OAKLAND -- While Caltrans and contractors scurry to prepare the new Bay Bridge for opening on Sept. 3, state inspectors are also busy below decks, painstakingly scanning 656 feet of welds at the base of the new span's huge tower using a procedure much like what a doctor uses to examine a baby in the womb: An ultrasound machine, lubricating gel and a handheld transducer probe.
Technicians are looking for cracks, bubbles, lack of fusion or other defects in the 4-inch thick welded vertical joints between the base's steel shear plates and stiffeners, which keep the four tower legs aligned during a big earthquake.
The testing and repairs of the 20 internal welds -- located within a massive steel diaphragm just above the waterline -- are the only major unfinished items on the signature 525-foot tower.
Every inch of weld with a significant defect must be dug out, rewelded and reinspected as defined by American Welding Society standards.
It's a big job.
It is the world's longest known continuous electroslag weld. If combined and stretched into a single 1-inch ribbon, the Bay Bridge's weld would reach the top of the Transamerica Pyramid building three times over.
The weld inspection and repair process is taking far longer than engineers estimated, and the work will likely continue for months although it hasn't affected the rest of the construction schedule.
The tower is the dominant feature on the new eastern span -- a 2.2-mile self-anchored suspension bridge and concrete skyway between Yerba Buena Island and Oakland -- that will replace the seismically vulnerable 1936 cantilevered span.
The weld repairs won't delay the bridge opening and the span is perfectly safe in the interim, said Bay Bridge materials science engineer Mazen Wahbeh.
He cannot say specifically what portion of the welds will eventually be replaced, but he puts the figure at less than 5 percent. That figure is well below the extra holding power designers built into the tower base. They used oversize, 33-foot high steel stiffener panels arranged into a diaphragm pattern and welded and bolted together.
Ironworkers from locals 377 in San Francisco and 378 in Oakland made the welds two years ago for bridge contractor American Bridge/Fluor Enterprises.
The unusually long time gap between the welds being applied and the welds being tested is due to job priorities and recent changes in the electroslag welding process, Wahbeh said.
"Since the bridge can open with or without the plate welds finalized, the contractor is prioritizing the remaining work," Wahbeh said. "And with the new weld methods, we have had some growing pains."
The narrow-gap electroslag process involves feeding electrically charged wire and a chemical flow agent through a guide tube into the vertical space between the steel plates to be welded. The arc creates a molten puddle that fills the gap.
It is used for very thick joints where the traditional single-pass arc welding method is impractical or would take far too long, Wahbeh explained.
Space is tight in the bottom of the tower, and the preheating required in conventional welding would have generated untenable working conditions and warped adjacent plates.
Time was the biggest factor. Engineers estimated the job would take six months using the conventional welding process; electroslag took 60 days.
Each of the 20 welded joints took 41/2 hours to complete at a rate of 11/2 inches per minute, according to the May 2012 edition of the Welding Journal.
"Even with the extra inspections and repairs, which we would have had in either case, we are still saving time," Wahbeh said.
The American Welding Society adopted electroslag standards for bridges in 2006 and they were incorporated last November into the Federal Highway Administration's Steel Bridge Design Handbook.
Despite industry advances, the Bay Bridge design team required American Bridge/Fluor Enterprises to first prove it could produce solid electroslag welded joints on full-size plates.
The contractor, Portland-based Electroslag Systems, Technology and Development, and Portland State University drafted and tested their process in a laboratory and then on a full-scale mock-up in Oakland.