The 181 Fremont Tower currently rising above all others in San Francisco, is the only all steel building designed to be LEED Platinum and the most resilient tall building on the West Coast. When complete, the 56 story mixed use tower will be the second tallest building in San Francisco (810 feet – 24 metres).
Key to the success of the building’s resilience performance is the introduction of a novel damped megabrace lateral system. Since the tower is slender and lightweight, wind accelerations posed a potential issue in the condominium levels. Therefore, an innovative viscous damping system was developed to fit within the architecturally expressed megabrace design. The damped megabrace system is able to generate approximately 8% of critical damping, which has a significant effect in reducing both wind and seismic forces.
In addition, the megacolumns were designed to uplift at their bases in the MCE event in order to significantly reduce the tension demands in the foundation and the megacolumns. The bases of the megacolumns were pre-tensioned with anchor rods extending below into the foundation so that uplift would not occur in wind or smaller earthquake events.
To achieve this level of resiliency, the tower is designed to achieve a “Gold” rating using the REDi Rating System (REDi 2013). This requires the building achieve immediate re-occupancy and limited disruption to functionality after a 475-year earthquake. At this performance level the structure will remain essentially elastic and that the non-structural components need to be designed for more stringent force and displacement requirements relative to the code. The ultimate performance of the tower at the maximum-considered earthquake (MCE) was verified following the methodology of PEER-TBI (2010). The owners, Jay Paul Company, were provided with additional recommendations for implementation of preparedness measures that would aid in achieving the enhanced performance objectives.
The 181 Fremont tower will not only enhance the San Francisco skyline, but also provide much needed office and housing, while protecting occupants and the building itself through unprecedented resilient seismic design.
181 Fremont – The design and construction of the first resilient tall building by design: Jason Krolicki, ARUP San Francisco, Associate Principal, Arup San Francisco, Tall Buildings Lead is a guest of honor and will be presenting a session at the Quebec Steel Conference on October 25th, in Quebec City.