Testing timber

Around the world, architects, planners, and environmentalists are betting that wooden towers are the future of building design. But in the United States, building codes prohibit their construction. Until the authorities are convinced that timber is safe, would-be wood builders will have to wait — or help with the convincing.

Hans-Erik Blomgren, a structural engineer in Arup’s Seattle office, has opted for the latter. In his eight years with Arup, he has explored the potential of wood as a lightweight, affordable, and renewable building material. Now, by designing and testing new methods of timber construction, he’s helping lay the groundwork for the next generation of wooden buildings.

Wood benefits

Timber’s appeal stems largely, though not entirely, from its impressive environmental qualities. Wood sequesters carbon dioxide, is naturally renewing, can be recycled for a variety of uses, and demonstrates excellent thermal performance.

Its advantages become even clearer when embodied energy is taken into account. Wood is essentially created with solar energy — it’s nurtured by sunlight and water. Compare this to the process required to produce steel and concrete, which is more energy intensive to produce, and clear differences emerge.

Until the authorities are convinced that timber is safe, would-be wood builders will have to wait — or help with the convincing.

A modern industry — elsewhere

While tall wood buildings have existed for millennia, the advent of cross-laminated timber (CLT) in the 1990s made them viable in the modern construction industry. CLT is prefabricated, lightweight, and quick to install, making it an ideal building material. Austria, Germany, and Switzerland now have mature CLT manufacturing industries, but the material has only begun to make inroads in American and Canadian markets (via British Columbia and the Pacific Northwest).

Structural safety

Current US building codes prohibit wood construction above five to six stories, partially due to concerns about structural safety. Changing the codes will require convincing authorities that timber buildings can perform well in all conditions, including earthquakes.

In recent times, tall (i.e., seven stories or higher) wood buildings have been built only in countries with little seismic activity. Although researchers in Italy, Japan, New Zealand, and Canada have studied timber seismic systems, none have been realized at these heights.

Recognizing the knowledge gap in this area, in 2013 the US National Science Foundation (NSF) awarded a research grant to develop and test CLT shear walls for use in regions of high seismicity. The research team comprises five universities, the US Forest Products Laboratory, FPInnovations, and Arup.

Such efforts are critical, Blomgren believes: scientific evidence will lead to code changes, which will encourage more designers, contractors, and clients to adopt the technology. This process has already occurred in other countries. Austria updated its building regulations to allow building tall with wood, enabling a number of highly regarded developments to spring up. UK policy accounts for carbon sequestration and carbon offsets, which have helped the market share of structural timber to grow.

“Building codes need to change, becoming more agnostic when it comes to materials,” Blomgren said. “There is always inertia in the construction industry, and it has yet to reach a tipping point in North America.”

The big day

In April 2015, after months of development, the research team held a public viewing of a component-level test of its wall system. The idea was to take a structural member — in this case, a 16-by-4-foot CLT shear wall — and examine its behavior under simulated seismic activity. “Structural engineers always design against how something fails,” said Blomgren. “We need to know the limit states of the developed system.”

Test panel

The team needed to demonstrate that the panel would maintain its structural integrity during an earthquake and regain its proper vertical position after the shaking subsided. It ultimately selected a design solution that embedded a pretensioned rod in the center of the CLT panel, helping to restore it to vertical after rocking from side to side.

CLT panels

The team inserted a steel rod between layers of CLT panel, then pulled the rod and attached it to the building base.

While previous tests had been open to only the research team, the public was invited to this round — a showcase of progress made. Contractors, building engineers, structural engineers, building officials, and architects gathered in the Washington State University Composite Materials and Engineering Center, while NSF officials watched live via webcast.

During the test, observers watched as a machine applied pressure to push and pull the top of the panel.

Credit: Arup and Green Ideas

During the test, a machine applied force to the top of the pretensioned panel, pushing it farther each time.

The panel began to rock, and the wood showed signs of crushing at the points where the stress was greatest.

As the applied force increased, the stress on the rod grew. The panel’s bottom corners also experienced higher levels of crushing as the rocking increased.

As the applied force increased, the stress on the rod grew. The panel’s bottom corners also experienced higher levels of crushing as the rocking increased.

At key stages of the test, the machine stopped while the engineers marked the locations where the wood had begun to crack. They then repeated the experiment several times, pushing the top of the panel farther each time — eventually reaching up to 18 inches of displacement.

The outcome: success. After the machine was pulled away, the panel returned close to its original position. The tests showed that the team’s design could maintain load-bearing capacity under highly seismic conditions, then return the panel to its original vertical position.

The team graphed the movement at the top of the panel compared to the force applied at the top of the panel, demonstrating that the design successfully maintained strength during very large displacements.

Next steps

Having proved that the design is sound, the team is now focused on “taking the principles learned at the component level and applying them to the full building system level,” said Blomgren.

A submission for a further stage of NSF funding is now being developed. If the application is successful, the team will construct a multistory mock-up and test it on shake tables to provide evidence of building-scale structural resilience.

Credit: Arup

A follow-up test demonstrated that the team’s solution worked for two coupled panels positioned side by side.

With luck, these and similar initiatives will bring timber towers to a city near you in the not-too-distant future.

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