By Michael Scancarello, PE, and Andrew Ruff
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Mass timber structures are becoming more frequently studied as a leading choice for high-end residential and commercial structures for their sustainability and aesthetic benefits. Perhaps often overlooked, mass timber also belongs in the conversation when it comes to multi-family residential projects, including those classified as “affordable housing.” Providing 69 units of affordable housing along with ground floor retail while funded through typical methods for affordable housing projects, the 340+ Dixwell project is one project that demonstrates this possibility. While this article does not spend time addressing the funding or specifics of affordable housing, it will present challenges and solutions to successfully delivering a cost-effective mass timber structure in this typology.
The 340 and 316 Dixwell buildings are each four-story structures comprised primarily of 5-ply cross-laminated timber (CLT) floor panels and 3-ply CLT roof panels that are supported on CLT bearing walls at unit demising and partition walls in addition to cold-formed metal framed (CFMF) bearing walls at most corridor walls. The exterior walls, while typically non-load bearing, are also CLT. At the ground floor retail spaces, glulam timber columns and beams support the bearing walls above while steel framing and a composite slab on deck are used above exterior covered parking areas to minimize structural depth. On this project, the following key design considerations helped achieve the project’s ambitious goal of demonstrating the feasibility of mass timber affordable housing.
Know Your Construction Type
Typically, the building’s construction type classification as defined by the building code is not something that structural engineers will be particularly familiar with, and it is usually selected without substantial input from the structural engineer. However, when the architectural goals include exposing the structure without the use of applied fire-resistant materials, knowing the options and limitations with mass timber systems is important. One of the first conversations between the structural engineer and architect (and likely code consultant) should be centered around selecting a construction type that can meet the project’s goals as efficiently as possible. In many cases, the construction type will drive the decision-making for structural layouts.
This project was designed and permitted before the 2021 IBC provisions for mass timber in construction types went into effect, and was permitted as Type VA, with a 1-hour fire rating. However, it is likely that even had the new construction types within Type IV been available, the same code path would have been followed. The excerpt from the International Building Code (IBC) Table 601 summarizes some of the key structural considerations based on construction type.
As can be seen in this table, both Type IIIA and Type VA require a 1-hour fire rating for most components of the structural system while each of the new construction types—including Type IV-C, which generally allows for exposed timber surfaces—require a minimum 2-hour rating for the primary structural systems. When fire rating is provided by the code prescribed methods that account for charring of the timber frame, it is the author’s experience that 3-ply panels will typically be insufficient for meeting a 1-hour rating for most span conditions. However, at shorter spans, thin 5-ply panels will often be sufficient and even at more moderate spans this 1-hour fire rating will only moderately decrease the maximum permissible spans of 5-ply panels.
When the fire rating requirement increases to 2 hours for exposed panels, the result is frequently a significant amount of wood fiber increase or a requirement for panel specific test data acceptable to the Authority Having Jurisdiction (AHJ). Reliance on fire testing at early stages of design and permitting often requires early selection of suppliers, which can be a challenge for projects that rely on public funding sources. Much like floor panels, when using CLT bearing walls exposed on one or both sides, the difference between a 1-hour and 2-hour rated structure can result in significant wood fiber increases.
With glulam framing members, it has been shown that a 1-hour fire rating can be achieved for glulam beams exposed on three sides that meet the traditional “heavy-timber” minimum sizing without significant impact on member sizing. However, like CLT, when a 2-hour fire rating is required, it is likely to control the design for most efficient beam sizes. (Glulam members with a non-uniform layup have additional requirements that may increase cost over an unrated structure even when timber volumes do not change.) Finally, while a structural system with any rating requirement also requires rated connections, 1-hour fire ratings are more easily achieved without significant impact on member sizing or aesthetics.
The footnotes of IBC Table 601 are important to understand. Footnote c, which has been clarified in the 2021 IBC, allows for the use of heavy timber for roof construction in many situations. Heavy timber requirements are generally less restrictive than those of a prescribed one-hour rating and were used at this project to reduce the panel thickness—and cost—of the roof structure.
Design for Flexibility
A key factor in successful mass timber projects is the early selection of a mass timber supplier. However, waiting as long as possible to commit to a particular manufacturer also has advantages. In some cases, like affordable housing, the timing of bidding and selection may also be dictated by funding streams. In the case of this project, a substantially complete design was needed prior to onboarding a supplier. This requirement allowed for a competitive bid process which, while a requirement here, is an approach more familiar and comfortable for many owners and developers accustomed to utilizing conventional structural systems.
