Record-Breaking Residential CLT Building Announced
A new residential building in Switzerland is expected to become the tallest cross-laminated timber building in the world upon its completion.
Designed by Danish architectural firm, Schmidt Hammer Lassen (SHL), the timber building will stand 100 meters tall (about 328 feet) on a former industrial site in the city of Winterthur, near Zurich. According to reports, the structure will be comprised of four building sections of different heights with a terracotta façade applied to its exterior.
Project officials have named the building “Rocket&Tigerli” for its industrial history, which produced locomotives of the same name. In a company statement, it was reported that SHL utilized a construction system developed by the Swiss company Implenia and the Swiss Federal Institute of Technology of Zürich, ETH.
In adopting this system, the building’s concrete core has been replaced with wood, thus reducing the weight of the overall beam. Not only does the material replacement make it possible to build taller constructions, Implenia officials say it also is meant to ensure that the entire building process achieves a lower amount of embedded carbon.
Once completed, the residential CLT building will stand more than 30 stories high and will surpass the construction of the Ascent, which is a 25-story apartment tower under construction in Milwaukee. The structure is also slated to surpass the current world’s tallest CLT structure, the Mjos Tower in Norway.
Competing for World’s Tallest CLT
In March 2019, the Council on Tall Buildings and Urban Habitat confirmed that Mjøstårnet by Voll Arkitekter in Brumunddal, Norway, was the world’s tallest timber building.
The 85.4-meter-high (280-foot-tall) tower was built using CLT, which is billed as a sustainable wood material for tall building projects. The building has succeeded the 53-meter-high (roughly 174-foot-tall) Brock Commons Tallwood House in Vancouver (which is a hybrid wood and concrete structure) and the 49-meter-tall (roughly 161 feet) Treet building in Bergen, Norway (which is another all-timber building).
Construction for the Mjøstårnet, an 18-story timber high-rise, began in September 2018. The plans revealed that the structure would encompass 11,300 square meters (roughly 121,632 square feet) of space and contain various apartments, the Wood Hotel, offices, a restaurant and a 4,700-square-meter (roughly 50,590-square-foot) swimming hall.
The following year, construction began on the 25-story Ascent building in Milwaukee. Upon completion the tower is slated to stand 284 feet tall and house 259 apartment units, along with retail, parking, a swimming pool and fitness center.
Proposed in October 2018, the initial design of the high-grade timber-based mixed-use tower involved 21 stories and was predicted to be one of the tallest buildings in the world of its kind, according to its developer, New Land Enterprises LLP (Milwaukee).
The then-predicted 238-foot tower would feature laminated timber—columns and beams created by pressing layers of wood together—encompass 410,000 square feet and was designed by architectural firm Korb + Associates (Boston), with Thornton Tomasetti as the project’s structural engineer.
Months later, at the end of January, the Milwaukee City Council’s City Plan Commission unanimously recommended that a requested rezoning at 700 East Kilbourn Avenue move forward, a first step forward in making the residential tower a reality.
In March 2019, the rezoning for the residential tower was unanimously approved by council and, in May, the United States Department of Agriculture’s Forestry Service division awarded the project with an undisclosed grant. In total, the division awarded 41 grants totaling $8.9 million, suggesting that an average size grant would total roughly $217,000.
By August, another proposal was sought by city zoning approval to add two floors to the Ascent tower, bringing the structure’s total height to 23 stories.
However, in September, the Milwaukee Plan Commission and the Zoning, Neighborhoods and Development Committee unanimously approved changes to the project, making the structure even taller. According to Rocky Marcoux, executive secretary of the Plan Commission, changes to the project were regarding a redesign for the anticipated parking area.
In addition to the architecture and development teams, Fond du Lac-based C.D. Smith Construction Inc. and Milwaukee-based Catalyst Construction are leading construction work, while Swinerton Mass Timber, of Portland, is in charge of the mass timber material.
