Behind the Scenes of Central Lofts: Designing Multi-family Housing with CLT
As the cross-laminated timber (CLT) panel installation on Central Lofts nears completion, our team recently reflected on the process of designing this mixed-use building. Central Lofts is the first building designed by Jones Architecture that utilizes CLT as its primary material, and among the first CLT projects to be reviewed for a permit by the City of Portland. Below, we share insights and observations from behind the scenes.
Identifying cross-laminated timber as the primary structural system was an initial key decision made early in the design process. Cross-laminated timber, or CLT, is composed of prefabricated timber panels that are compressed and glued together in perpendicular layers. Due to its prefabricated nature, CLT has the ability to greatly accelerate the construction timeline. Central Lofts is located at the center of a vibrant commercial hub in Portland’s St. Johns neighborhood and the shorter construction schedule enabled by CLT minimizes the impact of construction on the surrounding community. The prefabrication process allows R&H Construction to complete the CLT installation of this four-story building within a compact five week period.
In addition to the schedule benefits, the aesthetics of CLT make it a good fit for Central Lofts. The ability of CLT to remain exposed as a finish material was appealing. Studies have shown that interiors that incorporate natural elements such as wood can improve the mental health and well-being of building residents. The exposed CLT ceiling and the visibility of the wood panels throughout the building also tie the building to its northwest heritage. Timber is a building material with roots in the region: it has historically been grown and manufactured in the Pacific Northwest. As CLT gains popularity, there is opportunity for this historic regional industry to evolve.
Early Design Phases
The decision to use CLT informed and inspired the overall design of the building, starting in the early design phases. We identified building systems and materials that could be quickly installed in order to support the accelerated CLT timeline. For example, we chose not to use a concrete shear or lateral system because this would have slowed the project timeline. Instead, we used exterior wood-framed shear walls which could be constructed quickly. Cembrit, European siding panels, and a curtainwall glazing system, were also specified for their ease and efficiency in installation.
The width of the units is a direct result of optimizing the CLT panel size. This influenced the façade design and the proportions of the glazed and solid areas of the façade. The mechanical system for the building was selected with the façade in mind. We used mini-split instead of PTHP units in order to keep the perimeter of the building free of visual interruption and mechanical penetrations.
Early detail development –curtainwall attachments and floor to ceiling assemblies, for example– were key to ensure that the project met all current building code and acoustical performance requirements. A number of standard details, and the supporting testing data, were not applicable to this emerging construction type. This offered an opportunity to work with experts in the field, such as WoodWorks, to develop innovative, and newly code-compliant, details.
Construction Document Phase
The Construction Document phase involved close coordination between the MEP and structural engineers and our architectural team. This is important on all projects, but critical on projects that feature exposed CLT. Because the ceilings are exposed, the placement of every duct and pipe needed to be carefully considered.
The compact footprint of the building and the desire to keep the ground story commercial space free of bearing walls led to a very tight structural design. Consideration had to be made for how each CLT panel was spanning and how it could be supported. Each panel is unique structurally. One of the prominent features of the building, the projecting oriel windows, were surprisingly complex from a structural perspective because CLT inherently spans in one direction.
Another unique feature of the building is a centrally located scissor stair, composed of a pair of interlocking CLT stairs in a double helix configuration. Two separate stairs are required by code and must maintain independent fire rated enclosures. We used CLT panels as the stair stringers, and because of the thickness of the panels, they also provided the required 2-hour fire rated separation. Our team worked with Froelich Engineers, the project structural engineers, to determine how to support the panels without compromising the rated enclosures or resorting to bulky supports beneath the stair runs. The scissor stair optimizes the small footprint of the building, increasing the efficiency of the floor plate by cutting down on common circulation area.
The lack of tested and listed details for CLT buildings meant that we had to be innovative in our approach. An acoustical engineer was required to review our floor-ceiling assembly and floor edge details to ensure that the details met code requirements for sound transfer. We also collaborated with the firestopping manufacturer to develop fire rated penetration details and to provide engineering judgements. A building code appeal was approved to allow the engineering judgements as substitutions for listed details.
Central Lofts was among the first CLT projects to be reviewed by the City of Portland. Prior to this project, local CLT projects were only reviewed by the State of Oregon. The permitting process was intensive due to the lack of local precedents and tested details or code provisions for CLT and required many revision requests, particularly structural detail changes.
As one of the first CLT projects to be approved by the City of Portland, there was notable collective learning taking place. We welcomed the opportunity to communicate directly with the multiple plan examiners to coordinate design solutions to respond to their, at times conflicting, concerns. The project was approved by the City of Portland in 2018.
Selection of the CLT manufacturer
Structurlam, an APA rated manufacturer, was chosen due to their ability to meet the project’s schedule needs. The structural and design parameters for CLT are manufacturer-specific. When the preferred CLT manufacturer, Structurlam, was identified, our team tested all initial design assumptions to verify that they remained valid prior to moving forward. Structurlam met the design criteria, and our team was able to move forward without significant design or time-consuming permit revisions.
As a result of prefabrication and the compressed on-site construction schedule, CLT significantly accelerates the architect’s Construction Administration responsibilities. The project team strategically issued a number of RFIs and submittals at the front end of the project in order to maintain the schedule and to anticipate and resolve conflicts in advance instead of in the field. The Covid pandemic also had a significant impact on the schedule: the design team and R&H Construction had to work quickly to resolve supply chain issues that occurred throughout the process. Materials were quickly selected in order to avoid price escalation and supply chain concerns.
Throughout the Construction Administration phase, CLT shop drawings went through several formal rounds of reviews. These reviews were a collaborative approach and involved the larger project team, including the design team, Froelich Engineers, and R&H Construction. Because CLT is prefabricated, panel, column, and beam sizes and locations needed to be accurate within a tight tolerance, much smaller than with conventional construction. Similarly, the placement and size of every penetration had to be carefully considered and accounted for to ensure accuracy in the field. Penetrations had to be individually considered to determine if structural support was required.
The CLT installation on Central Lofts will be completed by early June 2021, with occupancy in the fall of 2021.