When it comes to seismic risks, there is an interesting (and mostly true) statement: “Earthquakes don’t kill people. Buildings do.” In this light, wood buildings have done well to ensure life safety of occupants even when heavily damaged in a seismic event. Light-framed wood buildings do get damaged in large earthquakes but will not easily collapse if proportioned correctly (i.e. no soft stories).

Traditional Light-framed Wood Construction

Mainstream wood construction in the U.S. is currently still light-framed wood systems, and it has been like this since the mid-1900s. This building type consists of dimensional lumber framing and wood panel sheathing, which performs very well in earthquakes from a life-safety standpoint for two main reasons:

• as a natural material, wood is much lighter than steel and concrete and has intrinsic flexibility, making it more resilient to earthquake loading; and
• the redundancy in light-framed wood building load paths makes it very robust against collapse. Even when most structural components are heavily damaged, the building system can still manage to remain standing by developing alternative load paths.

All buildings, regardless of materials used, are at risk of costly seismic damage. In the past two decades, major research and engineering projects have made strides in improving light-framed wood building seismic performances, including:

• CUREE-Caltech Wood Frame Project (1995 - 2000) supported by FEMA after the 1994 Northridge Earthquake in California. This was the first systematic investigation into seismic performance of as-built light-framed residential buildings. Wood shear walls were tested in the lab, effects of non-structural finishing materials were assessed, numerical models were developed for earthquake response prediction and large-scale shake table tests on full wood building structures were conducted. In short, this project established a foundation for quantitative understanding of wood building response in earthquakes. Before this project, such comprehensive research efforts were only seen in steel and concrete systems.
• NEESWood Project (2005 - 2009) supported by the National Science Foundation. This project took learnings from the CUREE project and pushed it to a higher level. The objective was to develop performance-based seismic design methods for mid-rise wood buildings that are not only safe but also have reduced damage during large earthquakes. A direct displacement design (DDD) method was proposed and verified using a full-scale six-story light-framed wood building on the E-Defense shake table in Japan. The project attracted great attention because it was the world’s largest building tested on a shake table to date. From NEESWood, the engineers know with certainty that they can design and build a six-story light-framed wood building that will only have drywall cracks (easy to repair) in a 2,500 year earthquake.
• NEESSoft Project (2010 - 2013) supported by the National Science Foundation. While NEESWood opened doors for new construction of multi-story buildings, the NEESSoft project examined existing wood building retrofit opportunities, focusing on “soft-story buildings” that have large openings on the first floor. Significant media attention was drawn to the full-scale four-story apartment building tested to collapse at the end of the project. But the important outcome is the retrofit techniques developed in this project and verified in full-scale testing before the collapse.

By 2014, earthquake engineering for light-framed wood building evolved to a point that:

• multi-story wood buildings up to six stories can be constructed to withstand large earthquakes with limited damage;
• existing soft-story buildings can be retrofitted to withstand large earthquakes without collapse; and
• a comprehensive set of design and analysis tools for light-framed wood building has been developed and validated through large shake table tests.

With more than two decades of research and development, light-frame wood building system design has met or even exceeded the seismic performance levels of typical steel and concrete systems for mid-rise construction. However, the size and height limit of wood construction is limited in the International Code Council’s (ICC) International Building Code (IBC), and there are still challenges to overcome as these advanced seismic design and construction techniques continue to transfer to the built environment.

Mass Timber and Tall Wood

In the 1990s, a new innovative wood product called Cross Laminated Timber (CLT) was invented in Europe. It was first applied to small projects such as vacation houses in mountain resorts. After the year 2000, bigger buildings were attempted, such as the nine-story Stadthaus Building in London and the 10-story Forte Building in Australia. It is very clear from these projects that a high level of prefabrication enables an extremely efficient construction process with greatly reduced time and labor requirements. More importantly, it was discovered through testing that wood components with large volume (also known as mass timber) can survive fire for extended period of time. If appropriate protection or sacrificial wood layers are implemented, a mass timber building may survive fire without losing load bearing capacity. This opens the door to even more opportunities for wood building design, including larger and taller wood buildings, than what is currently allowed for light-framed systems.

Around 2010, CLT started to gain traction in North America, boosted through an interesting TED talk by Canadian architect Michael Green on “Why we should build wooden skyscrapers.” A few notable mass timber buildings constructed and planned in North America include the six-story Wood Innovation Design Centre in Prince George, BC; the 12-story Framework Project in Portland, OR; and the 18-story Brock Commons Tallwood House at University of British Columbia. Worldwide, many more ambitious tall wood building projects have been announced, including conceptual designs pushing for wooden sky scrapers over 30 stories. An ongoing “mass timber movement” has set in motion and is still gaining momentum today.

Research and testing the seismic performance of mass timber construction continues today. While CLT is recognized as an engineered wood product in the 2015 National Design Specification for Wood Construction, there is currently no codified seismic design approach in the U.S. specifically for CLT-based lateral systems. For this reason, existing mass timber buildings often utilize steel or concrete lateral systems for earthquake loading.

A comprehensive review paper in 2014 summarized the history of seismic research on CLT systems, with a few major efforts and advancements listed below:

• SOFIE project (2004 - 2009) was carried out by the IVALSA Institute of the National Research Council with the support of Provincia Autonoma di Trento. As one of the major CLT seismic research efforts in Europe, the project established performance and design metrics for platform-style CLT buildings under earthquake loading. The project involved connection and wall testing, as well as multiple large-scale shake table tests in Japan. SOFIE project has had a major impact on seismic design of CLT buildings in Europe.
• Currently, through a series of research development led by FPInnovations, platform CLT panel walls have been recognized by the National Building Code of Canada (NBCC) with a fairly conservative set of seismic design parameters. So currently there is a codified procedure for seismic design of CLT buildings in Canada.
• An ongoing research project funded by the U.S. Forest Service’s Forest Products Laboratory seeks to determine the seismic design parameters for CLT shear walls. The project aims to generate design parameters compatible to ASCE 7-16 – Minimum Design Loads and Associated Criteria for Buildings and Other Structures requirements that can be used by engineers to design platform CLT buildings in the U.S.
• Most recently, the NHERI TallWood Project was funded by the National Science Foundation to develop resilience-based seismic design approach for tall wood buildings at eight to 20 stories. A full-scale two-story mass timber building with resilient rocking wall system was just tested in 2017, which survived 14 earthquakes without major damage. The project team is planning to design and test a full-scale 10-story wood building on a large outdoor shake table in 2020, followed by a large-scale fire test of the structure (also referred to as the Shake & Bake 2020 test).

Beyond these developments related to seismic design, mass timber buildings are also being considered by the ICC as new construction types for the next IBC update cycle (with public voting currently underway). These mass timber systems offer superb earthquake performance and great potential to expand the wood building market in larger and taller building sectors.

What the Future Holds

With its natural beauty, sustainability and resiliency, wood is a good material for building against earthquakes. With proper design and better quality control, light-framed wood buildings will continue to perform better in seismic events and dominate low-rise commercial and single-family residential construction in the U.S. Innovative mass timber buildings will also emerge in big cities and then spread out as the production and code acceptance of this new material grows.

Photo: New seismically-resilient mass timber building prototype tested in NHERI Tallwood Project, 2017. Courtesy Shiling Pei, Colorado School of Mines.