Carbon Logistics

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Amazon’s DII5 delivery station in Elkhart, Indiana, the first large scale, owner occupied, mass timber logistics facility in the U.S., was conceived as a test and learn working laboratory to implement sustainability strategies. More than 40 different sustainability initiatives were implemented across building, site, fleet, and MEP systems, including specific structural initiatives to reduce embodied carbon.

The delivery station project comprises a 171,000-square-foot warehouse and office block flanked by two 550-foot-long van-loading canopies. Mass timber was central to the facility’s carbon-reduction strategy, with the project using more than 500,000 board feet of cross-laminated timber (CLT). The building and site are pursuing International Living Future Institute’s (ILFI) Zero Carbon Certification.

Industrial and logistics facilities have been the fastest expanding segment of U.S. commercial real estate since 2020, and their future construction will continue to be a significant source of U.S. carbon emissions. Historically, industrial buildings have prioritized speed-to-market over sustainable initiatives, but at the speed and scale industrial facilities are being developed, even small carbon reductions add up to significant carbon savings across the full market sector. The new Amazon Delivery Station highlights how material changes, such as using CLT, can be pivotal in reducing carbon footprint, without compromising operations or speed of delivery.

The design team was given clear goals: maximize embodied carbon reduction without exceeding the project budget. ZGF Architects with help from KPFF performed a comprehensive whole-building and site life cycle assessment (LCA) to identify the largest sources of both embodied and operational carbon.

The LCA identified the concrete elements of the structure, such as footings, slab-on-grade, and tilt-up concrete walls, as the largest contributors to the structure’s total embodied carbon. While no viable alternatives have been identified for footings and slab-on-grade, perimeter concrete tilt-up walls were identified as the primary focus for reducing embodied carbon.

The design team brainstormed alternate wall systems, ultimately selecting perimeter CLT walls because of their modular construction, durability, and sustainability benefits. The CLT provided a durable interior wall face and back up for the rainscreen exterior assembly. To maximize the impact of the CLT walls, KPFF designed and detailed the panels as CLT shear walls. This approach eliminated any need for a secondary perimeter lateral force-resisting system, such as steel-braced frames, which would only have increased the building’s carbon footprint.

While mass timber is popular in Europe and Canada, its adoption in the U.S. has been slowed by U.S. code development. CLT shear walls were particularly ambitious for this project, as the state building code for Indiana was based on International Building Code (IBC) 2009, which was established well before mass timber language entered the building code. The team worked closely with the state authority having jurisdiction (AHJ) to approve an alternate means and methods request (AMMR) to use the prescriptive code requirements of IBC 2021 for the design and use of CLT as both horizontal diaphragms and lateral force-resisting systems, a process that would not be required in other U.S. locations that referenced more recent building codes.
While the team investigated changing standard industrial criteria to reduce materials, clear height was deemed essential to operations. For this reason, two different CLT shear wall strategies were used, each optimized for its unique requirements.

Due to the tall floor-to-ceiling heights and high regional wind loads, it was not possible for a single 3-ply panel to span from floor to roof. Instead, the CLT panels were laid out with a horizontal orientation. Working closely with the regional CLT supplier, Sterling, the team optimized the wall-layout while coordinating the structure with the dock-door locations. By simplifying panel layouts to limit fabrication complexity and using panel sizes that allowed the plant to streamline production, the cost per square foot of the panel was reduced. This provided room in the budget to allow further adoption of CLT at the adjacent van-loading canopies.
The CLT panels consisted of sustainably harvested, SFI-certified Southern Yellow Pine 3-ply panels. The panels are 8-foot-tall by 17-foot-wide and span horizontally between steel wide-flange columns. These columns function as strongbacks to provide out-of-plane support for the panels and support the weight of the roof. While CLT-bearing walls were explored, they would have required thickening the CLT to five or seven ply, increasing both the fiber-volume and the cost of the wall system.

All field welding near CLT was eliminated. Instead, the project used thousands of screws to tie the walls together. The CLT panels were fastened directly to the strongback with screws through the flange into the CLT in order to transfer out-of-plane wind loads and in-plane shear across the panel joints. Columns at the ends of CLT shear walls had a dual function to support roof dead loads and serve as hold-downs for seismic overturning. The horizontal panel-to-panel connection utilized a plywood spline set into the routed edges of the CLT and connected with self-tapping wood screws. The base of the CLT was protected by a moisture barrier and designed to bear directly on an 8 inch concrete curb. The panels were locked in place with proprietary steel angles screwed into the wall and fastened to the concrete with cast-in-place anchor bolts. The different construction tolerances between cast in place concrete and prefabricated CLT resulted in some gaps between the curb and wall. Wood shims were used to ensure good bearing and close gaps. During design, multiple base details were evaluated with contractor feedback, and the shimmed connection proved the most constructible for horizontal panels.

Beyond the sustainability benefits of this system, the occupant experience is also greatly improved. The biophilic properties of the exposed CLT softens the interior feel of the warehouse. This new wall system also provided more options for daylighting. Larger clerestory-like windows were installed at the top of the wall in bays not required for shear walls. This dramatically increased available natural light.

