Wood and Mass Timber Structures: Light Frame, Heavy Timber, CLT, Glulam, and Fire Performance

Evaluation and selection of wood structural systems for building projects, including light wood frame, heavy timber, cross-laminated timber (CLT), glue-laminated timber (glulam), and their fire performance characteristics under IBC construction types.

ARE · Project Planning & Design

Overview

Wood is the oldest structural material in American construction, and it's also one of the most rapidly evolving. From conventional platform framing in residential buildings to cross-laminated timber panels reaching 18 stories tall, wood structural systems span an enormous range of scale, cost, and complexity.

For the ARE, you need to do more than identify these systems. You need to evaluate them against each other, weigh their cost and availability, assess their load capacity, and determine how they shape building design and configuration. That evaluation extends to fire performance, because IBC construction type classifications directly control where and how wood structures can be built.

This topic covers three major categories: light wood frame construction (platform and balloon framing), heavy timber and glue-laminated members, and mass timber systems including CLT. You'll learn how each system transfers loads, what spans and heights they support, and how fire-resistance ratings work for combustible materials. The connections between structural system selection and IBC construction types (especially the Type IV subcategories IV-A, IV-B, and IV-C) are central to making sound design decisions on the exam.

Selecting among these wood systems requires weighing span requirements, project scale, budget, regional availability of fabricators, and how the structural choice shapes allowable building height and exposed-wood aesthetics.

Essential

Wood structural systems fall into three broad families, each with distinct load paths, span capabilities, cost profiles, and fire behaviors. Selecting the right system means matching these characteristics to your project's size, occupancy, budget, and code constraints. ## Light Wood Frame Construction Light wood frame is the dominant structural system for residential and small commercial buildings in the United States. Two framing methods define this category. Platform framing is the modern standard. Vertical studs extend only one story high, and each floor platform acts as the base for the story above. This approach simplifies construction sequencing because each completed floor provides a working surface for the next level. Platform framing also limits vertical fire spread, since floor assemblies interrupt stud cavities at every level. Typical members are 2x4 or 2x6 studs at 16 or 24 inches on center, with floor joists ranging from 2x8 to 2x12 depending on span.

Professional

An architecture firm in the Pacific Northwest receives a commission for a mixed-use building: five stories of residential over ground-floor retail, with a total building height of 65 feet. The client is interested in mass timber for its sustainability credentials and aesthetic appeal. The project budget is moderate, and the local jurisdiction has adopted the 2021 IBC. The structural engineer presents three options: light wood frame over a concrete podium (Type V-A over Type I-A), all-CLT mass timber with exposed structure (Type IV-C), and CLT with partial encapsulation (Type IV-B). Each option carries different implications. The light frame option is the most familiar to local contractors. Platform-framed upper floors on a concrete first floor is a proven approach for five-story mixed-use buildings. Construction cost per square foot is lowest, and the supply chain for dimension lumber and engineered wood products is well established.

TLDR

Platform framing: studs go one story at a time, floor platforms break vertical fire spread; standard for residential up to about 5 stories (Type V-A sprinklered)
Balloon framing: continuous studs foundation to roof; fire hazard from uninterrupted cavities; requires fire blocking; used mainly in stairwells, great rooms, and gable walls
Engineered wood products: I-joists (54% of new home floors), LVL beams, PSL columns, and trusses; do NOT qualify as heavy timber regardless of total size
Glulam: laminated lumber beams/columns spanning 100+ feet in arch form; graded by combination symbols (24F-V4); char rate ~1.5 in/hr
CLT: cross-laminated panels for floors, walls, roofs; 5-ply common (~6-7/8 in thick); delamination risk if adhesive not heat-resistant
Type IV-A: tallest mass timber (18 stories, 270 ft), full noncombustible encapsulation, no exposed wood
Type IV-B: up to 12 stories/180 ft, mostly encapsulated, limited exposed wood (40% ceiling, 20% walls)
Type IV-C: up to 9 stories/85 ft, fully exposed mass timber permitted
Heavy timber minimums: 8-in columns, 6x10 beams, 3-in decking; miss these and you drop to Type V
All Type IV-A/B/C buildings require NFPA 13 sprinklers throughout

ELI5

Think about building with wooden blocks. Small thin blocks? That's light frame construction, the kind used in regular houses. You stack thin sticks close together (studs), put a flat floor on top, then build the next story on that floor. Pretty simple, and it works great for houses and small apartment buildings. Now imagine you swap those thin sticks for big chunky logs. That's heavy timber. Those big pieces are hard to burn through because fire has to eat through a lot of wood before it reaches the core. Picture holding a match to a toothpick versus a thick branch. The toothpick catches fire instantly. The branch just gets a black crust on the outside while the inside stays solid. Mass timber takes this idea further. Instead of one big log, you take a bunch of regular boards and glue them together in layers, alternating the direction each layer faces.

AREProject Planning & Design

Wood and Mass Timber Structures: Light Frame, Heavy Timber, CLT, Glulam, and Fire Performance

2 min read207 words

Wood and Mass Timber Structures

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