26 July 2021

The rise of mass timber tall buildings in the US

TRADA image

With 1.5 million people moving into cities each week, the global imperative to plan and build urban environments that are resource-efficient and resilient to climate change has never been more urgent.


Currently, the construction and operations of buildings accounts for nearly 40% of the global carbon emissions load, and studies indicate that 70% of the infrastructure needed by 2050 is yet to be built. Thus, in order to address the question of how to provide housing, commercial, office and other spaces in dense, vertical cities while minimising the emission of greenhouse gases, the architecture, engineering and construction (AEC) industries must critically consider and evaluate choices behind designs, construction materials and processes.


The rise of the mass timber tall building, in which engineered timber products perform primary loadbearing functions for multi-storey developments, may provide a significant opportunity to address many of the problems facing the building industry. Constructing more tall buildings with structural timber – a renewable, natural material that actively stores carbon in its dry weight, in a process known as carbon sequestration – could usher in a paradigmatic shift across the city-building industry, increasing its sustainability and reducing its contribution to greenhouse gas emissions.


What is mass timber?

Mass timber buildings, which are distinguished from light timber buildings based upon larger cross-section size, use whole sawn timber pieces or engineered timber products to act as the primary loadbearing structure. Innovations in the timber industry have resulted in a number of engineered timber products, in which smaller wood members are joined with nails or dowels or, increasingly, fused with a water-resistant adhesive, allowing for large enough cross-sections to achieve the structural parameters required to be defined as a mass timber building.


These products, which can form a durable, loadbearing structural panel, column or beam include cross-laminated timber (CLT), nail-laminated timber (NLT), glued laminated timber (glulam), dowel-laminated timber (DLT), structural composite lumber (SCL), and wood-concrete composites.


Carbon sequestration

As the AEC industries work globally to reduce the amount of carbon emissions generated through building projects, timber is an outlier – a material that, while growing, actively removes and stores carbon from the environment, and for an extended length of time. On average, 50% of a tree’s dry weight is carbon. A single Douglas fir will sequester 966kg of carbon over a 20-year span. In the US, forests and other non-agricultural lands absorb a net 13% of the country’s global carbon emissions.


Forest age vs. efficiency

All forests represent carbon sinks to some degree, but the most efficient ones are abundant with young trees, defined as trees that are younger than 140 years. Younger trees sequester carbon at greater rates (~25%) than older trees, so cutting down trees to use in construction, then replanting, has a positive carbon impact.


Construction efficiency benefits

Apart from being a renewable building material, timber’s high strength-to-weight ratio means that transporting building materials to the building site results in lower carbon emissions, as compared with a building using heavier products. A mass timber building has the potential to be up to 75% lighter than a concrete building of the same proportions, which in addition to potentially being more carbon-efficient, can also reduce foundation sizes. 


The psychological equation

In a study conducted in Japan, participants’ diastolic and systolic blood pressure decreased in response to a room with a higher ratio of wood than a control room with no timber interior cladding at all. In a 2011 Kelz et al. study conducted in Austria, pupils were taught lessons in a classroom with a timber interior, as well as a conventional, non-wood classroom, over the course of a school year. The students in the timber classroom displayed indicators of a more activated parasympathetic nervous system, which acts to reduce stress levels and regulate healing and recovery. 


Tall mass timber today

While mass timber was popularised in Europe in the early 2000s, it wasn’t incorporated into the International Building Code (IBC) until 2015. Key projects have helped reshape the modern day building code in North America around the use of timber as a structural material. In Vancouver, Brock Commons Tallwood House was completed in less than 70 days using prefabricated components. The project didn’t satisfy local codes at first, so site-specific codes were developed. In 2018, Washington became the first state in the US to update its state building code to permit mass timber buildings up to 18 storeys tall, paving the way for engineered timber mid- and high-rise buildings throughout the country.


Poised to be the tallest timber composite building in the world once complete in 2022, Ascent in Milwaukee underwent rigorous fire testing to demonstrate adherence to local standards. As part of this testing, the building’s timber floor slabs and adhesive needed to be able to maintain structural integrity after exposure to intense fire for three hours. The materials were tested at the US Department of Agriculture Forest Service’s Forest Products Laboratory, in Madison, Wisconsin, where they were able to survive the testing period without losing structural stability.


In March 2019, the Council on Tall Buildings and Urban Habitat verified the completion of the world’s tallest all-timber building: Mjøstårnet in Brumunddal, Norway at 85 metres. This inauguration coincided with an amendment to its height criteria, the official guidelines upon which tall buildings are measured, to include timber as a recognised structural material. Following this, the Council conducted a global audit of tall timber projects, which it keeps regularly updated.


Tall mass timber construction has the potential to be part of a sea change in the tall building industry, ushering it towards a more resource-conscious, low-carbon paradigm that delivers on the high standards of safety and quality. Furthermore, as discussed, structural and technological innovations in mass timber have transformed it into a high-performance material, with the potential to be on par with conventional construction materials.


About the authors

Emily Torem, Staff Writer, CTBUH

Daniel Safarik, Assistant Director, Research and Thought Leadership, CTBUH


This excerpt is taken from the recently published book, Timber Rising: Global Perspectives on Mass Timber Advances for the Tall Building Industry, produced by the Council on Tall Buildings and Urban Habitat in conjunction with the US Department of Agriculture Forest Service.


This is a new release from the 2020/2021 Industry Yearbook Online. Download the article, including further reading and a full list of references, here