Buckling Restrained Braces In Mass Timber Projects: Three Case Studies

For high-seismic regions, force-resisting systems are necessary.

Mass timber construction has experienced a significant increase in adoption in recent years due to its architectural appeal and sustainable nature. As its use has spread into regions of high seismicity, the need for reliable and economic seismic force-resisting systems compatible with this type of construction has arisen. Lateral systems incorporating Buckling Restrained Braces (BRBs) complement the desirable benefits of mass timber, and several methods have been developed for their incorporation in mass timber buildings. Three of these methods will be highlighted: a timber/steel hybrid system which collects lateral forces over large areas and concentrates them into steel BRBFs that are able to handle the relatively high resulting forces; a timber BRBF system which resists the lateral forces on a more local level but which must develop the BRB demands into timber members, rather than steel; and a novel timber shear wall system utilizing vertically-oriented BRBs as hold downs at the base of the wall. Each system has its advantages for certain building configurations and will be illustrated through completed projects. 

BRB In Mass Timber/Steel Hybrid System:
San Mateo County Office Building 3 (SOM)

County Office Building 3 (COB3) is a new government center for San Mateo County in California. The 5-story building totals 208,000 square feet and utilizes a mass timber/steel hybrid system with a conventional BRBF lateral system. It will be the nation’s first net-zero energy civic mass timber building. 

The building was designed in conformance with the 2018 International Building Code (IBC) with State of California Amendments (CBC 2019) and is classified as a Type IV HT (Heavy Timber) construction per fire-resistance rating requirements of 2019 CBC Table 601. 

The new county office building, characterized by its H-shaped plan, is designed with an open-floor office concept in mind, with large open structural spans and a completely exposed timber structure. The building is located not only in a high seismic region (Seismic Design Category D), but also in a site prone to liquefaction, necessitating the need for a robust lateral force-resisting system. Due to the required long spans, the structural lateral force-resisting system is concentrated from the main timber structure into 4 ‘cores’ of the building consisting of conventional steel buckling-restrained braced frames (BRBF). These elements are shown in Figure 1.

Figure 1. Plan View of COB3 Timber/Steel BRBF Hybrid Structure

Due to the high seismic loads of a large floor plate relative to the available size of the steel frames, as well as current code restrictions in the use of timber shear walls in California, a BRB system was utilized to take advantage of the high Response Modification Coefficient, R, as given in ASCE 7. The COB3 structure consists of a traditional timber/steel hybrid design with the BRBs in frames consisting of steel columns and composite steel beams (the steel ‘cores’ of the building). In contrast, the timber structure consists of glulam columns and beams with a CLT floor system.

Lateral loads in the timber structure are transferred between CLT floor plate panels using plywood splines which splice each adjacent panel together, allowing the transfer of in-plane loads. In addition to the splines, steel drag members are utilized throughout the floor plates to act as chords and collectors. Chord and collector members are connected via screws to the top faces of CLT panels. Collector members are then welded directly to the core steel columns to transfer axial loads into the braces, while CLT panels, which rest directly on the steel beams of the core, are connected via screws to transfer shear forces. These elements are indicated in Figure 1, and a view of the overall structure is shown in Figure 2.

Figure 2. Steel BRBF and Timber Construction, COB3.
Photo Courtesy of Cesar Rubio Photography

With the steel cores of the building situated at the reentrant corners of the H-shaped building, the timber structure cantilevers more than 60ft in plan. As CLT systems tend to deflect significantly due to a combination of panel shear and bending, steel connector slip and plywood spline slip, the design criteria for the drift at the core was limited to 0.7%, allowing for an additional 1.3% drift in the cantilevered floor plate and thus remain under the code limited 2% total drift at the edge of the building. During the analysis of the BRB system, it was found that the braces, when sized for strength, did not need to be altered significantly to satisfy the more restrictive drift design criteria. This is made possible by the significant difference between R (8.0) and the deflection modification factor, Cd (5.0).

BRB In Mass Timber Frame: Terminus at District 56 (Aspect Engineers)

Terminus is an example of a mass timber structure with a novel lateral force-resisting system using BRBs entirely within timber frames. The Terminus project, a 5-story office building in Victoria, British Columbia, Canada, completed in 2021, presented an opportunity to explore a range of lateral force-resisting system options to find one that was best suited for the structure, the architecture, and the extremely high seismic demands of Vancouver Island. 

