Mass timber construction has garnered a great deal of attention recently, especially when it comes to constructing tall buildings. Over the past decade, tall wood buildings have popped in Europe, Australia and Canada. Initially, these tall wood buildings were topping out at around 10 stories. This is changing rapidly as a race to build the tallest has recently emerged with taller structures being proposed and announced.
The concept of tall wood buildings and mass timber construction hasn’t taken off in the U.S. like it has in other parts of the world, but that is slowly starting to change. Currently, the tallest modern mass timber building in the U.S. is the seven-story T3 in Minneapolis that was just completed last year. This is set to change as interest has grown and a handful of tall wood projects are underway or expected to start construction this year.
What is mass timber?
Mass timber is a term used to describe a number of large engineered wood products that typically involve the lamination and compression of multiple layers to create solid panels of wood. Mass timber construction uses solid wood panels to frame a building’s walls, floors and roofs and is seen as a sustainable and more carbon-friendly alternative to steel and concrete.
Types of mass timber products
Cross-laminated timber (CLT). CLT is made up of layers of dimensional lumber stacked perpendicular and glued together to create structural panels. CLT panels are typically made of layers of three, five or seven and because they offer two-way span capabilities can be used for floors, walls and roofs.
Nail-laminated timber (NLT). NLT is made by stacking layers of dimensional labor on end and fastening them together with nails or screws. NLT is commonly used in floors and roofs and can also be used to construct elevator shafts.
Glue-laminated timber (glulam). Glulam is made from stacking dimensional lumber on edge and bonding them together with moisture-resistant adhesives. Glulam is commonly used for floors, beams, columns and arches.
In addition to the three most popular mass timber products we outlined, others include dowel-laminated timber (which is similar to NLT in composition), laminated veneer lumber, laminated strand lumber and wood-concrete composites.
Modern mass timber buildings that have popped up in the U.S. recently like the 87,500-SF Design Building at UMass Amherst that was just completed last month. There’s also the Candlewood Suites on Redstone Arsenal which is a 62,688-SF hotel in Huntsville, AL that was developed by Lendlease.
Other examples include the Albina Yard, a four-story, 16,000-SF commercial building in Portland, OR and the Bullitt Center in Seattle which used glulam columns and beams in its structural frame and NLT panels for its floor decks. nd lumber, parallel strand lumber and wood-concrete composites.
So, while mass timber buildings aren’t a completely foreign concept, it isn’t exactly commonplace either and we haven’t really seen tall wood buildings cropping up here in the U.S. That’s all set to change this year thanks in part to the 2014 U.S. Tall Wood Building Prize Competition sponsored by the USDA, the Softwood Lumber Board and the Binational Softwood Lumber Council. Two winners were picked to split the $3 million prize money back in 2015, and the first of those two projects are expected to start next month.
Framework will be a 12-story mixed-use building that will include office, retail and affordable housing located in Portland, OR. The building was designed by LEVER Architecture and is being developed by project^. The building will feature glulam columns and CLT floor panels and ceiling frame and post-tension rocking CLT shear-walls to handle seismic activity. All testing on materials has been completed and the design has been approved. Building permits have been applied for and construction is expected to start once they’ve been issued.
475 West 18th was the other winner of the Tall Wood Building Prize Competition and will be a 10-story luxury residential building in Manhattan. The project was designed by SHoP Architects and is being developed by Spiritos Properties. The structural frame will consist of glulam beams and columns with floors made of a CLT and concrete composite. The 475 West 18th project is still going through approvals before construction can begin. The project is expected to achieve LEED Gold certification.
Carbon12 is an eight-story condo building designed by PATH Architecture going up in Portland, OR. The building will use CLT as its structural system and feature an underground mechanical parking system.
Roadblocks to using mass timber for tall buildings in the U.S. include restrictive building codes and a lack of manufacturers creating mass timber products. Currently, most building codes in the U.S. don’t allow wood construction over six stories. Even for buildings six stories or less, building codes haven’t specifically recognized mass timber systems.
In order to build taller than six stories using mass timber, you have to use a performance-based workaround to prove to the local authority having jurisdiction, typically through third-party testing, that the mass timber products can perform as well as currently allowed materials such as steel and concrete.
The 2015 IBC did start recognizing CLT products manufactured to ANSI/APA PRG-320: Standard for Performance Rated Cross Laminated Timber. This could be changing in the next few years as the ICC Board approved an Ad Hoc Committee on Tall Wood Buildings with the goal of making code changes to the IBC by 2021 to allow for tall wood buildings.
The other issue is that there are only a handful of manufacturers producing mass timber products in the U.S. Until as recently as 2015, there wasn’t anyone in the U.S. making CLT products approved for architectural use. That changed when D.R. Johnson in Oregon became the first U.S. certified manufacturer of CLT panels.
This meant that mass timber products have to be imported in which increases the price. The T3 project we mentioned earlier had their NLT panels made in Canada and the glulam products were shipped in from a European supplier.
Advocates of mass timber claim a number of benefits of its use over traditional steel and concrete for constructing tall buildings. If the timber is sourced responsibly it provides a completely sustainable and renewable building material. Carbon emissions from manufacturing mass timber products are a fraction of those created by the production of steel and concrete building materials.
The production of steel and concrete products account for about 8 percent of global greenhouse gas emissions annually. By using lumber all of the carbon absorbed during photosynthesis of the tree’s life remain trapped in the building product when using mass timber instead of creating more when using steel or concrete.
Mass timber has a natural tendency to char in a fire which slows the burning process and helps maintain structural integrity. The same thing happens when you throw a whole log on a fire and it takes forever to burn which is part of the reason people split their firewood. Mass timber’s resistance to fire can be further improved by encapsulating mass timber in gypsum board.
Tests have shown that cross laminated timber can achieve a fire rating of three hours. Mass timber products are also considerably lighter than concrete. The thermal performance of mass timber is also far superior to its steel and concrete counterparts which translates to lower heating and cooling costs.
As perceptions change, and more buildings are tested, proven and completed, mass timber buildings are expected to catch on here in the states the same way it has in Europe. Who knows, one day soon we could see a tall wood building boom similar to the skyscraper boom of the 1930s.