Fire-safety testing paves the way for timber towers
November 22, 2016
In the wake of extensive research into fire safety, a new kind of building will soon rise in the United States: the wooden tower.
Although tall wood buildings in countries like Norway and Austria have made headlines around the world, mass timber, as this type of construction is known, has yet to make significant inroads here. Wood dominates low-rise residential construction in the US, but is absent in buildings more than five stories tall.
Many designers, timber industry representatives, and government officials are pushing for that to change. Their motivations vary. Mass timber offers incredible aesthetic benefits and could create new jobs in rural areas. It also holds tremendous potential to mitigate climate change. Carbon-heavy steel and concrete production accounts for almost 5% of the world’s greenhouse gas emissions. Mass timber products require no fossil fuels to produce — trees grow with solar energy alone — and can be replenished through sustainable logging and tree farms. As an added bonus, it can sequester carbon held in trees, preventing it from seeping into the atmosphere when they decay.
In 2014, the United States Department of Agriculture and timber industry partners, enthusiastic about the combination of new jobs and reduced greenhouse gas emissions, launched the U.S. Tall Wood Building Prize Competition. The contest offered $3 million to teams of designers and developers who pledged to build a mass timber tower at least 80 feet high.
The government carefully structured the competition to address the primary barrier to this typology in the US: code compliance. Design teams opting to use steel or concrete can find ample documentation of safety standards in national and local building codes. When it comes to tall buildings aiming to take advantage of mass timber, however, the codes have little to say.
As a result, developers have to undertake the research needed to demonstrate acceptable levels of safety themselves, which can prove extremely expensive. The Tall Wood competition aimed to offset those costs, encouraging developers to try their hands at the typology.
Last September, Agriculture Secretary Tom Vilsack announced the winners: a 12-story mixed-use building in downtown Portland, Oregon, and a 10-story residential building in New York’s Chelsea neighborhood. Arup is working on both, applying the knowledge gained from the independent tests of cross-laminated timber (CLT) systems it has performed over the past decade.
The rise of CLT
CLT is made by gluing multiple two-by-six-inch wood beams together, then placing the resulting slab on top of an identical one oriented in the opposite direction. The process is then repeated until the desired depth is reached.
The resulting board, which some call “plywood on steroids,” is strong enough to be used for floors and walls. It completes the array of tools needed to build large structures completely out of wood, from façade to core.
The Tall Wood competition was explicit about its goal of jump-starting the technology in the US. “This competitive prize,” read the 2014 announcement kicking off the contest, “will showcase the architectural and commercial viability of advanced wood products like CLT in tall buildings.”
One of the largest regulatory (not to mention psychological) obstacles facing mass timber projects in the United States is fire safety. The US building code prohibits structures over six stories made of “combustible materials” — e.g., wood.
Fire engineers like David Barber, who works in Arup’s Washington DC office, have been working to change this by demonstrating that structural mass timber can offer as much fire protection as steel or concrete — and sometimes even more.
Wood chars when burned, providing a natural layer of insulation that protects trees from forest fires. When builders use steel, on the other hand, they have to apply artificial insulation, spraying the metal with fire-retardant foam or encapsulating it with fire-rated board. If they opt for concrete, they have to add additional inches for fire protection.
Structural mass timber can offer as much fire protection as steel or concrete — and sometimes even more.
A plethora of research on the fire performance of load-bearing wood materials like timber and glue laminated timber already exists, much of it conducted by the United States Department of Agriculture from the ’60s to the ’80s. But new materials like CLT are a different story. Although they’ve been tested internationally, little research has been carried out in the US. Because state and municipal building codes typically require construction materials to be vetted on American shores, this presents a significant barrier to implementation.
Barber and his colleagues are stepping in to fill the gap, examining, among other things, CLT’s ability to maintain its structural integrity when exposed to flames. To test this, they place weight on a panel, then put it in a furnace to see how long it holds up before breaking. Using the results of these tests, they can work with regulators to determine how much wood is required to match the level of structural safety that US building codes demand.
The two US projects are helping to resolve another unknown: how the hardware used to connect mass timber (and particularly the pieces that link beams and columns) will perform in fire.
Currently, these connections are neither standardized nor well researched — particularly with respect to American building codes, which are more stringent than those in many European countries. The New York and Portland project teams hope to remedy this by researching and designing wood connections and fasteners that address regulatory demands, structural safety requirements, and aesthetic considerations.
Similar to the CLT tests, this process involves placing full-size beams and columns into a furnace, adding weight to simulate the structural load they would carry in a building, and burning them until they collapse.
The engineers record information about temperature, movement, and char depth, then use this data to optimize their designs.
Building a tall timber future
In keeping with the spirit of the Tall Wood competition, all research that Arup and its partners are undertaking will be available to other designers and developers in the future. Ideally, this will enable tall timber buildings to pop up in cities across the United States.
Questions or comments for David Barber? Contact firstname.lastname@example.org.