In all, Stress Engineering employed more than 75 suppliers and subcontractors in manufacturing the slip joint. It cost $1.5 million—not including design and testing—and was procured directly by the SFPUC. "For a typical pipeline job, everything would go through the contractor, and most of the risk would be on them in terms of the vendors supplying the materials to the job," Wade says. "But because of the specialized nature of these components, we needed to plan for some special provisions to make sure there is a proper balance of risk between the owner, designer and contractor."
Freedom of Movement
The water agency also procured the 72-in. ball joints, which are the largest known to be manufactured in the world, according to fabricator EBAA Iron Sales in Eastland, Texas. The company's line previously topped out at 48 in. SFPUC also purchased a full-size prototype and subjected it to rigorous testing.
Composed of ASTM A536 ductile iron, each ball joint weighs in at over 17 tons and is tested hydrostatically to 200 psi, higher than the pipeline's normal operating pressure of 125 psi. The ball joints were tested at 12° displacement, which exceeds the 8° of movement expected during an earthquake, says Czarnecki.
Between the ball joints, two sliding supports allow the pipe to move freely. Mirror-finished stainless-steel plates on the bottom of the pipe sit atop PTFE-coated supports anchored to the floor. Between the supports over the main fault hazard zone, 1-in.-thick unsupported steel pipe stretches 147 ft.
A cladded-pipe element that connects the north ball joint with the slip joint was procured by Rados. Held in place by steel frames, the 50-ft-long continuous piece of pipe is free to compress into the slip joint with "absolutely no binding," Czarnecki says.
Planning and installing the 46-ton cladded pipe was one of the most difficult tasks on the project, Pelletier says. Fabricator XKT Engineering, Vallejo, Calif., welded a ribbed steel frame on four quadrants of the pipe exterior to provide flat platforms upon which long sheets of highly polished stainless steel could be attached using flush bolts. The apparatus allows the pipe to slide freely through the frame. On the 12 ft of cladded pipe closest to the ball joint (where the largest loads will occur), the stainless-steel sheets are replaced by Hastelloy, an expensive alloy of nickel, molybdenum and chromium.
"One of the problems we faced was that nobody could make a prediction of what the coefficient of friction for stainless steel or Hastelloy is going to be 50 years from now, when it needs to work," Czarnecki says. Since stainless steel can corrode under certain conditions, the team added the sections of Hastelloy, "which is about the best corrosion-proof material you can get," he adds.
Shape-Shifting Vault
