The selection of a cross-laminated timber addition was critical to overcoming site constraints in a $20 million adaptive reuse project at 69 A St. in Boston’s Fort Point. Photo courtesy of Warren Patterson Photography
Top of mind for conscientious development teams is leveraging creative design and construction solutions to reduce unfavorable environmental consequences, while satisfying program, schedule, budget and aesthetic requirements.
The good news is that wood—used in construction for more than 10,000 years – has metamorphosed into a choice option for green construction thanks to innovation, technology, and rigorous testing.
Cross-laminated timber (CLT) methodologies have been around for nearly a century, but their popularity didn’t gain momentum until the mid-1990s in parts of Europe. Cross-laminated timber is an engineered wood product typically consisting of three- to nine-layered lumber boards that have been stacked and glued at 90-degree angles to deliver superior structural rigidity in both directions. Composed predominantly of soft woods such as black spruce, pine or fir, the panels are manufactured in a wide range of thicknesses, widths and lengths.
By the 2000s, CLT (also referred to as mass timber) had gained popularity and was being used in building systems such as single-family and multi-story housing. CLT’s advantages were soon recognized in North America and four manufacturing plants had opened by 2015.
Code Has Been Cracked
That same year, the American Wood Council incorporated CLT standards into their National Design Specification for Wood Construction, triggering the expansion of CLT use into the heavy timber construction classification and inclusion in the 2015 International Building Code. The code also mandated that structural CLT products meet the requirements specified by ANSI/APA PRG 320: Standard for Performance-Rated Cross-Laminated Timber.
The code change made CLT a viable option in the assembly of exterior walls, floors, partition walls and roofs of non-residential buildings. Subsequent code updates have included a 2018 IBC reorganization of heavy timber provisions, and significant changes in the 2021 IBC and International Fire Code for tall mass timber construction of up to 18 stories. In addition to CLT, mass timber products covered under Type IV construction in the 2021 IBC include nail-laminated timber, dowel-laminated timber and glue-laminated timber, also referred to as glulam or GLT.
This versatile material is so timely, important, and relevant to climate change because it is an excellent alternative to carbon-intensive materials like concrete and steel, plus it is sustainable and renewable. According to the Think Wood 2021 Mass Timber Design Manual, wood products are 50 percent carbon by dry weight, meaning mass timber can sequester carbon well into the future, reducing the global warming potential of a building. CLT paneling is also thermally efficient, requiring one-third the heating and cooling energy of traditional building systems. This makes mass timber buildings well-suited to energy-efficient construction and the rigorous standards of Passive House and Net Zero-ready design.
CLT offers many advantages. It contains fewer concealed voids and is naturally fire-resistant because it chars first. In a fire, the char layer forms a protective outer layer while the core retains integrity, allowing time for safe evacuation. Because of its dimensional stability and rigidity, it also provides seismic durability in an earthquake.
Historic South Boston Renovations
Although it is 15 times lighter than steel and five times lighter than concrete, CLT has comparable strength and can span in two directions. As a result, foundations do not need to be as large and cranes can be smaller, making CLT an ideal solution in a tight urban setting, or for an addition to an existing building when fortifying the foundation and vertical structure would be cost-prohibitive. It was this quality in particular that inspired the first CLT addition to an existing building in Boston.
Nate Turner
Situated in South Boston’s Fort Point neighborhood, 69 A St. is a 1920s era industrial building that once housed the Standard Rivet Co. Constructed of brick and heavy timber wood beams and columns, the structure had three floors encompassing 29,000 square feet and a basement. The $20 million adaptive reuse project crowned the historic building with a 2-story, 16,000-square-foot addition, elevating the building to five stories and increasing usable space to 45,000 square feet.
The decision to use CLT for the addition was influenced by the long and narrow building configuration, site constraints, cost and construction schedule; the expansion would not have been feasible otherwise. The lightweight nature of CLT allowed the design team to retain the existing structure without additional reinforcement, which would have had budget and schedule impacts. Interior floors, walls and ceilings of the addition were left exposed, with the wood adding rustic warmth to the modern, open space design. The exterior walls were clad in a metal rainscreen panel system with exposed metal fasteners that referenced the building’s original use.
Incorporating mass timber products allows developers and property owners to reduce footing and foundation costs, increase sustainability and maximize aesthetic appeal to tenants.
Nathan Turner is an associate partner and senior project manager at Margulies Perruzzi.
