Widespread acceptance of Mass Timber Construction (MTC) has been hindered in the past by the spectre of fire. Now, a breakthrough MTC connection configuration developed and tested at Lakehead University in Thunder Bay, Ont. may provide a welcome level of assurance.
Professor Sam Salem, chair of Lakehead University’s Department of Civil Engineering, and graduate student Cory Hubbard were recently granted Canadian patent protection for an innovative mass timber connection system that achieves one hour of resistance from fire without any additional protection. The system is currently patent pending in the United States.
Salem’s MTC credentials run deep. He is a voting member on the Standing Committee on Structural Design of the Canadian Commission on Building and Fire Codes and a partner/research collaborator with the Ontario Mass Timber Institute (MTI).
The fire safety of individual MTC components, and even complete buildings, has been studied for years. Recognition of MTC’s popularity has resulted in code amendments based on building size and type, in certain cases requiring fire-protective cladding overtop of MTC slabs, beams and columns, and controlled exposure of exterior timber surfaces.
Over time, mass timber has been used in ever-taller structures in Canada and around the world.
MTC connections have been a particular point of focus. Studies in the past have revealed while bolt-and-plate connections offer a simple and strong connection in MTC buildings, their fire performance when unprotected is minimal. The interaction between glulam components, adhesives and metal plates and bolts introduces several variables that need to be carefully considered, making it difficult to predict the connection’s failure mode.
Salem and Hubbard’s investigation centred on the use of embedded rods rather than plates for column and beam connections.
Specifically, the new configuration utilizes two fully concealed, mechanically fastened steel rods embedded inside the glulam beam sections. Salem explains being completely concealed inside the wood section and not exposed to fire timber has the advantage of offering superior fire performance compared to other connection systems.
In their academic paper detailing their studies, Salem and Hubbard explain with the combination of the long anchorage length of the two mechanically-fastened steel rods and a small washer at the juncture, the fully concealed glulam beam-end connection of the 314mm x 135mm test pieces successfully achieved a one hour fire resistance without the application of additional fire protection. The longer rod anchorage length also allows the larger internal surface area of the wood prism to better resist shear stresses, as well as longer length for the rods to bear against the wood at high rotation values of the connection.
The research was funded through a grant from the Natural Sciences and Engineering Research Council of Canada. Salem and Hubbard tested their connection system in Lakehead University’s world-class fire testing and research facility, heating it over the course of an hour to more than 1,000 degrees – the fire temperature that brought down the World Trade Center.
Buildings built from mass timber may never escape concerns over fire, admits Salem. However, this connection system offers significant comfort.
“Fire is a serious hazard for all buildings regardless of the construction material in use. Advantageously, and unlike light-frame wood construction, mass timber, like the glulam sections utilized in the innovative beam-to-column connection in this research, char on the outside when exposed to fire while retaining strength and slowing combustion.”
Salem also says the system’s concealed design and performance in fire, combined with no requirements for any additional ugly fire protection, will please both architects and engineers.
The one-hour fire protection rating of this system is suitable for the MTC connections in building less than four storeys. Salem told the Daily Commercial News the next step will be to test larger MTC component connections in an effort to achieve the two-hour fire rating needed for taller buildings. Exploration of commercial possibilities will then follow.
John Bleasby is a Coldwater, Ont.-based freelance writer. Send comments and Inside Innovation column ideas to firstname.lastname@example.org.