What are the structural and material specifications of Skanska's mass timber school?
Skanska’s groundbreaking on a 300,000-square-foot mass timber high school represents a landmark shift in public K-12 infrastructure. By utilizing cross-laminated timber (CLT) and glue-laminated timber (glulam) structural systems, the project demonstrates how large-scale educational facilities can dramatically reduce embodied carbon, accelerate construction schedules, and enhance biophilic learning environments.
Why this matters: While mass timber has gained rapid adoption in commercial office spaces and multi-family residential projects, public educational facilities have historically faced structural, budgetary, and regulatory barriers. Skanska's 300,000-square-foot project proves that mass timber is no longer a niche aesthetic choice for small pavilions, but a viable, high-performance structural alternative for large-scale civic projects.
To scale a wood-framed structure to 300,000 square feet, structural engineers design a highly optimized gravity system. This typically relies on a standardized structural grid of 30-foot by 30-foot bays, which aligns with modern classroom layout parameters and optimizes the spanning capacity of 5-ply cross-laminated timber panels.
According to the American Wood Council (AWC), structural plans of this scale require precise specification of engineered timber elements to guarantee both gravity load performance and lateral force resistance:
- Floor and Roof Decks: 5-ply and 7-ply Cross-Laminated Timber (CLT) floor plates, typically fabricated from Spruce-Pine-Fir (SPF) or Douglas fir conforming to ANSI/APA PRG 320 performance standards.
- Gravity Columns and Beams: Glue-Laminated Timber (glulam) elements fabricated with structural adhesives capable of withstanding continuous exterior exposure, though designated for dry-service interior use.
- IBC Code Classification: Engineered under the International Building Code (IBC) Type IV-C construction provisions. This classification allows structural timber to remain largely exposed within the classrooms while maintaining a mandatory 2-hour fire-resistance rating (FRR) for primary structural frames.
Using prefabricated components allows for highly automated manufacturing tolerances. By utilizing computer numerical control (CNC) multi-axis routers, manufacturers pre-cut penetrations for mechanical, electrical, and plumbing (MEP) systems directly into the CLT panels off-site, which minimizes field-drilling and reduces on-site erection errors.
How does mass timber compare to traditional steel and concrete in school design?
Mass timber systems offer a structurally efficient alternative to traditional steel and concrete designs. While traditional assemblies rely on energy-intensive extraction and manufacturing processes, mass timber relies on renewable, carbon-sequestering softwood timber that is structurally optimized through advanced lamination processes.
Engineering analysis indicates that a mass timber structural frame is approximately 80% lighter than an equivalent cast-in-place concrete structure. This reduction in dead load yields cascading structural benefits, significantly lowering the required volume of concrete foundations and grade beams, which is a major advantage in jurisdictions with low-bearing soils or high seismic activity.
| Performance Metric | Mass Timber Structural System (CLT/Glulam) | Traditional Steel with Composite Deck | Cast-in-Place Concrete |
|---|---|---|---|
| Embodied Carbon Profile | Net-negative; sequesters carbon within the wood fibers | High initial carbon footprint due to fossil-fuel-intensive smelting | Extremely high carbon intensity; calcination process emissions |
| On-Site Construction Speed | Up to 25–30% faster installation via prefabricated gravity systems | Standard sequential staging; requires separate deck pours | Slower cycle times due to formwork, rebar placement, and curing |
| Acoustic Performance | Excellent dampening properties when combined with floor toppings | Requires extensive suspended drywall and acoustic plaster drops | Highly reflective surfaces; requires intensive surface-applied treatment |
| Seismic Weight Ratio | Low (approximately 20% of the mass of equivalent concrete) | Moderate weight profile | High mass, which increases lateral seismic forces on the structure |
| Thermal Mass Performance | High natural insulation properties; reduces thermal bridging | Low thermal performance; highly conductive thermal bridging | High thermal mass, but slower thermal response times |
Furthermore, the integration of mass timber directly benefits the project schedule. Wood Products Council (WoodWorks) case studies indicate that prefabricated CLT panels can be craned into place directly from delivery flatbeds, which reduces the need for extensive on-site material staging and lowers local traffic disruption during school construction.
