As part of the firm’s robust internship program, our summer interns each complete a research project that they present to the firm. In this multi-part series, our 2023 cohort shares their findings; see all the internship posts here.
The opportunity to work at Hennebery Eddy this summer has been an unforgettable experience. The work environment and my coworkers have really impressed me; they’ve taught me so much and are always willing to impart their knowledge and methods. As I approach my thesis year at Portland State University, I have gained understanding and a new perspective on the realities of the built environment. I intend to take these insights with me into my final year of graduate school.
With my thesis in mind, I decided to research seismic resiliency and unreinforced masonry buildings (URMs) in Portland — and specifically on finding an alternative structural building material that contains a low-embodied carbon equivalent (such as mass timber) to seismically retrofit these URMs. Hennebery Eddy specializes in historic preservation and rehabilitation; therefore, I knew that by diving into this investigation, a new seed could be planted for considering alternative, sustainable materials to retrofit existing and historic structures.
There are currently more than 1,600 unreinforced masonry buildings in Portland, which puts the city in grave danger in the event of an earthquake of a magnitude greater than 5.0. Any building built between 1870 and 1960 using masonry materials (such as brick, stone, or clay) that has little to no reinforcing is referred to as a URM. The image below demonstrates the hot spots for ground shaking in Portland when the predicted “Big One” (Cascadia Subduction Zone earthquake) arrives. Red areas indicate violent shaking, while orange indicates severe. The white dots represent the existing URMs in the city.
URMs pose a serious risk to our area because the floors, walls, and parapets are inadequately fastened, resulting in very poor lateral resistance. During an earthquake, these buildings suffer from floor acceleration, in which the upper floors experience faster lateral momentum than the lower ones, causing the exterior walls to dissipate and fall to the ground. Another concern with URMs is what is known as the pancake effect; during an earthquake’s turbulence, this phenomenon manifests as floors collapsing on top of one another.
Today, new construction has very tight seismic rules that designers and architects must comply with — but the code does not account for the embodied carbon of typical reinforcing materials. According to LPAred, the in-house research department at design firm LPA, a structure that combines concrete comprises more than 70% of the entire building’s embodied carbon, while structures that incorporate metals comprise 24% of the building’s total embedded carbon. As these numbers show, it is past time to start thinking about different ways to strengthen URMs and capitalize on the embodied carbon of these existing structures while incorporating non-carbon-intensive reinforcements.
After much research and conversations with structural engineers and local mass timber fabricators, it was surprising to learn that the use of mass timber to reinforce URM buildings has not been implemented into practice … yet. But, just because it hasn’t been widely implemented doesn’t mean that it will not happen in the future. The main factor as to why it hasn’t become a mainstream practice is because mass timber is a relatively new technology. While it has seen a lot more momentum in the last 10 years, code regulations have been slow to keep up, due to the lag in testing. Today, the use of mass timber in new construction is code compliant as a structural material, but more testing is needed to use the material as a means of seismically reinforcing in an existing building.
In my research, I found that the scale of the building is an important factor when considering using mass timber as a URM-reinforcing material; it would work best in a building three stories or shorter. The building use type is another important factor to consider when weighing reinforcing materials. Although using mass timber to reinforce URMs is not practiced commercially yet, a structural engineer at Morrison-Maierle says it is not impossible — but, the design would likely need to double the amount of strengthening material used. The graphics below demonstrate some of the potential options for how an URM could be reinforced with mass timber.
In conclusion, more studies are needed to be able to safely use mass timber as a reinforcing material in URMs — but from this research study, the future seems promising. The key takeaways from my research include:
- Successful implementation of mass timber reinforcements means finding the right scope and building scale that does not exceed three stories.
- Mass timber solutions will likely use more strengthening material and will still need steel connections to fasten all the elements completely.
- More than one structural solution may be needed when considering mass timber strengthening, depending on the building and project.
I can see this emerging technology being applied to code regulations within the next 10 years — as a result, we should start thinking about it now. How can mass timber become an effective seismic strengthening solution to reducing the amount of embodied carbon in rehabilitated URMs? As designers and builders, we should not become complacent in only continuing to choose concrete or steel as the primary strengthening materials because this emerging technology has a bright future.
City of Portland
- Unreinforced Masonry Buildings
- Chapter 24.85 Seismic Design Requirements for Existing Buildings | Portland.gov
- Oregon HazVu: Statewide Geohazards Viewer
Mass Timber + Seismic Resilience
- Is Mass Timber a Good Choice for Seismic Zones? | ArchDaily
- Timber based systems for the seismic and energetic retrofit of existing structures – ScienceDirect
- Testing of a Cross-Laminated Timber Pier-and-Spandrel Seismic Retrofit Solution for Unreinforced Masonry Buildings – TallWood Design Institute (tallwoodinstitute.org)