Seeing the Trees for the Forest

To view the figures and tables associated with this article, please refer to the flipbook above.

The new National Conservation Legacy Center (CLC) in Missoula, Montana, serves as a living archive of the United States Forest Service’s (USFS) history and its ongoing conservation work of America’s natural resources. Inspired by the surrounding mountain landscape, the museum features unique architectural and structural elements including 16 tree-like columns and a faceted roof shape at the main lobby and portico. The building represents wood species and products from across the country, including sawn lumber, engineered wood, and mass timber, making it a showcase for advanced timber engineering and sustainable design. The museum encompasses approximately 20,000 square feet of space. Exhibit and archival spaces comprise the first floor while offices and supporting areas for the staff are located on the mezzanine level. A rooftop deck provides access to a fire tower adjacent to the building. Simpson Gumpertz & Heger (SGH), in concert with the project team, worked to stitch the various timber components together structurally to create a building-scale showcase for the new museum.

Design work for the current building began in 2021 and was ultimately permitted under the 2021 building code of Montana, which is an amended version of the 2021 International Building Code (IBC). The building construction type was classified as Type VB, non-rated, combustible construction. The building was designed to be fully sprinklered so mass timber elements could be exposed in locations, and these elements did not need to be designed for fire performance (i.e. structural sections designed for char layer development). Construction of the project occurred approximately between Fall 2024 and Spring 2026.

As Engineer of Record (EOR), SGH designed the foundations and gravity and lateral load-resisting structural systems along with select connections. While the center is one building system, the design team considered the building as two primary zones (Fig. 2): (1) a main building space housing the exhibition, archival, and staff spaces; and (2) a folded roof zone with tree-columns composing the enclosed lobby and exterior colonnade. The main building zone has a structurally insulated panel (SIP) roof supported on mass timber glulam beams and columns with miscellaneous steel elements at the exhibition roof. The accessible roof deck, and mezzanine level are composed of mass plywood panels (MPP) supported on glulam beams and cross laminated timber (CLT) bearing walls. The central elevator and stair lobby tower are composed of MPP floor panels supported on fully exposed CLT bearing walls around the shaft and stairs. A combination of CLT panel and LVL stud framed shear walls act as the lateral force resisting system of the main building.

The folded roof canopy zone is composed of faceted MPP panels supported on the signature heavy timber tree-like columns. The “branch” elements of the trees carry load axially and not in bending by being part of a closed 3D-truss system that includes the faceted roof as part of the system. Laterally, the MPP folded roof diaphragm is supported on CLT bearing walls located around the perimeter of the main building and museum lobby, and at the far end of the exterior colonnade.

The visible and functional tree-like forms exhibited on this project are both a continuation of a long tradition of such designs in structural architecture as well as a contribution to the museum’s larger aim of serving as a direct educational tool about the USFS.

A tree’s structure has been optimized to perform ideally to the environmental conditions it sees in the forest: primarily self-weight and wind loading. The wood cellular material is generally oriented longitudinally with respect to the trunk to provide compressive resistance for gravity loads. Further, as a tree grows upwards and branches outwards, new layers are added to the trunks and branches to produce optimized cantilevers.

As noted in the book, “Structures: Or Why Things Don’t Fall Down,” a tree deposits each successive layer of wood on its trunk in active tension. As noted, in ASTM D2555, clear wood specimens (i.e. small specimens without knots or defects) are stronger in tension than in compression so the prestressed tree trunk preloads its material against the main lateral wind load. The wood material on the leeward side of a tree experiences reduced tension rather than immediate compression. Not considered is the effect of knots and defects in the wood which can reduce the tensile strength as accounted for in the National Design Supplement (NDS) values for various structural grades. Often missed is that harvesting and milling of the wood material into lumber and timbers removes the natural optimization of pretension and branching while we then design with the wood material now in separate pieces.

Yet, designers have strived to incorporate the tree form in ways ranging from geometry alone to increasingly integrated expressions of material identity. These precedents directly informed the structural and architectural conversation early in the project’s design. Precedent examples include:

For the CLC and using tree-form precedents, the design team sought to celebrate the history of the USFS in both geometric form and material with the project’s tree-columns. First, the project team sought to showcase the most important U.S. tree species for USFS history by collecting raw heavy timber donations from across the U.S. that would be milled into the tree-column components. Ultimately, 16 different wood species ranging from sugar maple to redwood were collected with each “tree” being a specific species of wood. Based on parameters such as wood appearance, programming, and material durability, woods were designated either for exterior or interior tree-columns.

