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How tall timber structures can survive “the big one”

How tall timber structures can survive “the big one”

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Over the past decade, timber has been making a comeback in the building industry. Timber manufacturing has grown 10-fold, and the square footage of large timber buildings has nearly quadrupled in the United States. These buildings are not single-family homes, but offices, dormitories, and condominiums. They do not use typical 2x4s. Rather, these buildings use “massive” timber products such as cross-laminated timber (CLT), a panel-like material that is constructed by gluing timber boards together, alternating each layer 90 degrees to improve the mechanical properties. In our own backyard, the Peavy Hall forest science complex, serving the College of Forestry at Oregon State University, is built with CLT.

Massive timber products make wood a more predictable material by evenly distributing natural properties such as knots. Additionally, they allow for a larger product to be made than could be cut from a single tree trunk. Using the capabilities of massive timber, more than 44 wooden structures six-stories or above have been constructed around the world in the past six years. The tallest massive timber building in the United States, known as Carbon 12, has showcased major strides in the global forest products industry, and is located just 80-miles north in Portland.

Some people have wondered whether tall timber structures could persevere in an earthquake-prone region such as the west coast of North America. This region is along the Cascadia Subduction Zone, a 1,000-kilometer fault line in which a magnitude 9.0 earthquake may occur, often referred to as “the big one.” How do engineers know that massive timber structures could withstand an earthquake?

To answer this question, we need to know a bit about how buildings respond to earthquakes. Building response is dependent on many factors, but an important one is building weight. The seismic forces that engineers design for are proportional to building weight, which means that a heavier structure may experience a larger earthquake force. Wood is a light material compared to steel or concrete, which helps to reduce the overall demand on the building.

But just because the building is light does not mean it is earthquake-proof. In fact, there is a difference between a structure’s strength capacity to withstand an earthquake and its stiffness to reduce deformation. A lighter and less-stiff building may make occupants feel greater shaking, and create large movements that damage building components. Thus, stiffness must be considered in the design of timber buildings, and has been an important topic of research. For example, at OSU, a stiff mass plywood panel (MPP) developed by Freres Lumber of Lyons, has been tested to be used as an earthquake resisting system in buildings.

Furthermore, the energy transferred by an earthquake force to a building needs to go somewhere. Oftentimes, engineers will designate certain building components to be responsible for dispersing this energy, understanding that the components likelywill  be damaged, but ensuring that failure would not lead to safety concerns. Again, at OSU, testing of structural connections to best disperse energy in timber buildings has been ongoing for many years.

A different way to improve the seismic response of timber buildings was developed in New Zealand in 2005. The design places steel rods in parallel with massive timber products. These rods are post-tensioned, meaning a stretching force is applied so that they compress the wood to keep it in place. When subjected to an earthquake, the rods control the motion of the system, and pull it upright afterward. Think of a guitar string: The strings are tuned by pulling the ends, plucked so they vibrate to play the desired note, and eventually return to their original position.

A key piece of this system is that it re-centers the building. If a building is leaning after an earthquake, it can be so difficult to repair that it is less expensive to simply tear down and rebuild the entire building. Because of this quality, this design goes beyond basic safety requirements of our current building codes. This system has shown excellent performance in laboratory tests and in the real-world. After a magnitude 7.8 earthquake in Kaikoura, New Zealand, a post-tensioned timber building not only survived the earthquake, but was so well kept that it served as the city’s response headquarters. As it turns out, the first building to use this system in the United States is Peavy Hall at OSU.

So, tall timber buildings can survive “the big one” because our engineers are ensuring that the buildings remain light, have adequate stiffness, and have components in place to take the earthquake energy. In some cases, these timber buildings have gone beyond safety requirements to limit damage and downtime after an earthquake. At OSU, research is ongoing to continue to improve current systems and validate their performance over the course of 50-years.

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