eathing applied overtop the DLT panels can be calculated like a typical nailed plywood diaphragm per CSA O86 11.5 or Special Design Provisions for Wind and Seismic (SDPWS) 126.96.36.199.
DLT panels can be used as structural bearing walls with the DLT exposed on both sides, or sheathed one or both sides as a shear wall.
Smaller two-way spans or weak-axis cantilevers (up to two- to three-foot cantilevers or four- to six-foot simple spans) can be achieved in DLT panels by using screw reinforcement inside the panel. Screw reinforcement for a weak-axis cantilever can be designed using a strut-and-tie truss analogy to design angled shear and compression screws, and using the plywood sheathing as a tension flange.
Larger minor axis cantilevers can be achieved using steel or wood outriggers set atop or notched into the top of the panel.
Detailing for Moisture
When detailing DLT panels, designers need to account for moisture movement – wood expands perpendicular to grain when as moisture content increases. Incorporation of small gaps between panels deals effectively with this issue. The T3 Minneapolis structure had a floor plate which was 220 feet wide, and which experienced both snow and rain during winter construction. The gap between panels effectively dealt with expansion of the panels during construction, and there were no issues with moisture damage or remedial works required. The key is in detailing for the movement, and letting panels dry out after they get wet.
Use of OSB sheathing with a pre-applied moisture-resistant top coating and taped joints is a newer solution to this issue. Along with providing a path for the water to move off the floor plate, this strategy provides significant protection from moisture, and greatly reduces expansion of the panels.
If a gap is provided between panels, it can be filled in afterwards with lumber, or retained to create a visual delineation between panels – as it was on the T3 building. From a fire design perspective, the gap between panels is not an issue if a continuous topping layer or plywood spline is provided. Similar to a plywood spline between CLT panels, this continuous layer prevents air movement between floors, thus ensuring char development remains uni-directional (NLT Guide 3.3.2). The local authority having jurisdiction (AHJ) for the T3 building accepted permanent gaps between NLT panels for this reason.
DLT can be used in all types of combustible construction. From a fire perspective, DLT behaves the same or better than NLT.
Both the National Building Code of Canada (NBCC) and the IBC recognize NLT and provide guidance for both structural and fire design. No product-specific standard is required, as the structural design of each lamination element is covered by the building codes. It is resistant to fire, and has long met the requirements of heavy timber in North American building codes. NLT can be used in all types of combustible construction.
The fire resistance of mass timber panels is now widely proven – the char developed during a fire creates a self-protection layer. Research recently completed by FPInnovations showed that a 2 x 8 NLT floor panel with concrete topping can achieve a three-hour fire resistance rating under full load (Osborne, 2015).
The inherent fire performance of mass timber removes the need for intumescent coatings and dropped ceilings that would be required for a steel structure, allowing the wood to be exposed as a permanent soffit.
As with all mass timber systems, it is important to address acoustic separation between mass timber walls and floors, which can be achieved through appropriate detailing. Acoustic mat should be used in floor buildups with no dropped ceiling.
Timber-Concrete Composites with DLT
The stiffness of mass timber panels is important in long-span floor systems, and as can be seen in Table 1, consideration of floor vibrations often govern the required panel thickness.
A concrete topping is often required for acoustic performance, and this topping can be made composite with DLT panels to increase the panel stiffness. In Europe