An Alternative Rocking CLT Shear Wall System

Seismic-force-resisting systems (SFRS) using cross-laminated timber (CLT) shear walls lack the benefit of standardized guidance that includes external energy dissipation systems. The low seismic force reduction factor (R) values for mass timber shear walls in ASCE 7-22 are based on the nonlinear mechanisms in the wall panel connections. In a recent project in San Francisco, CA, Tipping Structural Engineers explored utilizing mild-steel butterfly fuse connectors with rocking CLT panels to reduce the seismic demand. The SFRS of the building consist of CLT wall panels connected with steel butterfly fuses to dissipate input earthquake energy. The CLT shear walls are connected to the foundations using steel pins at either end. This enables the connected CLT panels to achieve rocking motion under lateral loading. The butterfly fuses experience nonlinear shear deformation due to the relative movement of the CLT panels and dissipate energy. Rocking actions only occur on the central panels because the designed gaps below each of the two side panels allow them to pivot freely around the pins. The seismic response of the proposed system is characterized primarily by the rocking mechanism and the shear yielding of butterfly fuses. Since this lateral load resisting system is not prequalified in ASCE 7-22, a peer-reviewed performance-based approach was used to validate the design utilizing nonlinear pushover analysis. The validation studies demonstrated the effectiveness of the system design using a R factor of 8. The study was extended to include non-linear shaking simulations to explore the basic mechanics of this innovative system. Procedures based on the analytical model are provided to select the butterfly fuses in preliminary design.