Warriors Basketball Arena Stays in Lead at Halftime
The $1-billion project is more than halfway through construction, filling an 11-acre site hemmed in by dense development, including the Mission Bay campus and hospital of the University of California, San Francisco. The Chase Center arena, office towers and retail and parking structure sit at various depths in mud and fill from the city’s 1906 earthquake, just a few hundred feet from the bay. A structural concrete slab and 1,500 piles drilled into bedrock protect the arena and surrounding buildings from water, settling soil and seismic forces.
Construction began in January 2017 on the privately funded project, which includes the approximately 1-million-sq-ft arena and two 10-story office towers as well as 100,000 sq ft of retail. Managed by joint venture partners Clark Construction Group and Mortenson Construction, the project has completed 1.5 million craft hours, with 900 workers currently on site and a projected 8,000 workers overall. The arena itself is built around four 120-ft-tall concrete cores topped by a 350-ft long-span roof held up by four main 500,000-lb steel trusses. Magnusson Klemencic Associates (MKA) was the project's structural engineer.
The team’s 200-page monthly playbook for the design-assist project is a race to the roof, aiming to install the trusses as quickly as possible. The arena itself is sliced into sections, with each quadrant on a different mini-schedule to finish concrete, steel and other construction. It’s a demanding schedule, says Peter Bryan, the Warriors’ vice president of construction and development, but it is necessary to complete the project in time for the start of the 2019-2020 NBA season.
“Mortenson-Clark has orchestrated a pretty amazing dance out there, when you consider all the craftspeople, all the deliveries, all the equipment,” Bryan says. “Unlike an office building or other types of development, when we set our end date for a start of season, that’s fixed.”
The challenges surrounding the Chase Center development began with the site, as designers pondered how to arrange the arena, towers, plaza and parking on the limited area, says David Manica, president of Manica Architecture, the design architect. The variable bay mud complicated any excavation for underground structures, yet San Francisco’s strict height limits helped keep the arena and towers at 135 ft and 160 ft high, respectively.
“We were given a space that was about the size of four city blocks, and the soil conditions were quite poor there,” Manica says. “All the street edges had to be active and interesting and a front door. There wasn’t going to be a big fence along one edge of the site where all the trucks would come in and all the trash would come out.”
The answer was a 1,000-stall, two-story underground parking structure and a single underground entrance and loading dock for trucks. “It was a complicated, three-dimensional puzzle that we solved to separate all the services and parking from all the wonderful outdoor plazas and retail areas,” Manica says.
Wind patterns also influenced the site’s layout. “We could not create windstorms or tunnel effects that would cause wind coming from the northwest corner of the site to increase,” Manica says.
So while early concepts put Chase Center in the center of the site, with the other above-ground structures and features spinning around it, project designers moved the arena closer to the water and placed the two office towers on the opposite side. This opened up space for a protected 3.2-acre public plaza. The arena and office towers were also spaced at least 20-30 ft apart so the structures could shake and bend during an earthquake without hitting each other, Manica says.
Construction began before all designs were completed, a typical approach for sports arena projects, says Trevor DeLong, the project’s senior superintendent with Mortenson-Clark. When crews started digging, only the foundations were 100% designed.
“Because of the size and scale of the concrete superstructure and steel superstructure, it takes so much time to build. So there is time to lag the design concurrently along with that,” DeLong says.
Digging out 300,000 cu yd of soil for the foundations took five months, DeLong says. “First we needed to start stabilizing the subgrade—you literally can’t walk equipment on the bay mud or you’ll sink,” he says. “You can sink standing in it.”
So before beginning excavation, the team built an underground CDSM (cement deep soil mixing) wall by drilling into the ground with a triple auger that injects grout and mixes it with the soil, DeLong says.
“You have to do that first before you start digging the hole, otherwise you’re just going to be surrounded by a lake,” DeLong says.
The mud was also variable, full of random fill from the site’s previous history as a railyard. The team installed 1,500 piles to support the site, drilling to depths up to 150 ft, with the deepest under the arena’s concrete cores. Designers adopted a hybrid pile system after discovering that the sand in some locations didn’t provide enough friction to support only auger-cast, 24-in.-dia piles, he says. Weaker areas required the deeper, 36- and 48-in.-dia drilled shafts using rebar and concrete.
The site also had subgrades of various depths, DeLong says. The arena’s subgrade was about 10 ft deep, while the parking structure’s was at 24 ft and the practice court’s subgrade depth was 14 ft.
Working so close to the water, the project team chose to encase the site in a “bathtub” rather than install a dewatering system, DeLong says, which could risk dewatering areas outside the property line. Crews poured a concrete working surface that became the substrate for a waterproofing membrane on top of it.
Above the working surface is the project’s “structural slab on grade”—this concrete slab ranges from 12 to 16 in. thick across the entire 11-acre site, protecting it from the moving water and settling soil. Crews continue to pour the structural slab in bite-size pieces, about 700 cu yd at a time. Most of the slab can be poured before the long-span roof is completed, except for the slab beneath the arena’s basketball court.
Crews have been working since May to install the roof trusses with plans to set the last of the four main 500,000-lb steel trusses in early August. Only half of each truss can be raised at a time, then joined together and installed. The goal is to enclose the arena by Nov. 1, before winter rains begin, DeLong says.
“Everything we do in terms of sequencing and flow and what preceding work needs to happen is about what is going to be the quickest path to the roof,” he says. “That really drives the whole building.”
Moving to the top of the arena at such speed required bypassing areas still in design, such as the kitchen, commissary and underground mechanical rooms. Meanwhile, the concrete kept pouring, with eight different concrete subcontractors working on the project. With so many elevated decks and multiple grade changes throughout the site, the team was running three 1,000-cu-yd pours a week.
“We had several weeks this winter where we were pouring 4,000 cubic yards a week of concrete, with two ready-mix plants serving the job,” DeLong says.
All this noise and activity affects the project’s many neighbors, from UCSF students and residents at the surrounding condos to patients, families and staff at the UCSF Medical Center. Hospital helicopters frequently fly over the site, posing a challenge for placing tower cranes.
“The helicopters go right over the top of the job. All of the crane activity had to be planned and coordinated for height so as not to create crane conflicts but also stay below the flight path,” DeLong says.
Scheduling was also an issue, says Bryan. Daytime construction affected local businesses, but nighttime work disturbed the many medical students living across the way in Hearst Towers. The contractor amended its schedule, he says, to accommodate final exams.
Mission Bay was first developed as an enclave for biotechnology firms, so the project team holds monthly meetings with the biotech community, which includes companies like Bayer Corp. and FibroGen as well as UCSF. But with a project like this, drawing basketball fans, office workers and shoppers to the area, the neighborhood discussions won’t end with the construction.
“Those are efforts that we at the Warriors and Chase Center will be dealing with for the life of the building,” Bryan says.
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