The building is home to the head office of the project’s developer, Design Build Services (DBS), who sought a clean and modern mass timber look combined with resilient seismic performance. The high seismic demand of the region, with a short period 5% damped spectral acceleration of 1.32g, had the design team searching for a lateral system that was suitable for a mass timber project, namely that it was highly ductile, cost-effective, and still celebrated the exposed mass timber. Ultimately the team landed on using BRBs within timber frames, a combination that had not previously been built in North America and one that united the best of BRB with the best of mass timber. Housing BRBs in a glulam frame meant a refined and uniform architectural language of timber throughout the building and streamlined construction without sacrificing performance. A view of a typical frame can be seen in Figure 3.

Figure 3. Timber BRBF in the Terminus Project, Vancouver, BC. Photo Courtesy of Dasha Armstrong

The design and detailing of this novel system required careful consideration of the forces, the system’s true performance, and the exposed structure’s visual nature. One of the challenges was ensuring that the timber frame would not provide any rigidity or restraint that would limit the energy dissipation capabilities of the BRBs. In short: how to ensure the frame connections behaved as true pins and could accommodate the building drift. The design started with engineering first principles and good timber detailing practice: detailing the timber connections with numerous mild steel tight-fit pins. The ductile connection failure mode of the pins yielding would allow the joint to have some natural flexibility. The team further provided slotted holes in the steel plates so the connection assemblies would not be restrained and aligned the work points of all members to limit eccentricity introduced into the connection. These details can be seen in Figure 4. By a stroke of serendipity, researchers at the University of Canterbury studying BRB-timber structures had built and tested full-scale models of BRBs in glulam frames. The researchers demonstrated that a frame connected with bolted or pinned connectors could be approximated as a pinned frame that allowed the BRB to undergo the deformations necessary for the expected ductility.

Figure 4. Timber BRBF Connection Details

A second challenge the designers faced was resolving the high seismic forces within the timber frame elements. The connection forces, which included an overstrength factor of around 2, drove an increase in frame member sizes and connection complexity. Two strategies helped to keep the timber connection forces in check: (1) The columns, which were under significant axial forces, were continuous over three stories to minimize splices, leaving only a single massive column connection at the base to be detailed, and (2) a steel tie element was concealed between the CLT deck and the raised access floor. In addition to these strategies, frame locations were chosen to keep the BRB sizes at a minimum, allowing the connections to be designed for the smallest force possible. 

With the biggest technical challenges addressed, collaboration between the design team, the timber fabricator, and the BRB supplier proceeded smoothly, and installation was comparatively simple. Connecting the BRBs to knife plates that were pre-installed into the timber elements kept the BRB erection straightforward. 

Ultimately, this building represents the possibilities for mass timber with non-conventional material and system combinations. 

BRB As a Holdown for Mass Timber Shear Walls: Catalyst Building and Oregon State University Test Specimen (KPFF)

An innovative application of BRBs in a mass timber lateral force-resisting system is their use as hold-downs for mass timber shear walls. This timber-steel hybrid configuration utilizes each of the materials to their greatest benefit; namely, it takes advantage of mass timber panels for their stiffness and strength but relatively low ductility and BRBs for their stable cyclic behavior and force-limiting capability. The stiff mass timber walls limit story drifts but, with the inclusion of BRBs as hold-downs, can still be designed for relatively high ductility (e.g., a response modification coefficient, R, of 6 to 8). Since the maximum force exerted by a BRB hold-down in an overstrength condition can be reliably estimated, capacity-design principles can be straightforwardly applied to check the BRB-to-wall and BRB-to-substructure connections as well as the mass timber wall itself to preclude brittle failure modes.

Currently, mass timber shear walls with BRB hold-downs are not prescriptively permitted for seismic regions in ASCE/SEI 7, as referenced by the International Building Code. As such, current projects must pursue performance-based seismic design. However, the equivalent lateral force and modal response spectrum procedures using a response modification coefficient, R, have been shown to produce designs that are likely to satisfy the requirements of performance-based design and nonlinear analysis. Engineers pursuing this system can therefore be confident that their preliminary/proportioning designs will hold up to more sophisticated analysis later in the performance-based design process.

Figure 5. Mass Timber Shear Wall with BRB Hold-Downs at Catalyst in Spokane, WA. Inset shows threaded bars epoxied into a mass timber panel for a BRB-to-wall connection.

