 and referenced National Design Specification (NDS) for Wood Construction can be used in DLT, as only specified strength and stiffness for each lamination is required. Full-scale panel testing is not required to determine structural performance – a significant advantage to DLT as a product.
As a floor or roof deck, DLT is a highly efficient structural panel. Similar to NLT or GLT (glulam on flat), all of the wood fiber runs in the direction of the span. This provides the most efficient use of material for floor and roof systems which are typically one-way spanning between beams or walls.
From a structural perspective, each individual lamination spans between supports, meaning calculation of the panel stiffness and capacity is simple. The structural design of each lamination in a panel is covered by CSA O86 and the NDS and applicable grading rules. Structurally finger-jointed lumber is used for spans longer than 20 feet, meaning no reduction in strength or stiffness is required for longer panels. This is a big advantage over traditional NLT where butt joints in laminations require a 20 to 30 per cent reduction in panel strength and stiffness (refer to Nail Laminated Timber Design Guide, Table 4.1 and IBC 2015 2306.1.4).
The Uniform Building Code (predecessor to the IBC) has recognized laminated decking since 1927, and the NBC since its first publication in 1941. The requirement for minimum fastening of the boards together ensures that the panel acts as an element and not individual boards. The 3/4″ diameter hardwood dowels in DLT can create the same interlayer shear capacity between boards as the original 20 penny nail requirement specified in the IBC.
Machine stress rated lumber can be used to increase strength and stiffness of the panel.
Diaphragms and Shear Walls
Plywood or OSB sheathing atop the panel gives shear capacity to DLT panels for use as structural diaphragms in floors and walls. The sheathing also allows for simple nailed connections between panels with strips of plywood. The shear capacity of the sheathing 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 topp