This process can provide competitive pricing, but in the U.S., CLT is not sold as a commodity product in the same way as steel or concrete. Most manufacturers produce a variety of panels as slightly different products with different constraints tied to materials, manufacturing processes, and transportation limitations. This presents a different challenge for the design team than many may be used to.
When a supplier is onboarded early, the design team’s goal is to optimize their solution for the selected supplier. Conversely, in a competitive bid, the design team must ensure the design is compatible with as many suppliers as possible. In this scenario there is more responsibility on the design team to be familiar with the capabilities and strengths of multiple suppliers. In the case of 340+ Dixwell, the design team was able to rely on previous experience working with several of these suppliers as Architect and/or Engineer of Record (EoR), Delegated Design Engineer, or both. However, when that experience is not available, it is recommended to communicate with as many suppliers as possible at multiple steps of the design process. Even with ample experience, frequent feedback from suppliers can help the team ensure that their design does not preclude suppliers from bidding or put unnecessary constraints on them that would limit their ability to provide competitive pricing.
Leverage Material Strengths
As a simple spanning element, CLT is often not as efficient as other panelized timber elements like Nail-Laminated Timber (NLT) or Glued-Laminated Timber (GLT) decking which orient dimensional lumber stacked on edge, placing all fibers in the primary strength direction of the panels. However, by placing the panel fibers in alternating directions, CLT provides several distinct advantages that can make it an efficient choice (Figure 2).
In addition to providing dimensional stability in both axes perpendicular to the face of the panel, CLT panels also have significant in-plane shear strength, making them suitable to be used as diaphragm elements when properly joined together. The ability to use the CLT as a diaphragm and eliminating the need for the topping slab to be structural allowed the design team to study non-cementitious topping systems. Although ultimately not selected for this project due to budgetary constraints, a dry lay assembly could reduce the embodied carbon and reduce the number of “wet” products applied over the timber, potentially providing a schedule savings. With new products and data continuing to become available, this could become a beneficial alternative in the future for projects where a cementitious topping is not required by code.
CLT panels also provide flexural capacity in two directions. While creating true fixity across panel joints is very difficult, if supported frequently enough, it is possible to achieve two-way spans with individual panels. Some notable mass timber projects have leveraged this and utilize fully point-supported panels with closely spaced columns. At 340+ Dixwell, this attribute of CLT was used to provide beam-free corridors as well as beam-free zones within primary bearing lines, allowing MEPFP distribution to be kept tight to the ceiling. Early coordination between the structural and building systems were critical to ensure that a 10 foot-6 inch floor to floor height could be achieved while maintaining an 8-foot ceiling height within the portions of units with dropped ceilings. As part of the early design process, multiple framing options were presented for review and coordination, and the choosing by advantages method (a decision-making process, often used by the authors, taken from lean construction practices) was used to select the preferred choice. While the authors have frequently found that spanning CLT panels across the width of double loaded corridor residential buildings results in efficient layouts for non-bearing wall structures, because of the architectural desire for exposed CLT partition and demising walls it was determined that eliminating additional framing members and spanning the CLT panels directly between these walls provided the most advantages to the project. This decision then drove revisions to architectural layouts to improve the efficiency in panel selection.
The two-way spanning capability of CLT was leveraged at two primary conditions that repeated throughout the building. First, corridors were designed to be beam-free by utilizing corridor walls as bearing walls, a strategy commonly used in traditional light-frame construction. To accommodate the different capabilities of potential suppliers, these spans were confirmed to work in two different ways. For manufacturers who could provide the exterior laminations (and therefore primary strength direction of the panel) in the short direction of the panel, panels would be able to span directly across the corridor and either 3-ply or 5-ply panels would be sufficient. However, for suppliers who primarily supply panels with the outer laminations parallel to the length of the panel, spans were confirmed to be acceptable for the panel to span in the weak direction of the panel, including the impacts of fire rating. This flexibility prevented the need to have multiple short span corridor panels that would increase the number of pieces to erect.
