Building tall wood buildings has been happening in Europe and Canada for years — but it took a small group of intrepid Portland developers to push through the restrictive codes that limit mass timber construction in the US. The engineering, permitting and design breakthroughs that resulted have ushered in a new era of sustainable wood building in America.
A tour of the Bullitt Center in Seattle inspired Ben Kaiser to leave stick frame construction behind and only build with mass timber from that point on. He saw wood construction’s undeniable benefits and — most importantly — how it could make a meaningful difference in the fight against climate change.
Kaiser+Path’s first mass timber project was The Radiator, arguably one of the first all-timber building in the US of its scale in the past century. Its development demonstrated the financial viability of mass timber construction and helped kick off a wood building boom in Portland, Oregon.
Energized by its new mission, Kaiser+Path entered the 2014 US Tall Wood Building Prize Competition, which offered a $1.5 million grant to each of two winners. With only six weeks until the deadline, the team got to work on a proposal for the tallest CLT building in America, Carbon12.
Despite the competition’s numerous rules and short timeline, Kaiser+Path was uniquely positioned to enter. Already composed of trained architects, contractors and developers, the small firm could move quickly to pull the necessary materials together. Significantly, they already owned the proposed building site, which was a competition requirement.
In February 2015, they received a letter informing them that they were not selected as a winner of the competition. They decided to build it anyway.
Six minutes after receiving a letter that we were not selected as a winner of the US Tall Wood Building Prize Competition, we decided to build Carbon12 anyway.
— Kristin Slavin, Project Architect, Kaiser+Path
The building site is on a prominent corner along the N. Williams corridor, just seven minutes from downtown Portland. At eight stories high, it has sweeping views of the city skyline, four mountains and numerous bridges.
Did You Know?
Kaiser+Path led all aspects of the development; financing, design, construction and sale of 14 condominium apartments.
A Starring Role
Carbon12 was envisioned as a simple wood-and-glass jewel box, designed to showcase the beauty of wood while hiding the mechanical systems to maintain a clean finish. In the end, to achieve an acceptable energy performance for the skin of the building, there was a shift to solid walls clad in custom corrugated painted metal siding – a reference to tree bark – with floor-to-ceiling windows.
Even at eight stories, Carbon12 is uniquely inviting with muted natural finishes and a palpable connection to nature just outside the windows. The concept of an open loft space was born from the inherent flexibility in the structural system. Mass timber naturally creates adaptable space due to the long spanning capacity of the panels. By using glulam beams and columns, most walls within the unit can be removed or added as the owner desires.
Leaving the wood structure exposed throughout the building presented unique challenges, including how to mitigate sound transfer between units, where to run building systems such as ducts and sprinkler pipes and how to keep the space simple and elegant rather than rustic. Particular care was taken to design a layered floor system to improve acoustic isolation. Building systems were hidden above strategic soffits in the units, where fire sprinklers are also hidden with side-wall heads. The whitewash finish on the exposed wood lightens the space while bringing a more cohesive and modern aesthetic to the wood.
The orientation of the building provides a choice between city or mountain views while preserving the most light possible for each unit. The units primarily face either east or west with north and south facing balconies.
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See the acoustic assembly
Read the Report: Acoustic Assembly
To achieve the acoustic goal (an STC and IIC of at least 60) while keeping the CLT exposed on the ceiling, A Acoustics helped design an assembly that would exceed code.The design added layers of insulation, DensDeck, gypcrete, and an acoustic mat between the finish floor layer and the structure. It also added a perimeter insulation board around the walls to isolate the floor from the walls. All of this was done to isolate the walking surface from the structure so that the sound vibrations could not transfer through the structure to the next unit.
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Learn about the finish used to prevent staining
More Information: Finish Used to Prevent Staining
To prevent staining of the exposed wood during construction, multiple finishes were tested by turning a glass of red wine upside down on finished wood and moving it slightly every week to see how much it would stain. After several weeks, it became apparent that the JFB Hart finish was the most resistant to stains, as the red wine would wipe away with a damp paper towel. The CLT manufacturer applied this finish on all of the exposed sides of the wood in the factory so that it would be protected during construction. Once the building was dried in, the dirt was wiped away and the wood looked as good as new. To avoid residual oil from the bolts and connections seeping into the wood, galvanized steel connectors where the wood was exposed.
Product:
Inspired By The Forest
The inspiration for Carbon12 was nurse logs which are fallen trees that, as they decay, offer seedlings shade, nutrients, water and protection from disease. Their role is to nourish the next generation of timber.
Checking Diagram — The floor plan for Carbon12 was inspired by the checking of a log, with natural splits along an edge, creating space for light and air.
Inspiration Sketch — An image of a checked log sitting on an urban street corner was the cover of Kaiser+Path's 2014 US Tall Wood Building Competition proposal.
Carbon12 is meant to act as a nurse log, providing a platform for relationships to be built, for partnerships to take form and be a rallying cry to change the world.