The office block afforded greater flexibility to optimize grids and ceiling heights for mass timber. The roof-height was set to 12 feet from floor to roof, allowing a single wall panel to span vertically from the floor up to the top of the parapet without strong-backs or other supplementary steel or concrete framing above the finished floor. In addition to using the exterior walls for lateral resistance, a central wall reduced the diaphragm spans and created a beautiful feature wall along the main entry corridor. Occupants enter the building passing a round timber column constructed from a tree harvested during the site preparation.

Similar to the warehouse, panel-to-panel connections used a plywood spline and custom steel angles at the top and bottom of the panel. Without the presence of strongbacks, conventional hold-downs were used at the end of shear walls. While the office block is industrial, and finishes were limited, the presence of wood in the offices, classrooms, and lunchrooms instantly elevates the space. The large windows illuminate the timber and allow views of the busy site.

Both styles of CLT shear walls were designed in accordance with ANSI/AWC 2021 Special Design Provisions for Wind and Seismic (SDPWS), which allows CLT shear walls to be used with R = 1.5 for seismic design category B. This code path does not have defined design requirements. The alternate method path no. 2 for moderately ductile CLT shear walls in low seismic regions, from the Oregon Statewide Alternate Method 15 01, was used as a guide for designing the walls and connections. This approach resulted in the first code-compliant CLT shear wall warehouse building located in a seismic region in the U.S.

The LCA also identified the roof structure as a significant source of embodied carbon. Metal roof deck has high embodied carbon due to the energy-intensive manufacturing process. For the Indiana delivery station, the metal roof deck was replaced with a panelized wood roof that is composed of plywood and wood purlins. At the warehouse, this structure was paired with open-web steel joists. While a full glulam framed roof was evaluated for the warehouse, spans exceeding 50 feet would have required heavy members, driving up column and footing sizes and offsetting embodied carbon gains. The long span steel joists with a panelized wood roof proved to be a pragmatic hybrid that balanced carbon, cost, weight, and constructability.

In the office, where programming was more flexible, panelized roof elements were supported on glulam framing on a grid optimized for glulam efficiency. At the van-loading canopies, metal roof deck and steel beams were swapped for glulam beams and CLT roof set-on-steel columns. Perimeter fascia was added to protect the wood from wind-driven rain.

Where concrete systems could not be replaced wholesale, the project focused on responsible alternates to components:

Collectively, these measures support the project’s ILFI Zero Carbon Certification (ZCC) trajectory and demonstrate a viable pathway for industrial-building typologies to move decisively toward Net Zero.

While warehouse concrete tilt-up construction sequencing has been standardized, the Amazon mass timber warehouse required an original approach. For this project, the steel columns and open web joists were erected first, followed by CLT shear walls. This sequencing required interior temporary bracing of the roof until CLT shear wall installation was sufficiently complete, but it allowed a single mobilization of iron workers, thereby simplifying scheduling.

For the timber, the contractor implemented a robust moisture-management plan that included pre-applied waterproofing, panel edge protection, and on site best practices to account for Indiana’s precipitation and cold winters. The project used Sterling CLT panels, which are smaller than panels available from other manufacturers. While the smaller panels meant more pieces, they also meant a smaller lift and smaller crew to hoist the panels into place and fasten them to the strongback columns. Additionally, the panels were easier to manipulate in the high Indiana winds.

This design resulted in a surprisingly fast installation—a 60 percent reduction from the allotted install timeline. Fortunately, the manufacturer was able to work with the General Contractor, Graycor, and speed up production to facilitate faster delivery to the site. On future projects, additional schedule efficiencies may be achieved by prefabricating multiple panels to create “super panels,” reducing the number of lifts and waterproofing laps. Finally, the panelized roof system’s on ground assembly and large format lifts delivered both erection speed and jobsite safety.

As with any project that aims to change established norms, there are lessons learned. Some key takeaways from the DII5 project are:

The next 50 years will bring billions of square feet of new industrial space to the U.S. With this magnitude of construction, owners are likely to prioritize sustainable design and construction. Amazon’s mass timber delivery station demonstrates that a carbon first decision process can coexist with the speed and operational constraints of last mile logistics. Each time we make thoughtful trade-offs to lower-carbon materials, such as CLT shear walls, carbon is significantly reduced. Making such an approach standard practice can directly translate into massive carbon savings when replicated at scale.

The CLT lateral systems now codified in IBC 2024, SDPWS 2021, and ASCE 7 22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures create a clear path for mainstream adoption, reducing reliance on AMMRs and lowering procurement risks for developers and contractors. Such a shift sets the stage for CLT shear wall implementation—and other carbon reductions—to be routine, rather than one-off exceptions. Structural engineers can offer realistic solutions to the growing climate crisis by embracing low-carbon alternates, thus delivering logistics facilities that move packages without moving the climate needle in the wrong direction. ■

About the Author

Jessica Westermeyer, PE, SE, is an associate and licensed structural engineer at KPFF with a passion for mass timber. Her mass timber projects include a 2024 NCSEA SEE finalist (UW Health Science Education Building) and a 2023 DBIA National Merit winner (UW Milgard Hall). (jessica.westermeyer@kpff.com)