Transportation Hub Will Be a Safe Haven
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Issue: 06/29/2015 Transportation Hub Will Be a Safe Haven San Francisco "groundscraper's" robust steel frame engineered to allow immediate reoccupancy after a very strong quake 06/29/2015 By Scott Blair A crane on a trestle lifts an architectural steel assembly, including a 9‐ft‐tall cast node, into place along the perimeter of the $4.5‐billion Transbay Transit Center, which is taking shape in seismically active San Francisco. The node picks, followed by more steel tubes that link to form a four‐and‐a‐half‐block‐long exoskeleton, is repeated more than 300 times. The lateral‐load‐resisting system will allow immediate reoccupancy of the 1,425‐ft x 171‐ft "groundscraper" after the "Big One." The architect for the five‐level bus‐and‐rail hub calls the 304 expressed custom nodes, which weigh from 2 tons to 23 tons each, robust and muscular. "We are trying not to let them look light or delicate because they are anything but [that]," says designer Fred Clarke, senior principal with Pelli Clarke Pelli Architects (PCPA). Designated an essential facility by the city, the four‐and‐a‐half‐block hub must be available for "immediate occupancy after what we're calling a 975‐year earthquake," says Bruce Gibbons, a managing principal of structural engineer Thornton Tomasetti (TT). That is equivalent to a magnitude‐8 temblor on the nearby San Andreas fault, he adds. Using performance‐based seismic design (PBSD), TT followed stringent requirements set forth by the Transbay Joint Powers Authority (TJPA), the public agency set up in 2001 to manage the project. Under the plan, buses and trains will remain operational immediately after any disaster to enable transit of people and supplies. The building also will serve as a safe haven for the South of Market community. Work on the $1.9‐billion first phase began in 2010 and is on schedule for completion at the end of 2017. The excavation is one of the largest in the city's history. "This is a landmark project," says Amanda Gillespie, project director for the joint‐venture construction manager‐general contractor Webcor‐Obayashi under a "no self‐perform" contract. Below grade, the concourse and train platforms will serve future rail connections. Two levels of public space, retail and offices occupy both ends of the building, with a double‐height grand hall in the middle. Above that, a bus deck connects to nearby streets and the San Francisco‐Oakland Bay Bridge via ramps and bridges. A 5.4‐ acre roof park will top the 75‐ft‐tall building. Both the bus deck and green roof extend beyond the ground‐level footprint by about 23 ft. "We wanted to support the park at the edge while avoiding a cantilever," which led to an exoskeleton of inclined steel columns, says Michael Stein, managing director with Schlaich Bergermann & Partner (SB&P). The firm provided the original conceptual structural engineering when PCPA submitted its scheme for the 2006 competition. Strange Bedfellows The scale and complexity resulted in traditional competitors working together. With a project this big, there is a business necessity to have major general contractors performing as subcontractors, says Gillespie. Subcontracts are massive, so companies need substantial bonding capacity. Turner Construction Co. performs construction management oversight and quality assurance. General contractors Skanska USA Civil West, Balfour Beatty Infrastructure and Shimmick Construction hold major subcontracts. Managing another large general contractor is more difficult: A traditional subcontractor might be more cooperative in an effort to win future work, says Steve Rule, Turner's construction executive. To head off any potential disagreements, TJPA holds monthly partnering meetings. "It allows people to have open dialogue to get them all rowing in the same direction for the best interests of the project," says Dennis Turchon, TJPA's senior construction manager. The building is essentially a long structural tube, which limited options for either interior bracing or shear walls. Instead, each internal bay is a steel moment frame. Longitudinal seismic resistance is provided by the inclined columns and cast nodes, connected by shear links at the roof edge beam. The exoskeleton originally was designed with three‐dimensional basket‐shaped columns. They were value‐ engineered into inclined columns that bifurcate from a single point at grade and again at the bus deck, SB&P's Stein says. Nodes link steel members at each bifurcation and at the roof deck's spandrel beam. The mechanism works as an eccentrically braced frame (EBF), though the link occurs only at the roof, TT's Gibbons says. "As the ground moves and the inertia of the building kicks in and it starts swaying, movement— even at the bus deck—will induce sheer deformations in the links," he