Wood is humanity's oldest renewable structural material and remains the dominant framing method for residential and light commercial construction across North America. In recent years, advances in timber engineering have transformed wood from a material limited to low-rise light-frame buildings into Mass Timber—a high-performance structural medium capable of framing high-rise commercial flagships and multi-story institutional complexes under modern building codes.

On the ARE 5.0 PPD exam, candidates must master wood natural anisotropic material behavior, differentiate platform from balloon framing, enforce IBC fire blocking and draft stopping mandates, calculate Heavy Timber fire charring rates, and evaluate Mass Timber classifications (CLT, Glulam, DLT, NLT) under the 2021 International Building Code.

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Wood Orthotropy & Moisture Physics

Unlike structural steel, which is isotropic (behaving identically in all directions), wood is an anisotropic and orthotropic cellular material. Its structural properties, fiber stiffness, and dimensional stability differ across three mutually perpendicular geometric axes:

1. Longitudinal Axis (Parallel to Grain): Aligned with the vertical tree trunk. Wood possesses exceptional tensile and compressive strength parallel to the grain.

2. Radial Axis (Perpendicular to Growth Rings): Radiating outward from the central pith toward the bark.

3. Tangential Axis (Perpendicular to Grain, Tangent to Growth Rings): Tangent to the annual growth rings.

Under transverse loading, wood is extremely weak in tension perpendicular to grain. Connections that introduce cross-grain tension (such as hanging heavy mechanical loads from the bottom of wood beams without top-flange straps) frequently cause catastrophic splitting along the wood grain.

Moisture Content & Differential Shrinkage

Wood is hygroscopic, absorbing and desorbing moisture to reach equilibrium with surrounding relative humidity:

• Fiber Saturation Point (FSP): Occurs when all cell cavities are empty of free water but cell walls remain saturated with bound water (typically 28% to 30% moisture content).

• Dimensional Movement Below FSP: As wood dries below FSP, water leaves cell walls, causing physical shrinkage. Wood dimensional movement varies dramatically by axis:

  • Tangential Shrinkage: Greatest movement, typically 6% to 10% from green to oven-dry.
  • Radial Shrinkage: Moderate movement, roughly half of tangential shrinkage (3% to 5%).
  • Longitudinal Shrinkage: Negligible movement along the grain length, typically 0.1% to 0.2%.

• Architectural Detailing Implication: In multi-story wood platform framing, cumulative shrinkage across horizontal wood elements (sole plates, rim joists, floor joists, top plates) can cause vertical building settlements of $1/2\text{ to }3/4\text{ inch}$ per floor. Unless detailed with telescoping expansion joints, this movement shears rigid plumbing risers, cracks exterior masonry veneer ties, and buckles exterior cladding.

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Light-Frame Wood Construction

Light-frame wood construction utilizes nominal dimension lumber ($2\times4, 2\times6, 2\times8, 2\times10, 2\times12$) or engineered wood joists spaced at $16\text{ or }24\text{ inches}$ on center, covered with structural plywood or oriented strand board (OSB) sheathing panels ($4\text{ ft} \times 8\text{ ft}$) to form structural diaphragms and shear walls.

Platform Framing vs. Balloon Framing

• Balloon Framing (Historical): Wall studs run continuously from the foundation sill plate through two full stories to the roof rafters. Floor joists are notched or hung from an intermediate ribbon board let into the continuous studs. Balloon framing has been universally abandoned in modern construction because tall open stud bays act as continuous chimneys (flues) during a fire, spreading flames rapidly from basement to attic, and long straight solid lumber is difficult to source.

• Platform Framing (Modern Universal Standard): Each story is constructed as a separate structural platform. Subfloor sheathing is laid over floor joists, and wall framing studs (one story in height) are erected directly on top of the subfloor deck. Platform framing naturally provides fire stopping at each floor line, simplifies wall prefabrication on the deck, and uses shorter, economical lumber lengths.

Engineered Wood Products (EWP)

• Wood I-Joists: Composed of top and bottom solid sawn lumber or laminated veneer lumber (LVL) flanges grooved to receive a structural OSB or plywood web. Reduces variability in warping, crown and shrinkage compared with typical sawn members, while remaining moisture-sensitive; spans up to $26\text{ ft}$ with depths of $9.5\text{ to }16\text{ inches}$ ($L/18$). Holes can be cut in webs per manufacturer knockout guidelines, but flanges must never be notched, cut, or drilled.

• Laminated Veneer Lumber (LVL): Manufactured by bonding thin wood veneers ($1/10''\text{ to }1/8''$) with waterproof adhesive, with grain running parallel in all plies. Used for heavy floor girders, window/door headers, and ridge beams.

• Glue-Laminated Timber (Glulam): Individual dimension lumber laminations ($1.5\text{ in}$ thick for straight members, $0.75\text{ in}$ for curved shapes) bonded with structural adhesives. Glulam can be manufactured in virtually unlimited depths and lengths ($20\text{ to }100+\text{ ft}$), with higher-grade laminations positioned at top and bottom outer faces where flexural tension and compression peak.

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Fire Safety & Life-Safety Mandates for Light-Frame Wood

Under the International Building Code, light-frame wood construction is categorized as Type V (combustible interior and exterior framing) or Type III (combustible interior framing with noncombustible exterior masonry or concrete walls, typical of urban 5-over-1 or 5-over-2 podium buildings over a Type I-A concrete base). Because light-frame assemblies contain extensive concealed hollow spaces, the IBC mandates strict passive fire containment:

Fire Blocking (IBC Section 718.2)

Solid wood blocking ($2\text{-inch}$ nominal lumber), structural panels, mineral wool, or gypsum board must be installed within concealed spaces to cut off concealed draft openings:

• In concealed wall cavities horizontally at ceiling and floor levels.

