When it comes to seismic resistance, there are a number of myths that question the ability of wood to adequately perform in the event of an earthquake. However, its ductility allows it to deform plastically without breaking, absorbing and dissipating the energy generated by movement and vibration. Furthermore, unlike steel or concrete, wood is a lightweight material with a good strength-to-weight ratio, enabling it to withstand seismic forces without adding excessive load to the construction. This has been extensively verified in smaller-scale structures around the world, but how does a high-rise mass timber building behave in the face of an earthquake?
To dispel doubts, the Tallwood Project recently erected a 10-story building made of cross-laminated timber (CLT) at the University of California, San Diego (UCSD). The structure was tested on a shake table that simulated the 1994 Northridge earthquake in Los Angeles, magnitude 6.7, and the 1999 Chi-Chi earthquake in Taiwan, magnitude 7.7.
Mass timber buildings, constructed with layers of wood bonded together, are increasingly common in cities worldwide as they are seen as an environmentally responsible alternative due to their lower levels of embodied energy. This is especially the case in several jurisdictions in the United States, where updates to the International Building Code (IBC) have allowed for progressive increases in the heights of these buildings. The updates also provide detailed guidelines for design and construction, as well as requirements for fire resistance, load capacity, and seismic protection.
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Is Mass Timber a Good Choice for Seismic Zones?To gather concrete data and validate a seismic design methodology for tall wooden buildings, the US National Science Foundation (NSF) has allocated $17 million to upgrade the largest outdoor earthquake simulator in the world. Measuring 25 x 40 feet, this shake table is part of the NSF's Natural Hazards Engineering Research Infrastructure network and has the capacity to carry and shake structures weighing up to 2,000 metric tons or 4.5 million pounds, equivalent to 1,300 sedan cars. The simulator accurately reproduces the full 3D ground motions experienced during earthquakes, encompassing movement in all six degrees of freedom: longitudinal, lateral, vertical, roll, pitch, and yaw.
The Tallwood Building, designed by LEVER Architecture and fabricated by Timberlab using donated mass timber products, reaches a height of 116 feet. According to the architects, "The design, developed as a typical market rate prototype, is distinguished by its mass timber rocking walls, which allow the structure to rock and recenter itself during an earthquake, with no damage to the primary structural system. Post-tensioned steel rods extending the entire height of each rocking wall, as well as replaceable U-Shaped Flexural Plates (UFPs) at each floor level, absorb the force of a seismic event. The concept exceeds basic life-safety performance requirements by creating a resilient and easily repairable solution, avoiding the need to tear down the building following an earthquake."
During the tests, 800 sensors collected crucial data that will allow the development and calibration of computer models to help engineers design similar buildings in the real world. The top four floors will now be dismantled for further testing focused on the reuse of the material at the end of its useful life.
The project was supported by the US National Science Foundation and a consortium of universities, including Colorado School of Mines (lead), University of Nevada (Reno), Colorado State University, University of Washington, Washington State University, University of California (San Diego), Oregon State University, and Lehigh University. The project also received support from the US Forest Service, Forest Products Laboratory, and a number of industrial partners.
