Mass Timber Buildings

PMX 15, Part 1: How to design a timber building to code in a seismic region

In the first installment of our PMX 15 series, we describe the design choices that would help our timber building withstand a major earthquake.

By Mark Bauernhuber, Lily Huang, and Kristin Slavin

A rendering of PMX 15 outdoor space and entrance

A rendering of the entrance for PMX 15, Sidewalk Labs’ proto-model for a 15-story mass timber building. (Sidewalk Labs)

This is the first of two posts exploring our PMX 15 design. We recommend starting with the series introduction here. Check back next week for Part 2!

The Pacific Northwest is a North American leader in mass timber buildings, including the 18-story Brock Commons in Vancouver. It’s also home to one of the world’s most productive timber industries, showing the way forward for how to use sustainably harvested forest to build dense urban environments.

But the Pacific Northwest is a very high seismic region, meaning any timber structures built there need to be designed for safety during and after a major earthquake. To tackle this design challenge, the PMX 15 team chose to locate our hypothetical building in Seattle, which lies squarely in the earthquake impact zone. Structural designs that are seismically viable in Seattle will be representative of a variety of building locations in cities up and down the West Coast — and other parts of the world.

The seismic forces imparted on the building are directly proportional to the weight of the building. That works in timber’s favor when designing in an earthquake zone: the material’s lightweight nature actually reduces the seismic forces on the building by 10 to 30 percent compared to a steel or concrete counterpart. It also creates significant cost savings and sustainability gains from a smaller foundation.

But whereas PMX 35 had an all-timber structural system, further improving its sustainability benefits, PMX 15 was not allowed by code to achieve its seismic requirements using an all-timber lateral force resisting system. So we needed a design that could meet the code while still meeting our sustainability and manufacturing goals.

The Eccentrically Braced Frame

When designing any building for a high seismic region, the goal is to create what engineers call an “energy-dissipating system.” During a seismic event, the accelerations from an earthquake will create strong lateral (or side-to-side) forces on the building. If those forces become too strong, they can put the building’s connections, core, and foundation at risk of collapse. An energy-dissipating system reduces that risk by limiting seismic forces “felt” by the rest of the building.

For PMX 15, our team designed a lateral system with steel bracing distributed throughout the building to resist lateral movements and forces during seismic events. In the event of an earthquake, the ductile links connecting the steel beams would serve as an energy-dissipating element (often called a “fuse”). As the building rocks left and right, the links are designed to reduce forces and absorb energy. By directing the movement forces through the link connections in a controlled manner, the lateral system would prevent these forces from reaching other parts of the building, where more damage could occur.

In simple terms, the lateral system would help absorb the earthquake without breaking, keeping the building standing. Our calculations showed that the Eccentrically Braced Frame would withstand a 1-in-2,500-years earthquake, as required by seismic code.

Beyond the foremost objective of occupant safety, the Eccentrically Braced Frame has other advantages. One is speed of reoccupation. By concentrating damage in the ductile links and limiting impact on the remaining structure, the Eccentrically Braced Frame for PMX 15 readily enables repair teams to replace these links without tearing down the building — getting people back in their homes or offices faster. A typical concrete lateral system has a higher chance of needing to be demolished after surviving a high seismic event, given the challenges that concrete presents for on-site repair.

Another advantage of our Eccentrically Braced Frame is that it isolates the amount of customized bracing to the sides and the center of the structure. That opens up the vast majority of PMX 15 for manufactured timber parts — more than three-quarters, by our estimate — helping the building achieve its goals for carbon and construction speed. What’s more, we designed the Eccentrically Braced Frames to be fully compatible with our kit of parts, meaning they’re installed along with the timber parts for faster assembly.

The next-generation building code

In addition to the challenge of design in a high seismic region, Seattle also provided the PMX 15 team a chance to create a model that meets the very latest U.S. building code: the 2021 International Building Code. By recognizing the first new construction types in American building codes in decades, the 2021 IBC sets the stage for a profound leap forward in sustainable cities.

First, a little background on building codes. In North America, building codes tend to be highly prescriptive, requiring additional steps for designs that veer from pre-approved standards. This extra hurdle makes it difficult for developers to pursue innovative building approaches, even if those approaches have benefits to carbon footprint or project speed, such as factory-based mass timber. For example, prior IBC codes only pre-approved the construction of timber structures up to six stories. That meant few U.S. developers made the effort to build taller with mass timber, despite the technical and structural capacity to do so.

Following years of extensive safety testing and committee review, the 2021 IBC introduced a new set of pre-approved construction types for mass timber buildings, enabling developers to pursue such structures up to 18 stories without a special design review.

As an early adopter of the 2021 IBC, Seattle has the chance to provide a global example in next-generation climate-friendly buildings. Seattle’s code includes four types of timber structures, ranging in height from 6 to 18 stories (depending on other design considerations). We adapted PMX 15 to meet Code IV-B, allowing for some of the timber structure to be exposed on the building interior, which we think is important so residents get the biophilic health benefits of timber construction. (Of course, our approach can be fully encapsulated if that is preferred aesthetically.)

Under IV-B, some of a building’s mass timber components are encapsulated (meaning they are covered by material that provides additional fire protection), whereas other components can leave the timber exposed. In the case of PMX 15, most of the beams and columns have some exposure, especially in highly desirable areas like shared spaces and residential units, but the facades and most of the timber elements within the cassettes are encapsulated. In the case of the cassettes, we felt encapsulation made the most sense to address other considerations, such as acoustics and concealing the in-unit mechanical, electrical, and plumbing systems.

Taken together, this holistic, building-wide approach enabled us to leave as much wood exposed as possible while also meeting the prescriptive code requirements. All structural elements, whether exposed or encapsulated, live up to the 2-hour fire rating required to meet the code. (A side note: PMX 15 technically exceeds IV-B limits by three stories; we went this high to test other performance measures on the structural system, such as its seismic integrity, but the building could lose the extra three stories and still meet code.)

By designing to the 2021 IBC requirements, the PMX team charted a course for developers to meet the very latest building code with the greater predictability and precision provided by a factory-based construction approach. In the next post in this series, we’ll explain just what that approach means for the pursuit of more sustainable cities.

If you’re a developer or timber professional who wants to help us advance mass timber building design or offsite manufacturing, send us a note through our website or email