Mass timber construction has been widely touted by architects and builders for its relatively low carbon emissions, limited waste, and potential for rapid, on-site construction. But the benefits of engineered wood structural systems might be achievable with far less wood using an everyday building material: dimensional lumber, namely 2x4s, 2x6, and 2x8s.
This hypothesis is being interrogated at the 2021 Chicago Architecture Biennial, which runs through Dec. 18, by the local office of Skidmore, Owings & Merrill in partnership with the University of Michigan Taubman College of Architecture and Urban Planning faculty Tsz Yan Ng and Wesley McGee and their graduate students. Their efforts culminated in the SPLAM (SPatial LAMinated timber) Timber Pavilion at Epic Academy, a public high school in Chicago’s South Shore neighborhood.
The permanent pavilion is visually striking, an undulating lattice of spruce-fir-pine 2x members stacked 12 slabs high atop four concrete columns. At the center of the span, the number of slabs tapers to just a few. In some ways, the voids of the 3D grid resemble those of a Jenga tower, ponderous in their symmetry and absence. Yet, if the beauty of the expressed negative space is an ancillary effect of the project, it wasn’t the impetus. “Our real goal is to make a system that can be applied at scale and move the needle on the carbon problem in the construction,” says SOM design partner Scott Duncan, AIA.
In essence, the team has crafted a new variation of a structurally engineered wood system, spatially laminated timber with the help of modeling software to optimize the spatial arrangement of the slabs, the team can use fewer individual members to achieve the same level of stability as a flat plate structure, like cross-laminated timber. This translates to savings in space, material, cost, and, ultimately, carbon.
“There's already this great system out there, which is CLT,” Duncan says. “It's flat; it's—in a sense—unintelligent. … It's a flat plank. And anyone who has studied architecture or engineering or understands the principle of a vault knows there are inefficiencies, from a load perspective, when you have something flat like that.”
Recognizing the efficiencies that come with using replicated structural elements—an idea that has roots in Le Corbusier’s Dom-Ino model— many architects have turned to CLT as a solution for structural flooring in high-density residential construction, Duncan explains, mid-rise projects, such as the Timber Lofts multifamily apartment building in Milwaukee and the Carbon12 condominium tower in Portland, Ore., along with hyper-efficient custom homes, have been built with CLT, which fuses alternating layers of soft-wood into durable panels.
SLT, a Minimalist Riff on CLT
“We start with a simple, flat plate structure and run an analysis to find where the material is working hardest, the so-called hot spots,” says SOM director of structural engineering Benton Johnson, who oversaw the project’s structural analysis. “Then what the software will do is identify locations to remove material where it is under lower stress and has lower utility, and move those to locations of high stress and high utility. And we iterate on that process hundreds of times.”
Once the design is optimized, precisely cut–wood beams are fitted together with scarf-and-lap joinery. The roof of the pavilion is not just a roof, then, Johnson says, but a single-story prototype of the framing structure between floors of a larger building.
In a building industry whose collective operations account for nearly 40% of global, energy-related carbon emissions, timber has several well-documented advantages over materials like concrete and steel, Johnson says: “When you think about the life of a tree, it's taking in carbon dioxide, and it's putting out oxygen, leaving behind one carbon [molecule] in the actual cellulosic material. So, wood is actually about 50% carbon by weight.”
If short spans of low-grade, commodity lumber can be salvaged from deconstructed buildings, then they can be reused, thereby reducing deforestation and preserving old-growth trees. Still, CLT uses “more wood than you would ever want,” Johnson says, with slabs up to a foot thick. Meanwhile, traditional joist framing can be materially efficient but lack the structural performance and expansive capabilities of engineered wood systems.
“We took a step back and said, ‘What if we try to combine the best of both worlds?’ ” Johnson says.
SOM estimates the SPLAM Pavilion uses 46% less wood than a comparable structure built with CLT and provides additional room for HVAC and MEP systems. Compared to a traditionally framed structure of similar dimensions, the SLT system reduces the roof’s slab depth from 22 inches to 17 inches. The recovered area between floors can translate to increased floor-to-ceiling heights or, in a tall building, an entirely new floor.
“In height-constrained urban contexts, if you're able to add a floor into a building that's on the tall side, but still preserve the clear heights, that’s music to [the client’s] ears,” Duncan says.
A process called topology optimization guided the design, McGee says. Using software models that plot the direct and shear forces across the structure, SOM determined where commodity lumber could be positioned for the greatest material efficiency. The undulating grid closely followed the shear and bending-moment diagrams of stress.
Afterward, at Taubman’s Digital Fabrication Lab, McGee says, individual pieces of wood from a local lumber supplier were milled to accommodate the lap and scarf joints. Each joint was stress-tested with a tension machine with a 100 kilonewton load cell, and a CNC router cut the angled planes of scarf joints. A self-centering pneumatic gripping system helped accommodate subtle variations in the width of the 2x4s and orient them to withstand the direction of the force—a process that required iterating hundreds of possible design scenarios.
“It was a fluid process to work from the material, not just the tolerances of what is possible for fabrication, but what is possible for the design at the end,” Ng says. “What this system affords is that prefabrication can happen at a lab or at a manufacturing facility, so that it reduces time on site.”
All told, the pavilion took about three weeks to fabricate and 912 pieces of wood. After being cut and packed in pallets, roughly 60 uniquely shaped wood parts grouped in sequentially numbered families (like an IKEA kit) were shipped on an open flatbed to the site. McHugh Construction assembled the pavilion in roughly a week.
Moving forward, policy changes may help SLT gain a foothold as a scalable design and construction method for the floor slabs of low- and mid-rise residential and office projects. The 2021 International Building Code permits the use of mass timber in buildings up to 18 stories, and a handful of U.S. state and city jurisdictions have either incorporated the new standards into their codes or are considering doing so.
“If you look at the speed of typical framing, particularly balloon framing, it's pretty incredible these days,” McGee says. “This is definitely a different paradigm. But in a typical office building, where you have tens of thousands of members, you have to look at production from a different standpoint. It's much more akin to car manufacturing than it is to building manufacturing.”