— Ben Kaiser, Owner, Owner & Principal, Kaiser+Path
Carbon12’s design includes an elevator entrance directly into each home, generous balconies with views, and a mechanized parking valet that stores cars below ground.
Opening Doors: The Permitting Process
Initially, the City of Portland required that Carbon12 go through an extensive third party review process similar to a concrete building over 240 feet tall. The city had never reviewed a CLT project before, so it was clear that this route would take significant time and cost. Meanwhile at the state level, the Oregon Building Codes Division was launching a pilot program to establish a method for reviewing CLT buildings for local jurisdictions. After discussing it with the city’s structural engineer, fire marshal, and life-safety reviewer, the city suggested working through the state’s pilot program. In the end, the city was responsible for approving aspects of the building outside of the physical structure itself, while the state was responsible for reviewing the structural and fire/life safety aspects of the building through a performance-based analysis, with comments from the city.
By being involved in the review, the city was able to bring up specific concerns, while also observing how the state expected future CLT projects to be reviewed. Working with both arms of government enabled Kaiser+Path to share research, engineering and testing results so that the collective knowledge base expanded across both jurisdictions.
The combined work of Kaiser+Path, the city, and the state helped guide the ICC Ad Hoc Committee on Tall Wood Buildings in their review and approval of new building types to the 2021 International Building Code. These codes allow tall timber buildings up to 18 stories. The State of Oregon Building Codes Division adopted the provisions proactively in 2018 via a Statewide Alternate Method.
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Learn more about Carbon12’s building type
More Information: Carbon12’s Building Type
Building type dictates many critical aspects of a building, such as height and area allowances, types of materials allowed to be used, and fire ratings. However, Carbon12 as designed did not fit into any of the building types in the code, the 2010 Oregon Structural Specialty Code (OSSC). Therefore, we were required to do a performance based review to show why and how our design met the intent and safety of the building type closest to what we were proposing, Type III-A.
As Type III-A construction, the exterior walls were required to be built using non-combustible materials or fire-retardant treated lumber. After a cost comparison of steel studs vs. fire-treated lumber, we discovered that they were roughly the same price, but wood is a more insulative material. We chose to use fire-treated lumber because the thermal performance of the building was improved and our embodied carbon would be lower. Fire retardant plywood was used as required by the structural engineer for shear walls. This strategy helped to achieve the height and story increase. Additional requirements include the below:
- The structural frame, floor and roof require a 1-hour Fire Resistance Rating (FRR)
- NFPA-13 or NFPA-13R sprinklers, depending on use
- Two exit stairs with 2-hour FRR enclosures
- Emergency egress elevator connected to a generator for emergency power
- Learn more about the ICC Ad Hoc Committee
- Learn more about the Oregon Statewide Alternate Method
A Glass And Timber Showpiece
Both trees and people need light to grow and thrive. Special consideration was taken throughout the Carbon12 design process to ensure the units were filled with light and expansive territorial views.
Interior — Exposed wood and natural light fills the space, bringing the outside in.
Exterior — Light transmittance and coloring of different glass options were studied to maximize views of the wood ceilings as seen from the street through large windows.
As an engineer, designing exposed structural elements that are highlighted throughout the building is exciting. We can see the work we put into the project, and the exposed mass timber elements gives warmth to the environment we are creating.
— Mike Munzing, Munzing Structural Engineering
Structural Design
Mass timber is currently limited more by building code than by engineering capabilities. The US is decades behind Europe and Canada in CLT construction. The testing and structural values that are available in those markets were referenced when designing the building’s structure. Still, CLT has been met with initial skepticism by regulating bodies.
Due to the seismic concerns in the Pacific Northwest, an all-wood lateral system could not be used. Munzing Structural Engineering selected a core system that would compliment the speed of construction of mass timber, called a buckling restrained brace frame (BRB) system. BRBs are a pre-manufactured, bolted connection system with very high lateral resistance values.The BRB system was integrated into a structural steel core of beams and columns that allowed it to be erected in coordination, and in-step, with the CLT. It also allowed the most flexibility when routing building systems in concealed spaces.
Carbon12’s framing subcontractor has a steel division that fabricated and erected the steel core and installed the BRBs. They modeled the steel elements in 3D to integrate into the wood model that the manufacturer created to eliminate clashes between the two systems. Having one subcontractor responsible for the entire structural system enabled efficient scheduling of their labor, as well as reducing points of conflict between the wood and steel systems.
- Learn more about Munzing Structural Engineering
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Learn more about the BIM coordination on Carbon12
More Information: BIM coordination
Coordination is essential with mass timber construction. Having all parties involved early allows for detailed building information modeling (BIM), which shows how the elements of a building interact. With mass timber, 3D modeling enables pre-manufactured panels to be cut in the factory with precise placement for mechanical, electrical, and plumbing shafts. This significantly speeds up the process and makes the construction of the building more precise.
