The next generation of mass timber products could be made with timber rather than steel fasteners – with steel nails, screws and bolts replaced with hardwoods as timber engineers test joints and screws made with dowel-shaped timbers.
The push to replace steel fasteners with timber—especially in low—and mid-rise construction—not only cuts emissions but improves structural performance and speeds up assembly time.Â
In February, Wood Central revealed that the Australian government was supporting a new project to commercialise dowel-laminated timber (or DLT)—allegedly stronger, greener, and more affordable than cross-laminated timber—for use in low—and mid-rise construction.Â
Now, a study published in the Journal of Building Engineering, Monotonic testing of single shear-plane CLT-to-CLT joint with hardwood dowels, has tested 154 different single shear-plane experimental tests using four different CLT species used in DLT, including Douglas-fir, Grand-fir, Western-hemlock, and Spruce-Pine-fir, two hardwood dowel species, using two different dowel diameters.
Wood Central understands this research is the first type of single-shear or CLT-to-CLT joints used in floor-to-wall, wall-to-wall, and floor-to-floor connections.
The study states, “The objective is to evaluate the mechanical properties of the CLT-to-CLT joint, including yield and ultimate strength, serviceability and yielding stiffness, and ductility.”
The result is a new approach for calculating hardwood dowels yielding strength and elastic stiffness – providing beyond doubt that a hardwood dowel offers higher strength stiffness compared to a wooden screw.
According to lead researcher Aivars Vilguts, “hardwood dowels have become one of the most popular fasteners for wood-to-wood joints in recent years,” adding that “the most widely used hardwood dowels are Red Oak (in the United States) and Yellow Birch.”
Mr Vilguts, who is a postdoctoral associate at Virginia Tech in the United States, added that “experimental testing suggests that hardwood dowel joints have comparable or even superior performance than screws.”
The research, funded by the US government’s Harnessing Emissions into Structures Taking Inputs from the Atmosphere (HESTIA, found that Red Oak-based dowels were twice as strong as normal screws. Or, as Mr Vilguts puts it, “for the same shear loading in the half-lap joint, we would need to use on average twice as fewer hardwood dowels as screws.”
It also found that when tested under extreme conditions, such as earthquakes, the dowell-based connectors “still perform in an elastic stage without any yielding.”
DLT is made by stacking timber and securing it with hardwood dowels, and unlike CLT, it uses friction rather than glue to keep panels in place.
According to the Canadian Department of Forest Products and Applications, it is “more structurally efficient than CLT for one-way spans because all the boards face the same direction.”
Large panels can then be pre-manufactured for floors, walls and roofs – and can measure up to 3.7 metres by 18.7 metres, depending on shipping restrictions.
Without metal fasteners, CNC machines can easily automate DLT manufacturing with highly durable materials housing pre-integrated acoustic materials, electrical conduits, and other service interfaces.
