When the San Francisco-Oakland Bay Bridge was originally built in 1936, it was considered an engineering marvel. But the engineers and builders at that time were more concerned with building a bridge that would withstand the choppy waters and high winds of the bay than with the possible threat of earthquakes.

In October 1989, a 7.1 magnitude earthquake severely damaged the eastern span of the bridge. Located just a few miles from the San Andreas and Hayward faults, the bridge is expected to face another major quake within the next 30 years. A study conducted by Caltrans, the California Transportation Commission, determined that while the western span of the bridge could be retrofitted to withstand another quake, the eastern span needed to be replaced. And so began one of the most ambitious engineering projects in U.S. history. In addition to the challenge of replacing a bridge that is traveled by 280,000 vehicles daily without seriously affecting traffic flow, the new bridge will incorporate a host of innovative seismic safety technologies that will make this one of the engineering marvels of the modern world. The completed span will include some of the largest and heaviest components ever used in bridge construction and will be able to withstand an 8.1 magnitude earthquake. The eastern span is due to be completed in 2012.

ESAB is proud to have played a part in this innovative project. ESAB filler metals and equipment were used by Trans Bay Steel, a major steel fabricator in Napa, California, to construct a key component of the seismic safety system, a state-of-the-art hinge beam system installed at the expansion joints of the pre-cast concrete deck segments of the Skyway portion of the bridge. These hinge beams are designed to allow for expansion and contraction caused by temperature changes and other minor motion. One end is fixed, and the other end has a sliding stainless steel clad bearing surface. There is also a “replaceable fuse section” designed to shear in the event of a large earthquake, preventing catastrophic failure of the bridge.

TransBay was accustomed to fabricating and transporting heavy materials. This project, however, presented challenges even to this experienced fabricator, in turning thick A-709 HPS GR 70 steel plate, ranging from 65 to 100 mm thickness, into cylinders and cladding the sliding surface with a 316L stainless steel overlay. Caltrans had specified very tight tolerances for the job, and welding specifications had to meet AWS D1.5:1996 requirements for fracture critical material.

Creation of the hinge system began on an ESAB Avenger 2 cutting gantry with a rotating triple torch oxyfuel head that cuts and bevels in one pass. The plate was then rolled and welded into cylinders. Once the “can” was formed, the long seam was tack welded and then moved to welding stations, where automated  SAW welding completed the seam. The steel was pre-heated to 140º C using large natural gas torches. After the internal long seam was welded, the can was then moved to an outside station where the seam was back gouged and ground smooth to remove any impurities and ensure a good quality full penetration weld for the outer seam. The SAW welding procedure included multiple small passes, approximately 30 passes for the inside weld and another 30 for the outside weld. Because heat input is restricted for this material to less than 3.5 joules, the welding parameters were set at 625-650 amps and 34 volts and the machine travel at 770 mm/min. to achieve 3.3 joules/min. heat input. The process used a DC lead arc with an AC trail arc. TransBay used ESAB SpoolArc®^® 95, 5/32”-diameter welding wire and ESAB OK 10.62 flux for this process. SpoolArc® 95 is specially designed for single or multipass welding in applications where strength and impact toughness are required. These filler metals were tested to be sure they met the Caltrans requirement for an H4 hydrogen designator.

After the cylinders were welded, they were re-rolled to meet the 1-3 mm tolerance in roundness. Diaphragm plates were welded inside the sliding side to provide extra support for the stainless steel overlay and stiffening where the beam mounts to the concrete structure of the bridge. The fillet welds on these plates were also performed with the SAW process. The final specifications required 80,000-PSI yield strength, 90,000-PSI tensile strength, and minimum Charpy impact value of 30 ft.-lbs. at -30º C.

Each cylinder was 8 to 9 ft. long. Four to eight cylinders were welded together in two separate sections to form each side of the hinge system. The round seams were processed with the SAW method similar to the long seam.

One of the bigger challenges of the project was the need to apply a stainless steel overlay to the sliding section to facilitate movement and prevent corrosion. ESAB converted some of Trans Bay’s subarc welding equipment with a cladding head that uses a strip electrode 1/16 in. thick and 1-1/2 in wide. TransBay used ESAB stainless steel strip band with an ESAB 10.05 flux designed to produce a self-releasing slag and optimum bead shape. A 309 stainless steel underlay was applied first, followed with a second layer of 316 stainless steel. After the cladding, the cylinders were machined down to a total thickness of 5 mm of stainless steel and sanded to a final surface finish of 8 µm.

At the end of the fabrication process, all welds were examined by NDT methods, ultrasound and X-ray tested. The final weight of a complete hinge system ranged from 75 to 127 tons. More than 100 systems were built for the Skyway portion of the bridge.

“This was a completely new design, never built before,” says Trans Bay President Bill Kavicky of the hinge system. “We were the first fabricator in the world to attempt forming and rolling this particular grade of material in these thicknesses. There was no historical data or documentation when we started. We have since compiled volumes of information on how the material reacts at these thicknesses and diameters.”

For assistance with the welding and cladding operations, TransBay turned to BayArc Welding, their local distributor for ESAB Welding and Cutting Products. TransBay has a long-standing relationship with BayArc, who in turn has an excellent relationship with ESAB. The three companies combined their talents and knowledge in a unique manufacturer-distributor-fabricator partnership to develop welding procedures for the project.

“This job is an example of how things really should happen,” says Rich Deveau, president of BayArc. “Everyone sat down to discuss it. The approach was, ‘How do I do this job, and what’s the best solution for it?’” Deveau distributes exclusively ESAB welding products because he believes ESAB has better solutions than their competitors. “I get excellent support from them,” he notes. “I’ve never had any problems with them backing up their products. If we needed some testing done or samples, they were very easy to work with. ESAB is the total package – equipment, filler metal, fluxes – ESAB makes the whole thing.”

“The support and the relationship between BayArc and ESAB have surpassed anything I could imagine,” Bill Kavicky agrees. “It’s more of a partnership than a customer/vendor relationship.” In a project like this, with very tight tolerance requirements and many new challenges, Kavicky feels the support TransBay received from ESAB was critical to their success. “We were given open access to their weld lab,” he says. “We could try different fluxes, wires and welding processes. All the full support we received is worth a lot.”

For more information on the Bay Bridge project, including videos of the construction challenges, visit www.baybridgeinfo.org.

For a video highlighting the TransBay hinge system project, click here.

View our online guide to seismic-certified filler metals on our Seismic Website.