Mass Timber - Paving the Way to a Sustainable Future
Mass timber is experiencing a renaissance. Thanks to innovative technology, the old post and beam URM’s in Pioneer Square are being replaced by a newer framing style that consists of using small wood members to form larger solid wood formations to construct walls, floors, and roof systems. Currently, the most common forms of mass timber are CLT (cross-laminate timber), DLT (dowel-laminate timber), NLT (nail-laminated timber), MPP (mass plywood panel), & glulam (glue-laminated timber).
I had the pleasure of attending the four-part mass timber webinar series hosted by DCI Engineers several months ago. The four parts of the series were:
- Welcome to Mass Timber with guest hosts from TCA Architects & Truebeck Construction.
- Mass Timber for Developers with guests from Killian Pacific & Hacker Architects.
- Mass Timber: Breaking Down Type IV-HT Heavy Timber with guest hosts from SKS, Perkins & Will, & Hathaway Dinwiddie.
- Tall Mass Timber: The New Code with a guest host from Swinerton.
The purpose of this web series was to give an overview of the benefits, drawbacks, and overall beauty of mass timber buildings.
You may be thinking that using more wood would mean more deforestation. Well, you would be correct. Yes, more trees are required, but this is where sustainable forest management becomes a key factor. The suppliers who are harvesting the materials have learned to do it in a managed way that reduces the spread of forest fires and ultimately saves trees. Another note on sustainability is that within a building’s life cycle (assumed to be 100 years), the trees used to produce the timber will have regrown fully in their 60-year life cycle. Production of mass timber requires a significantly lower amount of embodied energy than other materials; the sum of all energy needed to produce a material consisting of the mining of raw materials, manufacturing, and transportation. To produce a 3-meter tall column with the same carrying load across all materials; timber requires 60 kilowatt-hours, concrete requires 227 kilowatt-hours, and steel requires 561 kilowatt-hours. An example given during the presentation showed that a hybrid type building consisting of 56% mass timber saves 774 metric tons of CO2, which is equivalent to taking 587 cars off the road for an entire year.
Wood is the only 100% recyclable building material we use. This means that at the end of the building’s life cycle, all of the timber can be recycled into new products such as particleboard, mulch, or even paper.
As an example, an apartment building called Forte in Melbourne Australia consists of 9 floors of CLT over 1 PT (concrete) podium. The CLT floors were constructed in 38 working days with a crew of 6 people. In contrast, the same 9 floors constructed using more traditional concrete and steel takes 16-18 weeks with a crew of 30 people or roughly 30 weeks with a crew of 20 for traditional stick framing. Using CLT in place of concrete and steel saved roughly 1,400 tons of CO2 emission, not to mention time in the schedule and labor costs. The entire project was complete in 11 months. Follow this link to see a time-lapse video of the Forte building being constructed.
Most people in the PNW have tendencies to want to get out and explore the natural beauty that we are surrounded by. Why not bring nature into your own home? Studies show that show people are happier, more productive, and have reduced stress when they are in an environment where they feel more connected to nature. Simply having exposed timber brings a more natural feeling into the building. Mass timber buildings are also leaning towards an overall more biophilic design, not only having the exposed timber but incorporating more natural lighting and ventilation as well as more natural landscape features to build a more organic environment.
Mass timber modules are more fire retardant than typical stick framing because the surface area is much greater than the volume of lumber, which allows the inside of the modules to stay cool while the outside chars. Charing is a burn that is slow and controlled at a predictable rate. Most timber modules are rated for 2 hours. Based on the building code where you are developing, this allows for a majority of the timber to stay exposed as the finished product. This requires fewer cover inspections and fewer materials. Removing not only the cost of raw materials - drywall, paint, and trim - this dramatically reduces the labor costs of installing those finishes as well. Not to mention a reduction in the schedule.
Resilience to Seismic Forces & Light Weight
Seattle is located over a large and deep basin and is prone to more intense earthquakes, although not frequent. Mass timber buildings allow the entire structure to be lighter, which in turn makes the building more resilient in the event of seismic activity. The typical mass timber building system is 5-ply timber with a 3” concrete topping slab (to reduce acoustic sound transfer) and when compared to a standard 8” post-tensioned concrete deck, the timber system for each floor is half the weight. With the timber system reducing the overall building weight the foundation system can be lighter. This allows for less excavation and foundation work, shortening the schedule even before the erection of the building begins.
Longer Lead Times
One of the drawbacks of prefabricated timber is that there is an associated long lead time. This can cause problems if a mistake is made or was not properly coordinated beforehand and a module does not fit as planned in the field. This factor also requires planning, design, and collaboration to be 100% complete before construction of the timber begins. Although labor costs are reduced during the erection of the mass timber, the preconstruction process is more intense and does require more collaborators.
Required Moisture Management Plan
An extensive moisture management plan is necessary to reduce water staining and damage to the timber. Keeping the timber covered and dry is an effective method of reducing the side effects of water, but typically it is most effective to erect the building in a dry weather period. Unfortunately for the PNW it rains for the majority of the year making it nearly impossible to erect an entire building without a day of rain. This requires more coordination and pre-planning to reduce the amount of water staining and damage during construction, before being dried in.
Initially Higher Cost
Mass timber products have a higher initial cost than other construction materials especially in the current market with lumber prices on the rise. Although timber has a higher initial cost, the speed of construction and lessened onsite crews reduce the cost so that it ends up comparable to the construction methods we are used to. However, there are some ways to help keep initial costs down too. Using standard sizes of modules versus custom sizing saves huge when ordering prefabricated pieces. Early design input and collaboration with all parties involved allow for fewer adjustments down the road. And utilizing modeling in the early stages for mechanical, electrical, and plumbing systems will reduce potential costs by the ability to use prefabricated modules rather than custom pieces.
Full Design Is Required
The timber systems arrive onsite prefabricated which requires the design to be 100% resolved before the timber is ordered. Although this makes preconstruction more intense, it does significantly reduce the amount of future coordination in the field.
More Parties Need To Be Involved
With mass timber buildings as the new wave of sustainable construction comes the cost of steep learning curves for contractors, developers, and subcontractors. Thankfully, there are mass timber experts – like the professionals who put on this informative web series – that, brought on during the early phases, can save the headache of coordinating and managing this unfamiliar building style.
This beautiful new construction style is paving a path towards an even more sustainable future. There is still much to learn about the details that come with mass timber, but with the experts out there willing to share their knowledge, these timber buildings can become the new standard approach in construction. And with many new projects in the design process in California, Oregon, and Washington, it will be exciting to see how these buildings shape our skyline and our environmental footprint in the near future.
Photo credit: DCI Engineers