Near the corridor, door openings between bearing walls were sized to allow for panels to span these openings while being supported on wall panels only, without any headers above the door. This arrangement was stacked on each floor to ensure no concentrated loads would occur above unsupported sections of the CLT. Additionally, these openings were aligned from unit to unit so that consistent panel layouts could be used throughout the building, maximizing repetition and reducing restrictions on panel layout (Figure 3).
The requirement to provide beam-free corridors and door openings without using headers placed two constraints on the decisions driving the layout of floor panels, slightly minimizing opportunities for efficiency. However, by grouping these constraints together in the center of the building, the area of impacted panels was reduced. Additionally, choosing corridor and door widths and locations that didn’t push the limits of individual panel widths, multiple solutions were available to allow suppliers flexibility in the final approach.
The Role of the Engineer of Record
In part due to the uniqueness of each supplier, it was determined that a delegated design would make the most sense for the final mass timber package. This would allow the selected supplier as much flexibility as possible to reduce costs by tailoring their solution within the constraints outlined by the design team. However, to sufficiently finalize the design without running the risk of changes that could not be mitigated during construction administration, it was important for the engineer of record to be very involved in developing the design of the mass timber systems by providing:
A range of expected wall thicknesses for each supplier so that appropriate tolerances could be built into the floor plan, allowing for slight changes in dimensions without impacting code required dimensions. The architect then set all unit dimensions based upon centerlines of the mass timber and ensured there was room for these walls to grow or shrink as dictated by the supplier’s available products while maintaining critical required interior dimensions and clearances.
A range of expected floor thicknesses to ensure adequate clearance for required MEP routing while working with the exterior wall detailing. The thickness of the floor assembly was used to set floor to floor heights. This project also aimed to aid the speed of erection by minimizing piece counts by using single panels where the width of the panel was the floor-to-ceiling height in the platform type construction, allowing for an entire wall segment to be comprised of one piece of CLT. Therefore, heights were also limited by the maximum panel widths a manufacturer could produce (Figure 4).
Guidance on the limitations of suppliers, particularly as it pertained to maximum panel dimensions that would impact the ability to use single panels as bearing walls and where the location of openings might dictate panel layouts. Corridor layouts were confirmed and door locations within units were set to provide maximum flexibility to systems distribution and panel layouts as described in detail previously.
Anticipated timber connection details at any locations where they would impact the architecture and at the interface between different trades, including connections to supporting concrete and steel elements. The authors attempted to allow the greatest flexibility to each supplier’s preferences, however the design team also had to be proactive to provide details that helped to dictate conditions that may increase the scope of one trade to simplify constructability and coordination between trades and/or improve the product that is ultimately delivered.
Providing guidance on floor plan inefficiencies that will impact costs. Early unit layouts typically had a framed drywall surface applied to one side of all CLT walls to provide acoustic separation and vertical routing of building systems. However, after selection of a code path and structural system, this drywall was standardized to be continuous on the same side of each wall for the full length of the wall and height of the building instead of alternating where architecturally preferred so that it could also be used to provide the fire resistance rating for exposure on one side of the wall. By using materials that would already be present, the total wood fiber requirements for the walls were substantially reduced. Typical bearing wall to bearing wall dimensions varied throughout the building in initial unit layouts. After the selection of the framing approach, unique layouts were moved to areas of the building where site constraints would require unique solutions, maximizing repetition in the primary structural layout and allowing for uniformity in most governing spans, therefore allowing for more panel thickness reductions.
Inexpensive Buildings Don’t Need to Feel Cheap
By leveraging the strengths of each unique material and following through on good design decision making, the new affordable housing units on Dixwell Avenue are proving it is possible to build high-quality affordable housing on a tight budget. It is possible to use mass timber as a cost-effective and biophilic element in a sustainable fashion on a project that will be certified to PHIUS passive house energy efficiency standards. It is also possible to introduce new materials to the market and build up a skilled local workforce to construct these buildings. Every project presents opportunities to learn from as we continue to improve what we build, how we build, and who we build it for. This project was informed by many before it and the authors hope its demonstrated successes, as well as its recognized challenges, will contribute to continued improvement in our industry. ■
About the Author
Michael Scancarello, PE, is a Project Director at Odeh Engineers, a member of WSP. He is passionate about sustainable structures and is an Embodied Carbon Champion in the firms commitment to SE2050.
Andrew Ruff is the Research Director at Gray Organschi Architecture in New Haven, Connecticut, and a Visiting Critic at the Yale School of Architecture.