• At required floor and ceiling levels and in specified concealed-space locations; the common 10-foot interval concerns horizontal concealed-space separation, not a replacement for vertical floor-level fireblocking.

• At all interconnections between vertical stud cavities and horizontal spaces created by soffits, dropped ceilings, cove ceilings, and stair framing.

Draft Stopping (IBC Section 718.3 & 718.4)

Draft stopping divides large concealed horizontal floor-ceiling plenums and attic spaces to prevent smoke and heat travel:

• Floors: In non-sprinklered buildings, floor-ceiling spaces must be divided into compartments not exceeding $1,000\text{ square feet}$. In sprinklered buildings, draft stopping in floors is generally waived.

• Attics: In non-sprinklered Group R-1 and R-2 occupancies, attics must be divided into compartments not exceeding $3,000\text{ square feet}$ or divided along tenant separation demising walls.

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Heavy Timber Construction (IBC Type IV-HT)

Heavy Timber construction (IBC Type IV-HT) utilizes large solid-sawn or glued-laminated timber elements without concealed combustible spaces. Heavy timber structures exhibit exceptional inherent fire endurance due to the predictable thermal physics of wood charring.

Prescribed Minimum Nominal Dimensions (IBC Chapter 6 / Table 2304.11)

To qualify as Type IV-HT, structural members must meet or exceed strict minimum cross-sectional dimensions:

• Columns Supporting Floors: Minimum $8'' \times 8''$ nominal ($7.5'' \times 7.5''$ actual).

• Floor Beams & Girders: Minimum $6'' \times 10''$ nominal ($5.5'' \times 9.5''$ actual).

• Roof Columns: Minimum $6'' \times 8''$ nominal ($5.5'' \times 7.5''$ actual).

• Roof Beams & Arches: Minimum $4'' \times 6''$ nominal ($3.5'' \times 5.5''$ actual).

• Wood Floor Decking: Minimum $3\text{ inches}$ nominal solid sawn tongue-and-groove splined planks or $4\text{ inches}$ nominal laminated decking, covered with a $15/32\text{-inch}$ wood structural panel or $1\text{-inch}$ finished flooring.

• Concealed spaces: Type IV-HT has specific limitations and exceptions. Consult the 2021 provisions for permitted concealed spaces and their required protection instead of treating all concealment as prohibited.

The Charring Phenomenon & Fire Resistance

When exposed to standard ASTM E119 fire temperatures exceeding $550^\circ\text{F}$ ($290^\circ\text{C}$), the outer surface of heavy timber pyrolyzes, forming an expanding layer of porous charred carbon. This char layer acts as a natural, highly effective thermal insulator with extremely low thermal conductivity:

• Predictable Char Rate: Structural wood chars at an empirical rate of approximately $1.5\text{ inches per hour}$ ($0.025\text{ to }0.030\text{ inches per minute}$ for Douglas Fir and Southern Pine).

• Reduced structural section: The char layer insulates wood behind it, but the fire design must account for effective char depth, the heated zone, residual section, adhesives and connections. The entire uncharred core cannot simply be assigned 100% ambient strength and stiffness.

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Mass Timber Typologies & The 2021 IBC Expansion

The 2021 International Building Code expanded Type IV construction from a single category (Type IV-HT) into four distinct construction types, creating a revolutionary framework for tall mass timber buildings:

For Group B with the qualifying sprinkler conditions in the 2021 tables, IV-A can reach 18 stories/270 feet, IV-B 12/180, IV-C 9/85, and IV-HT 6/85. Apply the detailed protection and exposure rules, not these dimensions alone. IV-A has extensive protection with specified exceptions; IV-B limits exposure, IV-C permits broader interior exposure, and IV-HT includes qualifying CLT and its own concealed-space provisions. Limits differ for other occupancies.

Engineered Mass Timber Products

• Cross-Laminated Timber (CLT): Fabricated by bonding alternating orthogonal layers (typically 3, 5, 7, or 9 plies) of kiln-dried dimension lumber with structural polyurethane adhesive. Alternating perpendicular grain orientations provide two-way spanning slab capabilities, high dimensional stability (reducing cross-grain dimensional movement), and high in-plane diaphragm shear capacity.

• Nail-Laminated Timber (NLT): Dimensional lumber stacked on edge and mechanically fastened with continuous nails or spikes. Excellent for one-way floor and roof decks.

• Dowel-Laminated Timber (DLT): Dimensional lumber stacked on edge and friction-locked with dried hardwood dowels. Contains zero metallic fasteners or glue adhesives, allowing rapid cutting with CNC timber routers.

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High-Performance Mass Timber Connections

Traditional exposed steel bucket connections with large external through-bolts conduct fire heat rapidly into the interior of the timber, causing early joint failure. Modern mass timber engineering utilizes concealed high-performance connections:

• Concealed steel connections can place insulating wood cover around hardware, but required cover, fasteners, plug details and fire endurance must be established by an approved design or test. A generic 1½–2-inch cover does not automatically create a 1- or 2-hour connection rating.

• Fully Threaded Self-Tapping Screws (STS): Engineered high-tensile steel screws installed at 45-degree angles to take combined shear and axial withdrawal forces, creating rapid, high-stiffness connections across mass timber panels.

Structural spans, member-depth ratios and preliminary capacities in this section are design-screening assumptions, not completed structural designs. Check loads, serviceability, fire rating, vibration, connections, bracing, durability and constructability with the engineer. IBC 2021 references ASCE 7-16; an exam-provided reference or a locally adopted edition may differ. Never transfer a numerical limit between editions or structural systems without checking its scope.