Mass Timber Building Construction

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).