Second, SGH conducted an in-depth parametric modeling study (Fig. 3) to optimize both the visual form and the structural performance of the structural tree members. This enabled the team to review the full range of shapes and branching configurations. The team developed each structural tree to reflect the unique properties of its wood species (i.e. structural branch sizes varies in some locations depending on the performance of the specific wood material).

The final form of the tree-columns (Fig. 4) became three tiers: a base trunk tier and two upper tiers of branch elements. The top tier of branches directly support the MPP faceted roof panels of the folded canopy above. Load is then transferred downward to a base trunk element composed of four heavy timber column members. While branches in a natural tree transfer forces in bending, timber moment resisting connections would have been both costly and complex in terms of design, fabrication, and erection on site. Instead, the tree elements here carry loads axially and not in bending by being part of a closed 3D-truss system that includes the faceted roof as part of the system. Steel plate bolted connections occur at branch nodes and a horizontal tension rod band provides resistance to prevent the structural trees from splaying outwards (Fig. 5). The branch extremities from adjacent trees share connection plates to the faceted roof, so the entire set of trees provides continuous, collective support, forming a visual and structural forest (Fig. 6).

Ultimately, the design team celebrated wood and all its variety with each structural tree being a different species, maintaining the structural logic and diagram, while also being honest with the origin of the wood (typically from supplier donations) as being cut to size. Hence the separation of the wood pieces at the trunk, the use of metal for the connections, tension rods, and embedded connector plates to the faceted roof.

The MPP folded roof canopy above the tree columns provides lateral restraint as the structural diaphragm over the lobby and exterior colonnade. Given the roof surface is not planar and the tree-columns are pin-supported at their base for gravity load transfer only, lateral wind and seismic forces are transferred through the angled facets in the MPP panels and supported by CLT shear walls around the perimeter of the lobby, at the main building exterior wall, and at the terminus of the colonnade.

The folded roof’s asymmetric shape in plan also required the team to carefully track design loads and deflections imparted by the canopy onto the main building. The connections to the main building were detailed to be hidden from sight as they do not contribute to the wood structure exhibit.

The main building appears to be more conventional but simply has less obvious innovation. The main building’s structural system consists of a mix of sawn and engineered wood products for the building-as-exhibit. Various products were placed, designed, and used where each made sense from a material, structural, and educational need:

Throughout the main building, glulam beams serve as the main framing for the floors (besides some steel at long span exhibition space areas). These provided consistent visual legibility as the classic modern-day timber beam, while allowing for a range of material systems to span between, ranging from MPPs, to structural insulated panels (SIPs), and CLT depending on location.

As illustrated in Figure 7, one challenge for the building was the design of CLT shearwalls around the lobby perimeter. While CLT shearwalls were not yet adopted in the 2021 IBC used for the overall project, the team, in conversation with the local authorities and the owner, designed the CLT shear walls using the provisions of the ANSI/AWC 2021 Special Design Provisions for Wind and Seismic (SDPWS) document as an early adoption approach.

Additionally, the CLT panels were developed further for the main vestibule as tall slender shear walls providing both lateral stability and showcasing facade-level elements. This design, along with the CLT supplier’s standard panel width of four feet, meant that height-to-width aspect ratio restrictions on the shear wall panels were a primary concern. Therefore, the design approach of the panels along with their connection design required close integration between SGH and KPFF engineers as a delegated design partner. Ultimately, the KPFF and SGH team developed a “combined panel” performance-based approach where panel-to-panel connections were designed to meet the 2021 SDPWS requirements as well as project-specific in-plane shear loads.

The new museum’s purpose to educate the public ultimately drove the use of this unique and large collection of wood material types and systems.

During design, the team coordinated the individual components so that each could function semi-independently while also contributing to the larger interconnected system. During construction, the team coordinated materials and trades to manage tolerances, planning, and assembly. ■

Paul Kassabian, PE, CEng, is a Principal with Simpson Gumpertz & Heger Inc. ([email protected]).

Michael Richard, Ph.D, PE, is a Senior Project Manager with Simpson Gumpertz & Heger Inc.

Emma Ramsden, PE, is a Consulting Engineer with Simpson Gumpertz & Heger Inc.

Blerina Doucette. E.I.T. is a Project Consultant with Simpson Gumpertz & Heger Inc.