With sustainability and green building at the forefront of design and new construction, many owners are exploring mass timber options to maintain quality while reducing their carbon footprint. Prefabricated and engineered for durability, mass timber checks the boxes when it comes to strength, acoustic, seismic, and fire performance and comes in different types. While the method depends on each project, there are some advantages and disadvantages owners must consider when planning new construction.
Mass Timber Types
The most common types of mass timber are cross-laminated Timber (CLT), nail-laminated timber (NLT or nail-lam) glued-laminated timber (glulam), dowel-laminated timber (DLT), laminated strand lumber (LSL), secondary laminated LVL (SCL), laminated veneer lumber (LVL), and wood-concrete composites (WCC)-hybrid options for wood high rises) and mass plywood panels (MPP). Although mass timber is not new, a few options are gaining more prevalence in construction across the United States.
Lighter, but equally as load-bearing as steel, CLT is gaining popularity as it is ideal for office, mixed-use, student housing, schools, multi-family, hospitality, industrial, and more. Formed using three or more layers of lumber placed and adhered in alternating directions, CLT’s high strength and dimensional stability make it a popular choice for high-rise construction. With its favorable aesthetics, cost effectiveness, environmental performance, speed and efficiency of installation, and design flexibility (it can be utilized for an entire building or combination of cores, walls, shear walls, diaphragms, shafts, floor/ceilings and roofs), CLT is on the rise. In addition, field modifications can be made on-site with simple tools, on-site waste is limited thanks to design and fabrication, and its structural and dimensional stability allow CLT to span in two directions—offering even more opportunities in design and application.
The other mass timber types seeing an uptick in use are NLT and glulam. NLT panels are constructed from dimensional lumber that is nailed together and is finding modern uses for an old method. Great for floors, decks, and roofs among other applications, NLT is a common choice for historical spaces and mid-rise residential buildings. Glulam is most commonly used for beams and columns. It is a popular choice for residential and commercial applications when owners would like to keep the aesthetic of exposed wood.
From predictability and strength to product performance and sustainability, mass timber in general offers owners many benefits. Perhaps the most attractive benefit of mass timber construction is the sustainability. Not only is the amount of wood quickly replenished, but it also leaves a much lower carbon footprint while building than traditional materials such as steel. Fast and easy to install, these engineered panels generate almost no waste on site. “CLT stores carbon and produces fewer greenhouse gas emissions during the manufacturing process. The wood fibers used in CLT are traceable from certified forests to the end user (consumer),” said JE Dunn Senior Project Manager Tim Ockinga.
In addition to sustainability, mass timber also offers superior fire protection. “The char layer on mass timber panels burns slower than a traditional steel or concrete building,” said Ockinga. “If designed correctly, buildings using these materials could actually be safer.”
In a construction climate where speed is crucial, mass timber is one way to shave time off the schedule. Using prefabricated panels, construction is approximately 25 percent faster than concrete with up to 90 percent less construction traffic. This translates to 75 percent fewer trade workers on a site for the mass timber structure--which makes it more conducive to adhere to current social distancing directives—as well as less site noise. Prefabrication results in installation efficiency and Because it is lighter than steel and concrete, it is a good solution for project sites with poor soils. In addition, each manufacturer's standardized sizing of mass timber equates to certainty of overall design, detailing, fabrication, and the mitigation of waste and costly mistakes, thus saving time and money on each project. MEP systems are easier to install in mass timber construction when the locations are modeled in the design of the CLT panels because it allows the penetrations to be cut in the factory during the precision CNC machine fabrication.
While mass timber is advantageous from a sustainability and reliability standpoint, it presents challenges of its own. The most notable hurdle is often cost. “Historically, much of the mass timber production and construction has occurred in Europe over the last 20 years,” said Ockinga. “This makes transportation to project sites cumbersome as well as expensive. On average, owners are looking at 3-10% higher costs to use a mass timber option.”
Cost aside, designers of a mass timber project must also consider the number of beams required in buildings using the material, as it affects other systems within the building. Because more beams are required to maintain floor-to-ceiling heights, designers must take into account how to get the mechanical system around them. “Design teams must get creative on where mechanical systems can dip below the beams, thus lowering the floor-to-ceiling height in those individual locations,” said Ockinga. As an example, the design and construction team for a project in Savannah, Ga. collaborated to provide a series of vertically stacked chases to house domestic and fire line mains, roof leaders, dryer and kitchen exhaust ducts, HVAC refrigerant line sets, etc. to account for these heights.
As more projects in the US utilize mass timber, some of the challenges will take care of themselves. “We are seeing an increase in both planned mass timber manufacturing domestically and experts in America who can install it,” said Ockinga. “As those plants and labor ramp up, costs will naturally go down as logistics and installation are easier to plan.”
Another tool that helps owners plan now and in the future of mass timber construction is JE Dunn’s digital preconstruction tool Lens®. Used to collaborate during the design phase, the platform provides real-time estimates for every decision. “We have a record of every project and related pricing in the database,” said Jason Hickam, national preconstruction and estimating director. “Thanks to some preliminary pre-construction work we performed on a large-scale mass timber project, we were able to capture those metrics and add a mass timber option in Lens. That means, with the click of a button, we can switch from steel and its price to mass timber to see what a change in materials would look like against the project budget.”
Because there are many mass timber options, the method provides plenty of opportunity for creative solutions to work within the project budget. During preconstruction of the current mass timber project in Savannah, Ga., the JE Dunn team utilized CLT panels for building cores, shafts and shear walls ilo CIP, thus reducing the building loads on the foundations. This resulted in less expensive foundations, and transfer slabs and quicker construction. To lean on the strengths of each mass timber type, the team used a post and beam construction type with GLT columns and beams and CLT wall, floor, and roof panels, resulting in a reduction in overall construction budget costs and installation timelines. “Though cost is sometimes a hurdle, the wide range of mass timber options means we can try different ways to make it work,” said Ockinga. “The flexibility in design to leverage different mass timber types—in addition to the superior sustainability—make the method attractive to more and more clients, and I think it will continue gaining popularity in the US in the coming years.”