Did you know that when mass timber burns, it doesn't weaken? It protects itself and maintains its strength even as its surface chars. This remarkable property was recently demonstrated in a three-hour fire test conducted at Southwest Research Institute (SwRI).
Photo shown above (from LinkedIn post, courtesy of Simpson Strong-Tie): Mass timber fire resistance test setup at Southwest Research Institute showing glulam beam and cross-laminated timber (CLT) assembly with insulation and instrumentation, part of Wisconsin construction management efforts for sustainable high-rise building safety.
On April 16, 2025, the first of three fire tests took place, evaluating a full assembly of glulam beam, column, and cross-laminated timber (CLT) deck. The test targeted requirements for Type I-A and Type IV-A buildings. Using standard, off-the-shelf connectors from Simpson Strong-Tie and WIEHAG, the assembly was subjected to fire exposure with intense heat.
Photo shown above (from LinkedIn post, courtesy of Simpson Strong-Tie): Construction worker installing Simpson Strong-Tie connectors on a glulam beam in a Wisconsin mass timber construction facility, preparing structural components for fire testing and high-rise building code compliance.
The result? The exterior was heavily charred, but the structural integrity was never compromised—proving the ability to meet or exceed strict fire code standards for high-rise construction.
Photo shown above (from LinkedIn posts, courtesy of Daniel Glaessl and Simpson Strong-Tie): Heavily charred glulam column and CLT deck after 3-hour fire resistance test at Southwest Research Institute, demonstrating structural integrity in mass timber construction and advancing fire safety standards in Wisconsin high-rise projects.
“The assembly is heavily charred. However, the structural integrity was not compromised at any point in time.” Daniel Glaessl, Partner & CPO at Neutral
The successful three-hour fire test demonstrates what’s possible. The Neutral-led mass timber team, including C.D. Smith Construction as general contractor and mass timber installer, Forefront Structural Engineers, Stora Enso, WIEHAG, Simpson Strong-Tie, Arup, Hartshorne Plunkard Architecture, and Thornton Tomasetti, is pushing the boundaries of efficient, low-carbon construction. These results support upcoming projects like Neutral.Edison in Milwaukee and is a step toward making tall timber structures safer, more efficient and sustainable across the U.S.
Mass timber’s fire performance is rooted in science. When exposed to fire, the outer layer of timber chars at a predictable rate, forming a protective barrier—the “char zone”—that insulates the inner core. Once a char zone is formed, it dramatically slows further burning and insulates the wood, allowing the timber to retain its structural capacity.
Engineers design mass timber columns, beams, and panels with sufficient thickness—sometimes adding up to 16 inches of material—to ensure that even after prolonged fire exposure, the remaining section can still bear loads. This approach ensures that mass timber assemblies can achieve the three-hour fire resistance rating required for the tallest, most demanding building types.
Projects like Ascent MKE, the world’s tallest hybrid mass timber building, further prove mass timber’s capabilities. For Ascent, independent third-party tests demonstrated that its horizontal floors and ceilings achieved a two-hour fire rating, its steel connectors a two-hour rating, and its glulam columns a world-first three-hour fire resistance rating—meeting and exceeding Type I building code requirements. These achievements are changing perceptions and opening doors for mass timber adoption in urban environments and high-rise developments. Go to the full Mass Timber Construction CASE STUDY for more details on fire testing and all that went into proving and planning the Ascent MKE project.
As more developers and cities embrace mass timber, the construction industry is witnessing a shift. It’s about delivering on mass timber's promise by building to new heights and meeting advanced performance expectations. From Ascent’s pioneering fire tests to the recent Neutral-led SwRI series, it's all part of proving what's possible. Together, we’re reshaping the future of high-rise design with tall timber that's safe, sustainable, and resilient without compromise.
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Date: Apr 30, 2025 | Author: Tracy Lisowe
#sustainableconstruction #masstimber #firetesting #highrise #fireproofdesign #greenbuildings #codecompliance #talltimber #Milwaukee #Wisconsin
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