As structural engineers, we play an important role in shaping the built environment – from developing structural designs with a sensitivity for architectural aesthetics to choosing the best material types for those designs. Versatile, attractive, and sustainable, the potential of mass timber in new construction across the United States is capturing the imaginations of many leading building professionals and sustainability advocates alike. Whether building tall structures or sprawling long-span complexes, the use of mass timber (see adjacent list of potential components in a mass timber structure) enables the development of high-performance buildings that are exceptionally strong, have a lower carbon footprint than concrete, are lighter and stronger than steel, and are able to achieve similar fire resistance ratings.
In contrast to light-frame timber construction, mass timber construction uses a variety of pre-engineered products comprised of laminated layers of wood (typically held together by glue, dowels, or nails) that create strong, structural load-bearing elements like large panels, posts, and beams.
The material poses many benefits for builders in terms of viability.
First, fire safety. During a fire, exposed mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage. Testing at the US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) laboratory subjected a two-story structure built using Cross Laminated Timber floors and perimeter walls. Glulam beams and columns supported the levels above. Interior partition walls were built using light gage metal framing. The test structure was built using industry standard CLT construction methods and techniques in accordance with the ICC Ad Hoc Committee on Tall Wood Buildings (TWB) proposed construction.
It lasted 3 hours and 6 minutes, well exceeding the requirement of by building codes. For Type IV-A, a 3-hour fire resistance rating is required for primary structural frame and exterior bearing walls. A 2-hour rating is required for floor construction. For Type IV-B and IV-C, a 2-hour fire resistance rating is required for the primary structural frame, exterior bearing walls, and floor construction.
Second, strength. Modified to be stronger, improvement in mass timber manufacturing means products like CLT can achieve spans normally left to concrete slabs at a fraction of the weight. On average, mass timber buildings can weigh as little as 1/5th that of comparable concrete buildings. This in turn reduces their column sizes, foundation size, seismic forces, and embodied energy. With strength and stiffness that approach that of reinforced concrete, conventional mass timber structures are able to dissipate energy far more efficiently than ordinary reinforced concrete structures. As such, it results in a significant reduction in lateral forces the building needs to resist due to seismic events.
Third, Speed and Savings. Mass timber involves less construction waste which saves time and money. This is especially important in urban development because it can be craned into place. Consequently, projects take up less space on city streets during construction. Fully coordinated shop and erection drawings help create an efficient flow on site, enabling mass timber prefabricated elements to be installed quickly. When used as flooring, there is no need to provide temporary shoring for several days as is typical with reinforcing concrete structures. With no down time for curing or obstructions caused by temporary shoring towers like concrete, follow-up trades can begin work shortly after the timber frame elements and panels are erected. A shortened construction schedule results in cost savings in construction and a faster start to revenue generation associated with the intended use of the building.
Fourth, Sustainability. The building industry accounts for nearly 40% of global CO2 emissions. The manufacture of concrete and steel each contribute to about 5% of that number. According to a study in the Journal of Sustainable Forestry, replacing steel with renewable mass timber would reduce carbon dioxide emissions by approximately 15% to 20%. Additionally, most of the energy needed to manufacture mass timber building elements is produced using energy derived from carbon neutral biomass combustion. With products like CLT, the stored carbon mass of the wood is also significant. Wood in the panels pull CO2 from the Earth’s atmosphere. Lower embodied energy from the manufacturing process combined with the seizing of atmospheric carbon means the material is in line with the AEC’s commitment to Sustainable Development Goals, Additionally, it could be a factor in LEED certification or other green building ratings.
Most commonly, large-scale mass timber projects involve the use of two specific products: Cross Laminated Timber panels (CLT) and Glue Laminated Timber beams/columns (Glulam/Paralam).
Cross Laminated Timber: CLT
Glue Laminated Timber:
Beam or Column: Glulam
Floor or Roof Slab: GLT
Structural Composite Lumber:
Laminated Veneer Lumber LVL
Parallel Strand Lumber PSL
Laminated Strand Lumber LSL
Oriented Strand Lumber OSL
Mechanically Laminated Decking:
Nail Laminated Timber: NLT
Dowel Laminated Timber: DLT
Mass Plywood Panels: MPP
In structural systems (such as walls, floors, and roofs), large scale, prefabricated, solid wood engineered Cross Laminated Timber panels serve as load-bearing elements. Because CLT is made with layers alternating at 90 degrees to each other, it has strength in two directions. CLT has two way spanning characteristics similar to a concrete slab.
CLT can be used for both walls and slabs since it can bear in-plane and out-of-plane loads. For wall applications, the lumber used in the outer layers is oriented vertically so its fibers run parallel to gravity loads. This maximizes the wall’s vertical load capacity. In floor and roof applications, the lumber used in the outer layers has fibers oriented parallel to the direction of the span. Each panel’s ability to resist high racking and compressive forces makes it especially cost-effective for multistory and long-span diaphragm applications.
With four appearance grades—framing, industrial, architectural, and premium—glulam is used for both its structural and aesthetic attributes. Glue Laminated Timber is made with layers all oriented in the same direction. It is used most commonly for one way spanning requirements such as columns, beams and trusses. For exposed applications, such as vaulted ceilings and designs with long spans, builders can take advantage of the wood’s natural aesthetic. Glulam’s contemporary and distinctive look can be seen on some of McLaren’s recent projects like The Clubhouse at Greenliegh at Crossroads in Maryland and Rockland County Highway Facility in New York.
The AEC community in the US is now beginning to embrace mass timber as a feasible building option for architecturally expressive and resilient structures. As a safe, viable alternative to steel and concrete, mass timber construction will also be increasingly used in taller buildings, particularly now that wood construction can span beyond six stories to mass timber hybrid systems of more than 18-stories. The International Code Council (ICC) has approved 17 changes to the 2021 editions of the International Building Code (IBC) and International Fire Code, allowing for mass timber buildings to be constructed this high.