adds. To prove the concept, TT produced new fragility curves for shear links by first plotting data from various university tests and then plotting curves "to predict how much ductility we would get out of the shear links before they failed," says Gibbons. TT's data has been since incorporated into a study by the Applied Technology Council. During the required PBSD peer review, the question arose about whether the system was a true EBF. In response, TT tested full‐size specimens to confirm the required ductility, even though the code doesn't require testing. The tests bore out TT's predictions. "Testing was a good idea because the building does rely heavily on the shear links," says Gibbons. During a seismic event, as the shear links yield, "the rest of the building has to remain predominantly elastic and be able to transfer forces through the [inclined columns] to the cast nodes and into the foundations," says Carlos de Oliveira, president of Cast Connex, which TJPA retained to oversee node production. Because the site slopes around 10 ft from east to west, the angle of the node nozzles—openings to receive the steel tubes—vary. That meant the casting foundry, Bradken, had to produce nodes in 75 unique geometries. Nodes weigh a total of 1,650 tons. After the foundry received node drawings, it did a solidification analysis, "almost like a sample pour in a theoretical environment," and finalized the design of the mold, says Wayne Braun, Bradken's director of business development. Next, technicians create molds from wood tooling and chemically bonded sand. Workers then cast 2,900° F metal into the mold through a channel so that the metal flows up from the bottom of the mold. This method prevented turbulence that can dislodge sand particles or create gas bubbles. Once it cools and gets shaken out of the sand mold and grinded, the node goes through nondestructive ultrasonic and magnetic particle testing to make sure no cracks formed during cooling. Engineers compare a 3D laser scan of the completed node to a design model to ensure everything will fit properly once the casting gets delivered to the steel‐member fabricator. Crews also qualified the first node to be cast of each of the 75 unique geometries by using X‐rays to look for shrinkage around the critical nozzle areas where tubes are field‐welded. The project, which includes federal funding, was required to adhere to the "Buy America" stipulation for permanent elements. But much of the structural steel, including the moment frame's 120‐ft‐long, 125‐ton transfer girders, were too large for U.S. rolled‐steel fabricators. As a result, four fabricators are building up the shapes from layers of plate steel up to 5.5 in. thick. "When you are dealing with such big, heavy members, you generate a tremendous amount of heat in welding operations," says Ryan Clayton, a vice president of Skanska. The firm holds the $189.1‐million subcontract to furnish 23,000 tons of structural steel—apart from the nodes—and erect the entire structural system. "Having to control all those weld distortions to deliver product that is within specified tolerances is a challenge," Clayton says. As an added wrinkle, American Institute of Steel Construction tolerances require even more stringent finishes for the exoskeleton's exposed structural steel, he adds. Once the nodes reach the structural‐steel fabricator, workers perform a final trial assembly to ensure proper fit‐up in the field, de Oliveira says. To minimize field welding, every node gets welded to at least one steel member at the shop. At the site, Skanska's crew of about 90 ironworkers temporarily bolts the assemblies before welding. Several orbital welders supplement the team, especially on the nodes. Some of the steel tubes and nodes take more than four days to weld, Clayton says. Mammoth Excavation Before construction, TJPA built a temporary terminal and razed the old one, built in 1939. Then, in late 2011, Balfour Beatty, under a $187‐million contract, began two years of work on the 1,650‐ft‐long, 182‐ft‐wide and between 55‐ft to 65‐ft‐deep excavation— installing shoring and buttressing. Crews removed over 640,000 cu yd of sand and bay mud and 2,000 40‐ft‐long timber piles, installed in the 1930s. Virtually all the material was recycled. Crews also found a mammoth tooth and an ancient human skeleton during excavation. Due to the proximity to the bay, the water table is 10 ft below grade. Under certain load cases, the building could achieve buoyancy, so crews installed 1,800 micropiles up to 100 ft long to anchor the building against floatation, TT's Gibbons says. Once 3‐ft‐thick perimeter shoring walls were done, Balfour Beatty crews installed dozens of 3‐ft‐dia tubular‐ steel buttresses across the site. Geotechnical engineer Arup monitored the excavation and surrounding buildings for movement, using over 2,000 sensors.