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by Alan Blevins, Webcor Senior Superintendent, Concrete

If the 2130 Post Street project were a game, it would be Operation. Get in, touch as little as possible, and get out. Of a $20.5 million contract, a mere $500,000 went towards scopes unrelated to the seismic renovation or code compliance. This project was truly a seismic surgery; weave in new structural components around existing MEP systems and finishes, then cover them up and blend them in.

The original 2130 Post Street concrete structure was built in 1968. Thanks to the Loma Prieta Earthquake (1989) and Northridge Earthquake (1994), we now know that concrete structures built before 1980 are non-ductile; essentially, the lack of proper rebar cages puts their columns at risk of failure (among other issues). 2130 Post Street had the same issues.

Tipping Structural Engineers designed a retrofit infrastructure to remedy these issues, including:

• Wrapping existing interior columns with Glass Fiber Reinforced Polymer (GFRP)
• Bolting 1-inch thick steel angles to interior columns to support the weak slabs
• Adding new exterior concrete shear walls to reduce building drift
• Adding new exterior concrete columns and collector beams to support existing column moment frames

Each of the above applications required delicate coordination with the design team, subcontractors, and UCSF (the building owner) to determine a plan that accommodated these structural improvements while leaving finishes in place. Many of the columns to receive the GFRP wrap abutted kitchen casework that was required to stay in place. This demanded a unique solution from the GFRP application that would increase the structural capacity of the column while only having access to three sides of the column. Webcor, Tipping, Simpson Strong-Tie and FD Thomas (FRP subcontractor) worked together to mock-up this condition in Simpson Strong-Tie’s Stockton facility. The test included strapping full-sized concrete columns to Simpson Strong-Tie’s shake table and analyzing the performance of a non-retrofitted column versus a column with a three-sided FRP application.

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The exterior concrete work also presented its own challenges. Prior to full design, Webcor consulted with UCSF and Tipping on the best options to retrofit the building while maintaining finishes and minimizing costs. The answer? Perform as much work on the exterior of the building as possible. But this option wasn’t without its own logistical challenges. How do you provide access to the exterior of the building while also leaving room for concrete forms? How do you hoist materials to the backside of the building when mobile cranes and street closures are too expensive? How do you dowel into the PT Decks every 8 inches without compromising the existing PT cables?

Webcor Concrete developed solutions for each of these: Use a heavy-duty scaffolding system that acts as scaffold access but is also strong enough to act as a vertical concrete formwork support; use a lightweight Spydercrane on the roof that can lift materials up one side of the building and drop them down the other; and coordinate with Hilti, the project’s tool/materials supplier, to develop an in-house scanning operation that saves time and money and reduces risk.

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2130 Post Street is a sign of things to come. Los Angeles and Santa Monica have led the way on retrofitting outdated structures, but San Francisco and Oakland are not far behind. Both cities currently have soft-story retrofit mandates, and mandates for mid-rise and high-rise structures are expected in the near future (https://www.enr.com/articles/46210-san-francisco-to-begin-seismic-code-update-based-on-landmark-tall-buildings-study). Carefully strategized, cost-effective retrofits will be in high demand, and Webcor has the right people to ensure they’re successful.

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