Two examples of mass timber shear walls with BRB hold-downs are the Catalyst building in Spokane, WA, and a 3-story building test specimen which, as of this writing, is being cyclically tested in Oregon State University’s laboratory. Catalyst is a five-story, 150,000ft² mixed-use education and office building completed in 2020, which uses a combination of planar and core-configured cross-laminated timber (CLT) shear walls. At each end of the planar walls and each corner of the core walls, vertically oriented BRBs connect the CLT walls to the substructure below. See Figure 5. More information on Catalyst’s seismic force-resisting system can be found in a paper presented at the 2021 World Conference on Timber Engineering by Zimmerman et al. The 3-story building specimen being tested at Oregon State University also uses vertically oriented BRBs, but they are instead connected to mass plywood panel (MPP) walls. This work has been summarized in a paper presented at the 12th National Conference on Earthquake Engineering by Araujo et al. Testing of this 3-story building specimen saw the shear walls successfully undergo cyclic demands of up to 4% story drift. See Figure 6, which shows the shear wall and connection detail used in this testing. 

Figure 6. Mass Timber Shear Wall with BRB Hold-Downs in 3-Story Building Test Specimen at Oregon State University. Inset shows a U-shaped steel bracket for a BRB-to-wall connection (Inset photo shown prior to floor erection).

While quite similar, Catalyst and the 3-story building test specimen have a few notable differences. In Catalyst, mass timber walls bear on the substructure over their full length; in the 3-story building test specimen, they pivot about a central bearing area. In the former configuration, the BRBs experience larger tension than compression displacements (i.e., wall panels rock about their toe), whereas, in the latter, similar tension and compression displacements occur (i.e., wall panels pivot about their center). Additionally, Catalyst and the 3-story building test specimen utilize different BRB-to-wall connection details. At Catalyst, threaded bars are epoxied into the edge of the CLT panels above with embedment depths exceeding 2’-6” in order to produce an aesthetically concealed connection. In the 3-story building test specimen, a U-shaped steel bracket is used with 45-degree fully-threaded screws installed into the face of the MPP walls above. These connection methods are shown in the insets of Figures 5 & 6. These two examples present only some of the design and detailing flexibility available when using mass timber shear walls with BRB hold-downs. Other configurations are expected as this system sees continued development and application. For example, research is currently underway at the University of British Columbia (UBC) and the University of Northern British Columbia (UNBC), where BRBs are connected with glued-in rods into the side of CLT panels. The utility of BRB hold-downs for tall timber design was shown by Tesfamariam in the 2022 Council on Tall Buildings and Urban Habitat Journal.

Summary

Buckling-restrained braces can effectively serve as elements of seismic force-resisting systems for mass timber structures. Successful projects have been completed using BRBs as hold-downs for mass timber shear walls, in fully timber frames, and in conventional steel frames within larger mass timber structures. In the San Mateo County Office Building 3 project, conventional steel buckling-restrained braced frames were integrated into an otherwise mass timber structure allowing for large open spaces and accommodating a high seismic mass and significant re-entrant corners. In the Terminus project, BRBs were used in frames with timber beams and columns, allowing for an aesthetic architectural appeal that highlighted the mass timber structure and BRBs. The Catalyst project coupled the inherent stiffness and strength of mass timber shear walls with the ductility and stable cyclic behavior of BRBs to create a novel system which, in the testing of a 3-story building, withstood story drifts up to 4%. Many other structures, completed and in progress, have used combinations of these systems, or others incorporating BRBs, to create reliable lateral force-resisting systems for mass timber structures. 

Acknowledgements

The authors would like to acknowledge the assistance of the
following individuals:

Mark Sarkisian, Eric Long, Peter Lee, David Shook with Skidmore, Owings & Merrill

Anne Monnier and Jack McCutcheon with KPFF Consulting Engineers

Mehrdad Jahangiri, Brendan Fitzgerald, Jackson Pelling with Aspect Structural Engineers

Dr. Arijit Sinha, Dr. Andre Barbosa, Dr. Tu Ho, and Gustavo Orozco of Oregon State University and Dr. Barbara Simpson and Gustavo Araújo of Stanford University (formerly of Oregon State University)

Dr. Solomon Tesfamariam of the University of British Columbia (UBC) and Dr. Thomas Tannert of the University of Northern British Columbia (UNBC)■

About the author  ⁄ Brandt Saxey, S.E.

Brandt Saxey is the Technical Director for CoreBrace. He is a member of the AISC 341 TC-9 Seismic Systems Committee, TC-6 Connection Design Committee, and M3 Seismic Manual Committee. (brandt.saxey@corebrace.com)

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