High-rise timber in Los Angeles?

Posted by Derek Mason

10th October 2023

Picture credit: Venti Views on Unsplash.  Picture caption: Will the LA skyline ever include high-rise timber buildings?

Counting down to an earthquake in 10… 9… 8… 7… Wouldn’t it be helpful if we did know in advance of an earthquake happening? When the real thing hits, there’s often very little – if any – warning, especially along California’s fault lines. But when you simulate an earthquake, as engineers at University of California San Diego did recently, a countdown is a natural part of proceedings.

These recent earthquake simulations happened on one of the two largest shake tables in the world – a table that’s been upgraded to produce an authentic ground motion. The table can move east to west, up and down, north to south, roll, pitch and yaw (twist or oscillate about a vertical axis). It’s also the world’s only outdoor shake table, making testing a full-scale building a little easier.

A 10-storey timber building was erected on the shake table – the tallest building to be put to the test in this way. In the aptly named TallWood project, one of the goals is to find out how a high-rise building made from cross-laminated timber would perform in earthquakes, since mass timber constructions are increasingly popular as an alternative to concrete and steel.

The building was designed to suffer minimal damage in an earthquake and to be easily repairable afterwards. It features a rocking wall system, where the building’s wooden wall panels are anchored down by steel rods. This seismic resilient structure enables the panels to rock back and forth, reducing the impact of the earthquake.

Engineers watched as the shake table was used to simulate two highly destructive 20th century earthquakes. The first test simulated the 6.7 magnitude Northridge earthquake, which occurred north-west of downtown Los Angeles in 1994. Next, a second test simulated the 7.7 magnitude Chi Chi earthquake that took place in Taiwan in 1999.

More than 800 sensors in the tower provided data, and GoPro cameras captured footage from inside the structure as it moved. The data captured means researchers will be able to develop and validate the computation models used by engineers to design buildings and bridges.

After the initial two earthquake simulations, researchers went on to run dozens of tests over several weeks. Shilling Pei, Ph.D., P.E., F.SEI, M.ASCE, an associate professor of civil and environmental engineering at the Colorado School of Mines and a principal investigator on the project, told Civil Engineering Online, “If you design the structure correctly, this timber material doesn’t crack or deteriorate. It’s very flexible and can absorb energy fairly elastically, and it’s only one fifth of the weight of concrete.”

The structural performance of the building behaved as predicted, with about 2% drift. Researchers had designed the building so the joints could rotate up to 3% drift without damage. Pei explained why this makes the timber high-rise building superior to other construction methods: “Well designed concrete or steel joints can likely tolerate 3% drift without collapse, but not without damage”.

What was more surprising was the performance of the non-structural system. Pei explained, “The façade of the building, including the windows, performed very well, and it wasn’t because they didn’t move. We had GoPro cameras looking at the corners of the windows and saw the glass leave the frames. But the frames never broke”.

After more than 100 earthquake tests, this 10-storey timber structure is still standing. Researchers are now digging deeper into the data they’ve collected to provide evidence to others that mass timber buildings are a viable option for areas at risk of earthquake.

I think it may be a while before the LA skyline is dominated by timber buildings, but it’s interesting to see these developments, especially considering the lower carbon-footprint of mass timber compared to carbon and steel. With the U.S. being the second biggest emitter of CO2 on the planet, as well as a country where earthquakes are a regular occurrence, let’s hope they do start to put some of this research to good use.

In the meantime, if you need assistance with the structural elements of an upcoming project, please do get in touch.

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