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Shaping cross-laminated timber walls for seismic isolation

WISCONSIN—The concept of isolating buildings from earthquakes using elliptically shaped bearings was first introduced decades ago, yet the idea has had little practical use despite its simplicity and effectiveness. Isolation protects a structure from overload during an earthquake by providing flexibility between the ground and supported building. With digital fabrication tools now prevalent throughout the construction industry, cross-laminated timber panels have made elliptical seismic isolation possible at a scale larger than the original vision.

Researchers at the Forest Products Laboratory built full-scale, one-story-tall prototypes of elliptically profiled CLT walls. One could imagine these walls making up the first story of a building and swaying during an earthquake. Eventually, the system settles to rest and preserves the supported building, much like a rocking chair provides a stable seat throughout motion. Testing showed that varying curvature controls the rocking frequency. Backed by laboratory data, mathematical models make this elliptical CLT rocking wall system predictable and, therefore, useful for design.

Since the International Building Code has incorporated mass timber in its new version, allowing for buildings up to 18 stories high, it is worth continually assessing options to make tall wood buildings safer. Eccentricity of the elliptical shape leverages gravity of the superstructure so that CLT

Cross-lamonated timber
Testing a cross-laminated timber elliptical rocking wall, used for improving earthquake safety, at Forest Products Laboratory in Madison, Wisconsin. USDA Forest Service photo by James Bridwell.

self-center (i.e., stop rocking), without reliance on tensioned restraints. Elliptical CLT rocking walls, therefore, offer a resilient alternative to conventional earthquake-resistant systems for low- to mid-rise buildings. This is just one of the many innovations wood products offer that have the potential of affecting everyday life.