What are the primary design challenges when engineering a 300,000 sq. ft. timber school?
Engineering a high-occupancy public school of this scale requires solving complex physical coordination challenges, particularly regarding sound transmission between floors, complex MEP service distribution, and fire-life-safety codes. Designers must balance structural, acoustic, and thermal performance while keeping the natural wood surface visually exposed to achieve biophilic benefits.
Acoustic Separation in Learning Environments
To achieve the acoustic privacy required by ANSI/ASA S12.60 (Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools), floor assemblies must maintain a Sound Transmission Class (STC) rating and an Impact Insulation Class (IIC) rating of 50 or higher.
Bare CLT panels do not have sufficient mass to block low-frequency impact noise, such as footsteps or moving furniture. Structural engineers solve this by designing a layered floating floor assembly over the structural CLT deck.
This multi-tiered approach effectively dampens both airborne noise and physical impact vibrations before they travel through the timber framing to the classrooms below.
MEP Routing and Exposed Timber Aesthetics
Because Type IV-C construction encourages leaving the timber ceilings exposed to promote student well-being, engineers cannot hide ductwork, conduits, and piping behind suspended drywall ceilings.
This requires rigorous three-dimensional Building Information Modeling (BIM) coordination during the schematic design phase.
Using this workflow, horizontal utilities are concentrated within dedicated corridors where drop ceilings are acceptable, and only minimal, perpendicular service runs branch into classrooms through pre-milled openings.
Fire-Safety and Charring Rates
The structural integrity of mass timber under fire conditions is governed by the predictable charring behavior of heavy wood members. When exposed to flame, the outer layer of a thick glulam column or CLT panel burns at a stable rate, forming a dense char layer.
This char layer acts as an insulating barrier that restricts oxygen flow and slows heat transfer to the unburned inner core of the wood.
Per calculations established in Chapter 16 of the National Design Specification (NDS) for Wood Construction, structural engineers assume a nominal char rate of 1.5 inches per hour of fire exposure. By adding sacrificial wood thickness to the exterior faces of structural members, columns and beams can withstand a 2-hour ASTM E119 fire test while carrying their full design load, all without the need for gypsum-board encapsulation.
FAQ
What is the cost difference between mass timber and steel/concrete for schools?
While the raw material cost for engineered timber is typically 5% to 15% higher than equivalent steel or concrete structural members, the total installed cost of a mass timber school is often comparable or lower.
This parity is achieved through a 25% reduction in overall construction schedules, fewer foundation piles due to the lighter structural weight, and reduced interior finishing costs since the structural timber serves as the finished surface.
How does mass timber support biophilic design in K-12 education?
Exposed timber structures bring natural elements directly into the learning environment, which aligns with the principles of biophilic design.
Empirical research in environmental psychology demonstrates that visible wood surfaces in classrooms correlate with lower heart rates, reduced stress indicators, and improved cognitive focus among students compared to traditional, synthetic-finished learning spaces.
What are the fire resistance standards for mass timber schools under the IBC?
Under the International Building Code (IBC) provisions for Type IV (Heavy Timber) construction, mass timber elements must meet stringent fire-resistance ratings.
By calculating charring rates (typically ASTM E119 testing standards), structural timber can easily achieve 1-hour to 2-hour fire ratings without losing structural integrity.
Does a mass timber structure require special protection during construction?
Yes, managing moisture is critical during the construction phase of a mass timber project. Exposed CLT floor panels and glulam frames must be protected from standing water and excessive humidity before the building is enclosed.
Contractors use breathable, water-resistant coatings on the timber faces during transit and erection, and they implement rapid water-removal strategies, such as floor squeegees and temporary drainage openings, to prevent swelling and staining.
How does mass timber contribute to school district sustainability goals?
Mass timber helps school districts meet ambitious climate action goals by dramatically reducing the embodied carbon of new school construction. Unlike concrete and steel, which emit significant amounts of greenhouse gases during manufacturing, wood acts as a natural carbon sink.
The carbon sequestered during the growth of the trees remains stored inside the structural frame of the high school throughout its multi-decade service life, directly offsetting the project's carbon footprint.
