Eighteen Stories of Wood, No Fire Test

Cross-laminated timber (CLT) and other mass timber systems are now permitted for buildings up to 18 stories under the 2021 International Building Code (IBC Types IV-A, IV-B, IV-C). The fire safety provisions for these tall mass timber buildings were developed primarily from furnace tests on individual CLT panels and small compartment tests — no full-scale multi-story fire test has ever been conducted on a mass timber building above 6 stories. Code provisions rely on char-rate calculations and gypsum board encapsulation assumptions extrapolated from small-scale data, but full-scale fire behavior involves phenomena (floor-to-floor flame spread via connections, char delamination cascading across large exposed surfaces, structural load redistribution under fire) that cannot be captured by component-level testing.

Mass timber is the most significant structural building innovation in decades, with a global market projected at $1.4 billion by 2027. It offers 25–45% lower embodied carbon than concrete/steel equivalents. However, insurance underwriters, fire departments, and building officials in many jurisdictions are reluctant to approve tall mass timber projects because the fire safety evidence base does not match the code permissions. Several insurers have imposed exclusionary clauses or prohibitive premiums on mass timber buildings above 6 stories, effectively blocking projects that building codes technically permit.

Small-scale furnace tests (ASTM E119) measure fire resistance of individual elements but cannot capture structural system behavior under fire. The few large-scale compartment fire tests conducted (notably the Carleton University/NRC tests in Canada and the APT Building fire test in Austria) used single-story or two-story configurations that do not replicate the vertical fire spread, connection behavior, and load path redistribution of tall buildings. Computer models (FEM fire simulations) fill the gap computationally but remain unvalidated against full-scale data. The fundamental barrier is cost: a full-scale multi-story fire test of a tall mass timber building would cost $10M–$30M and requires purpose-built test structures — neither industry nor government has funded one.

An intermediate-scale testing protocol that captures the critical multi-story fire phenomena (connection behavior, char delamination propagation, vertical fire spread through concealed spaces, structural load redistribution) without requiring a full 18-story test structure. This could involve a 3–4 story mass timber assembly tested with realistic fire scenarios and instrumented to validate the computational models that codes already depend on. The adjacent success of the Cardington steel frame fire tests (1990s, UK) — which validated steel fire engineering for a generation — provides a model for how a single well-instrumented test program can unlock an entire building technology.

A team could build a reduced-scale CLT connection assembly and test char delamination behavior under sustained fire exposure beyond the standard furnace test duration (which typically stops at the rated time). Alternatively, a team could instrument an existing mass timber building with fire-condition sensors (thermocouples, strain gauges) and model the expected fire performance, identifying the most uncertain parameters. Relevant disciplines: fire protection engineering, structural engineering, materials science, computational mechanics.