Keegan Carmichael and David Webster attended the WoodWorks Seminar on Mass Timber Construction on December 6 and filed this report:
Mass timber is best applied as a competitor to concrete construction. Timber is lighter, offers carbon reductions, and can be erected faster than concrete. The typical weight of mass timber is 34 pounds per linear foot (plf) and requires a crew of 5-6 individuals during framing, whereas concrete on average is 150 plf (source: WoodWorks) with an average of 50+ construction crew. Mass timber is often used in Passive House construction due to its thermal performance, but a benefit of this is the interior aesthetics of the exposed wood interiors.
Mass timber framing offers a variety of framing systems:
- Post + beam
- light frame + mass timber
- CLT (whole building)
Applications of mass timber in construction are:
- Nail laminated timber – Primarily used for gravity loads (Bullitt Center, Seattle, WA)
- CLT panels (Albina Yard, Portland, OR)
- Glu-lam beams (Richmond Olympic Oval, Vancouver, BC)
- T & G decking (various projects)
- Composite timber + concrete slabs (Radiator Building, Portland, OR)
- Structural composite lumber (Rocky Mountain Elk Foundation, Missoula, MT)
The only way to make a carbon negative building is to build with wood. Given the current height restrictions for wood buildings in the US, IBC 2015 offers alternative means and methods for structural CLT framing, which are explained in the CLT handbook. Though cross-laminated-timber shear walls are not currently accepted by IBC 2015, often this construction is reinforced with steel connections for shear applications. Currently there are two major manufacturers of CLT panels in North America; D.R. Johnson Lumber Co. in Oregon and Nordic Structures Inc. in Montreal.
Mass timber construction, which is categorized as type IV construction according to IBC, has many benefits: speed of construction, aesthetics, lighter weight, better thermal performance than concrete, disaster resistance, structural flexibility (such as a two-way slab), and even carbon reduction. In order to maximize the speed of manufacturing, architectural design should reflect the size of CLT panel modules, which helps eliminate construction waste. IBC 2015 limits wood construction to 6 stories for business type occupancy and 5 stories for residential occupancy, though Europe currently has several wood framed buildings above this benchmark. Several projects on the West Coast utilize a 2-story concrete plinth with wood framing above it, which totals 7-8-story mixed-use building depending on the occupancy. One note is that CLT must be protected from the environment, and typical construction costs are 25% more than concrete for projects in the Northeast.
Image courtesy WoodWorks