When St. Eugene’s Cathedral purchased an antique baldacchino sculpture for its altar, the parish received more than a work of art. The 24-foot-tall marble structure presented an engineering puzzle upon its arrival in Northern California.

The brittle, 80,000-lb structure had no positive connections between its 500 stone pieces to resist seismic forces, and no additional retrofitting could be visible from the front. Braces would mar the sculpture’s appearance, while installing metal rods could damage the marble.

“There was no way we could really reinforce it,” says Chelsea Drenick, senior engineer with ZFA Structural Engineers, the project’s design and structural firm. “That’s why we turned to base isolation. In case of an earthquake, we wanted to prevent the impact.”

The team installed a system with friction sliders, rubber bearings and viscous dampers. The team also upgraded connections between the sculpture’s stone pieces by reinforcing them with fiber-reinforced polymer and thickened epoxy. The curved marble beam above the columns transfers the lateral seismic forces to the columns and resists inherent torsion due to the sculpture’s irregular geometry. Plywood ribs were added on the back of the dome and arched facade beam as a lightweight way to stiffen the upper portion of the sculpture.

“The hardest thing was finding components that actually worked for this size,” Drenick says. The manufactured sliders, bearings and dampers available were designed for smaller objects or large bridges. The St. Eugene’s project required custom-made components that needed to be designed, manufactured and tested. A Japanese company made the dampers, and the components were tested at the University of Nevada, Reno. The end result was a more resistant sculpture designed to move within a concrete moat during an earthquake and separately from the building around it.

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