On Carbon12, Structurlam (the CLT manufacturer) modeled every mass timber element, plywood spline, steel connector, and screw which was combined with a model from Timberland of our steel and BRB core system. Before construction started, the team had weekly meetings to discuss the points of conflict of the building based using this model. Structurlam kept an Issue Tracker to memorialize each solution.
- See a shake table test of CLT floor and walls
- See blast testing of CLT structures
Wood Structural Diagram
Due to the precise manufacturing process of mass timber, tight connections and pre-installed connectors are possible. This level of exactness requires an exhaustive pre-construction process to avoid clashes in the field.
Unlike steel, wood is a poor conductor of heat. This allows the interior of wood to stay cool and retain its load bearing capacity. This insulating capacity of wood also keeps concealed steel connections from heating.
— David Gessert, Fire Protection Engineer
Natural Fire Resistance
Mass timber naturally protects itself from fire through the process of charring. The lack of oxygen inside a mass timber panel or product produces a charred layer of wood on the outside. This char layer insulates the interior of the panel or product from the heat. This insulating property allows the recessed steel connections that are inside the wood to remain protected.
Mass timber char rates are very predictable. Tested and calculated rates allow structural engineers to design the char layer into the structure, effectively building in a sacrificial layer in the event of a fire. The portion of the structure left uncharred will safely support the building after the code-defined fire rating period.
As part of the performance-based fire design, Kaiser+Path agreed to hire a fire protection engineer to put together a concept report and a design report. The concept report outlined the issues of code non-compliance within the building. Kaiser+Path worked with Arup as fire engineering consultants. Once the issues were identified, Kaiser+Path hired an additional fire engineer, Fire Protection Consulting, to analyze the building and detail how it was meeting or exceeding the original intent of the code. A second, more detailed, analysis of Carbon12 included fire and smoke modeling of the building to identify smoke alarm and sprinkler response times. This shows that the design of the building (even with the exposed wood surfaces) would allow for adequate exiting and emergency responder safety.
Similar to a large log in a fireplace, mass timber is so dense that it does not easily burn. Like trees in the forest, when exposed to flame, mass timber creates a char layer. The char actually insulates the inside of the wood and prevents further burning.
More On Design And Approval
Architectural Drawings
Includes the architectural drawings
Read moreArchitectural Drawings
These documents may not be a complete representation of the as-built or final approved configuration due Carbon12. All drawings and documents on this website and portlandmaps.com are for reference only, and the copyright owner does not release the work for commercial gain or replication.
For permits and related documents associated with Carbon12, please search the address “12 NE Fremont St” at https://www.portlandmaps.com/advanced/?action=permits
Structural, Civil, and Landscape
Has documents about structural engineering, civil engineering, and landscape architecture
Read moreStructural, Civil, and Landscape
For more information about the Structural Engineering, Civil Engineering, and Landscape Architecture documents, please refer to the permit documents at the link below through the City of Portland website. The permit documents for all recent projects within the City of Portland are available through portlandmaps.com. These documents may not be a complete representation of the as-built or final approved configuration due Carbon12 due to the project having been permitting through the State of Oregon BCD. All drawings and documents on this website and portlandmaps.com are for reference only, and the copyright owner does not release the work for commercial gain or replication.
For permits and related documents associated with Carbon12, please search the address “12 NE Fremont St” at https://www.portlandmaps.com/advanced/?action=permits
Acoustic Assembly
Has documents about the acoustic assembly, which was engineered to exceed code by 20%
Read moreAcoustic Assembly
The acoustic assembly was engineered to exceed code by 20%, which requires a minimum STC and IIC of 50. STC (Sound Transmission Class) measures an assembly’s ability to block airborne sound from travelling through a wall, ceiling, or floor. IIC (Impact Isolation Class) measures a floor/ceiling assembly’s ability to absorb impact sounds such as footsteps or dropped items. The higher the STC and IIC, the better the sound isolation between spaces.
Fire Report
Includes the Concept Report and Design Report for fire analysis
Read moreFire Report
The Concept Report analyzed compliance with code, and outlined areas that needed to be further explored in the Design Report.
The Design Report details how Carbon12 meets or exceeds the original intent of the code.
Design Review
Has Portland Design Review process and documents
Read moreDesign Review
While the design review process was not specific to wood buildings in Portland, it was a major factor in the schedule of the project. Because of all of the unknowns, Kaiser+Path risked developing the design of the project with no guarantee that they would have the funding for the research and development necessary to build the building as submitted. We were required to do a Type II process. While this shows a fairly concise process, changes throughout the process add time to the schedule, and the realistic timeline for approval is much longer than the minimum 56 day timeline.
Finishes and Appliances
Includes info about premium finishes and the automated parking valet
Read moreFinishes and Appliances
Carbon12 offers premium finishes, an elevator entrance directly into each home, generous balconies with mountain, city and treetop views, a private entry for owners, and an automated, mechanized parking valet that stores cars below ground.