One of the next-door neighbors in my office neighborhood is about to open for business, an event which will certainly continue the transformation of my work surroundings.
Certainly, no building is earthquake-proof, but the design of the new building is quite fascinating:
- This tower will be the most resilient tall building on the west coast of the United States
Arup’s structural engineers employed a holistic resilience-based seismic design approach to minimise damage in a 500 year earthquake and allow immediate reoccupancy after a seismic event, far exceeding building code criteria. Its iconic tapering form, small footprint, and location in the midst of the Transbay urban regeneration zone presented significant engineering challenges. Arup incorporated groundbreaking design solutions including an innovative viscous damping system within the architecturally expressed steel megabraces and uplifting megacolumns which significantly reduced seismic and wind demands and resulted in a steel material savings of approximately 3,000 tons.
- The Resilience-Based Design of the 181 Fremont Tower
The mega-brace system is three braces in one (Figures 3 and 4). The middle (or “primary”) brace is a steel box section and the two outer (or “secondary”) braces are comprised of built-up plates attached to two viscous dampers at one end. As the building flexes laterally in a wind or earthquake event, large (elastic) strains develop in the very long primary braces. The result is approximately 6 inches of lengthening or shortening in the primary brace between the connected nodes. Since the secondary braces are connected to the same mega-nodes via dampers, this relative movement is utilized to activate the dampers and dissipate energy. The system was tuned to optimize the wind performance. However, the damping additionally benefitted the seismic response of the tower by reducing the earthquake demands across several modes of vibration.
- San Francisco’s 181 Fremont will Become the Most Earthquake-Resilient Building on the West Coast
The REDi Gold Rating that 181 Fremont - which Arup was the structural engineer, geotechnical engineer, and resilience consultant for - achieved includes enhanced structural and non-structural design to limit damage, improved egress systems, contingency plans to reduce post-earthquake recovery times, and development of a tenant’s resilience manual of recommendations to keep their space earthquake-ready. A building with a REDi Gold Rating can expect its repair costs to be cut by approximately 10 times compared to code-designed buildings and can also reduce the expected functionality downtime from 18 months to less than a few weeks.
- The Skyscraper Center: 181 Fremont
The other key structural innovation of the tower is the notch at the center; this notch creates turbulence that helps reduce the aerodynamic pull of the wind, allowing the design to require less steel to resist lateral wind forces.
What makes people think they know what a "500 year earthquake" is for the Bay Area? I'm skeptical.
Still, it's a beautiful and interesting building, and all indications are that it has been carefully designed and built.
It won't be the second-tallest building in San Francisco for long, though...