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The City of Portland, Ore., is now home to the tallest mass timber hybrid building in the United States. Carbon12, the first building in the country to use timber framing for residential development at this height, stands tall at 95 feet and eight stories. A hybrid system of cross-laminated timber (CLT) floor and roof diaphragms and steel core, Carbon12 demonstrates the possibilities of mass timber hybrid construction.

Structural design

The primary structure of Carbon12 consists of a glulam post and beam with CrossLam CLT floor panels, steel buckling-restrained brace (BRB) frame as the lateral force-resisting system, and light frame partition and exterior walls. The glulam beams and columns and CLT floor panels are exposed in most areas, allowing residents to experience and understand the structure of the building, so often concealed in residential construction. Beams and columns are connected with pre-engineered Ricon beam hangers, which are recessed beneath a sacrificial char layer of the timber to provide a 1-hour fire resistance rating.

The mass timber components, supplied by Structurlam, are integrated with a steel core to create a hybrid structure. This combination of structural systems is unique in multistory residential construction and serves to minimize the use of wet materials onsite and decrease the overall construction schedule. The BRB system allowed for most of the steel fabrication to take place offsite and be bolted together floor by floor, at the same pace as the wood construction, with minimal onsite welding. This allowed for reduced staging/shoring similar to traditional wood construction.

In addition to its lighter environmental footprint, the structure of Carbon12 is also significantly lighter in weight than its post-tensioned concrete counterpoint. Consequently, the lateral system did not need to be as robust, and the steel braces in the building’s core were significantly smaller in dimension. The light weight of mass timber also serves the building in a seismic event. During an earthquake, Carbon12 will need only to withstand a force proportionate to the weight of its structure, roughly one-quarter the weight of an analogous concrete structure.

Testing

As Carbon12 is a tall wood building in a high seismic zone on the West Coast of North America, it may be subject to dynamic loading scenarios throughout its service life. A typical post-and-beam glulam structure will experience story drifts limited to 2 percent by design codes. The story drift will also subject the respective post-and-beam connection to additional stresses, resulting from gravity load and rotation of the beams against the post.

To ensure that the connections at major glulam structural members would withstand the inter-story drift demands of a major loading event such as an earthquake, testing was completed on the KNAPP RICON S VS Connectors under design load.

Prefabrication and assembly

Fabrication of the mass timber structure began in the Structurlam production facilities in Penticton, British Columbia, Canada. Glulam beams and columns were manufactured and CNC framed for steel connections based on the CadWork 3D model. RICON beam hanger connections were pre-installed into beams and columns prior to shipment to the site. Custom steel column connections were test-fit in the shop and delivered as part of the mass timber package. The mass timber package was shipped from Structurlam to the site on 22 flatbed trucks, sequenced for installation.

Following excavation and construction of the foundation, three levels of the steel core were installed. Glulam posts, beams, and CLT floors followed. Glulam beams and columns and CrossLam panels were lifted by a tower crane directly from the truck bed into position, with each component labeled and designated for a specific location in the structure. Construction proceeded with three crews under a single subcontractor — one each for the steel core, the mass timber structure, and the conventionally framed exterior walls.   

During one of the wettest winters on record in Portland, construction of Carbon12 suffered only minor schedule impacts, while neighboring concrete construction projects were put on hold until the conditions allowed for pouring. Speed of assembly for the mass timber structure increased as the crew became more comfortable with the process. By the fourth level, all glulam was installed in one day, and all CLT panels were installed the following day. The crane was onsite for two and a half months, and the structure above ground was complete in eight weeks.

Unique features

On top of a rare hybrid design, Carbon12 features structural components unique to a building of its kind. The structure sits on drilled piers, a deep foundation system that is essentially a large-diameter concrete cylinder constructed by placing fresh concrete and reinforced steel into a drilled shaft. The robust foundation allowed for a 20-foot-deep basement, which houses a mechanical parking system. The only one of its kind in the U.S., the steel conveyor belt system allows drivers to type in a code and retrieve their car within a matter of minutes. The garage stores 24 cars within a two-story system.

Future of CLT

As an environmentally responsible building material, mass timber sequesters carbon dioxide from the atmosphere by storing carbon in the structure during the entire life of the building, while promoting sustainable forest management practices.

Carbon12 offers a glimpse into the possibilities of building with mass timber, and the associated benefits. CLT paves the way for a new paradigm of construction centered on renewable building materials, 3D modeling and coordination, cutting-edge prefabrication technology, and quick, clean, quiet assembly with minimal onsite waste.


Erica Spiritos is mass timber specialist with Structurlam Products LP        (www.structurlam.com). Mike Munzing is principle at Munzing Structural Engineering (www.mstructural.com).

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