Innovative wood products have enabled timber construction to reach record-setting heights with Milwaukee’s Ascent. The 493,000-square-foot, 25-story, mixed-use tower ushers in 21st-century possibilities for low-carbon, high-density living. Learn more here: https://t.co/jgjP2flX9k pic.twitter.com/mIWRsdYlGD— Softwood Lumber Board (@LumberBoard) April 19, 2022
In July 2021, it was reported that general contractor C.D. Smith was working to link large pieces of timber together for the project. According to Urbanmilwaukee.com, the mass of timber is being provided by two separate European suppliers. Reports indicate that each piece of wood has already been specified for its placement in the building. For example, if there is piping or electrical components slated to go through the material, those necessary holes have already been created.
As the structure gains height, the width of the load-bearing pieces shrinks, further supporting the decrease in weight needed to support the entire building.
Although not all CLT building are competing for height or braggers’ rights, many other commercial projects have adopted the sustainable material. In August 2021, officials behind what is projected to be the tallest mass timber building in the United States reported that construction was on schedule at Intro Cleveland, a mixed-used building in Ohio.
The $150 million structure, which replaced an old strip mall, is slated to host about 300 apartments, ground-floor retail space and a top-floor event venue. Reportedly, about 80% of the 36,000 square feet of the ground floor already has tenants committed.
Reportedly, the timber used for the project is blond hem-fir and spruce trees harvested in the Austrian Alps. The structure was designed by Hartshorne Plunkard Architecture of Chicago.
At the end of that same year, Boston-based Shawmut Design and Construction announced its plans to use cross-laminated timber (CLT) for a mixed-use office building in Los Angeles.
According to reports, Shawmut is working in conjunction with Santa Monica, California-based real estate firm Redcar Properties and Portland-based architecture and design firm Lever Construction on the 75-foot-tall, 125,000-square-foot mixed-use project.
Once completed, the structure will be one of the tallest CLT structures in Los Angeles.
Mass Timber Guides in the Industry
In August 2020, The International Code Council, along with the American Wood Council, released a joint publication: Mass Timber Buildings and the IBC, which aims to provide an overview of requirements for mass timber construction as found in the 2015, 2018 and 2021 International Building Code.
The document highlights changes beginning with the 2015 International Building Code that allow for construction of mass timber buildings with larger heights and areas than was permitted for buildings of wood construction types (Types III, IV and V) prior to the 2015 IBC.
The document also reviews the 2015 IBC recognition of CLT, the reorganization of heavy timber provisions in the 2018 IBC, followed by the changes in the 2021 IBC and International Fire Code for tall mass timber construction.
The following year, the AWC announced that it had completed testing of connections used in CLT diaphragm. The latest tests results showed that the connections performed better than expected in higher-than-expected shear capacities.
The tests were conducted in support of new criteria for design of the CLT diaphragm design approach included in the 2021 Special Design Provisions for Wind and Seismic. They focused on the structural performance of the individual connections of the diaphragm, designed in accordance with the National Design Specification for Wood Construction.
The connection testing was conducted at Weyerhaeuser’s ISO 17025 accredited test facility in Federal Way, Washington, and included lateral, withdrawal and bearing strength per standard methods. The 2021 SDPWS is referenced in the 2021 International Building Code.
Around the same time, the Softwood Lumber Board announced that the WoodWorks – Wood Products Council and resource firm Think Wood have released the inaugural Mass Timber Design Manual.
The free, interactive resource aims to offer a comprehensive collection of the most up-to-date information on topics from mass timber products and design best practices to taller wood construction and sustainability. Funding for the manual was provided by the SLB.
And more recently, last month, researchers from the Massachusetts Institute of Technology developed a set of computational tools to assist architects and engineers to better avoid construction materials noted to have “embodied carbon.”
According to researchers at MIT, wood generally produces a much smaller carbon footprint as compared to steel and is noted to perform very well under forces of compression. However, steel is known to out-perform wood and other materials when it comes to tension.
From an emissions standpoint, Josephine Carstensen, an assistant professor of civil and environmental engineering and coauthor of a paper on the research, believes that architects and engineers should be opting for timber materials if the structure being built doesn’t have any tension. Carstensen also noted, that in selecting the wood material for these particular structures, steel could then be used for applications where its properties provide maximum benefit.