This Skyscraper’s Innovative Structure is Changing the Game for Earthquake Design
Source: Metropolis Magazine
by Laura Raskin
The most remarkable thing about 181 Fremont—San Francisco’s third-tallest tower, designed by Heller Manus Architects—is not the penthouse’s asking price ($42 million). Rather, it’s an innovative yet unglamorous structural detail: a viscous damper system that far exceeds California Code earthquake-performance objectives for buildings of 181 Fremont’s class, allowing immediate reoccupation after a seismic event.
When Arup engineers first evaluated Heller Manus’s design, the steel superstructure was excessively heavy—as heavy as OMA’s CCTV headquarters in Beijing, with its gravity-defying cantilever. That weight had negative implications for the skyscraper’s ability to weather seismic forces. Arup’s engineers, led by Ibbi Almufti, who heads the firm’s risk and resilience practice, set about reducing the amount of steel to create a lighter (and therefore more flexible) structure.
Consequently, they employed a viscous damper system to reduce the amplitude of earthquake-induced vibrations. The story-height dampers look like pistons and attach to the ends of two secondary braces that sandwich the primary diagonal steel braces in the commercial floors. As the building sways, the primary braces compress and lengthen like a spring, while the secondary braces activate the dampers, which absorb the earthquake’s energy. PTFE pads at the intersection of the braces and floor plates also help the braces glide through floors and prevent them from buckling.
The system is especially relevant as earthquake-prone San Francisco is seeing a boom in tall-tower construction. “For tall buildings, 181 Fremont is one of the safest, probably in the world,” says Jeffrey Heller, president and founder of Heller Manus. In addition to installing the damping system, Arup engineered 181’s remaining structure to REDi Rating System standards—resilience-based guidelines that Almufti created with input from academics, engineers, architects, and other stakeholders. Says Almufti, “People who want to do better [now] have an alternative.”
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