February 2012

Electroslag Welding on the New San Francisco/Oakland Bay Bridge

The narrow gap improved electroslag welding process was used to make 20 welds in the new San Francisco/Oakland Bay Bridge tower base. Ironworkers from Local 377 (San Francisco) and Local 378 (Oakland) completed the 20 welds in June and July 2011. Each single-pass weld was 10 meters (32.8 feet) high and between 100 mm and 60 mm (3.9 and 2.4 in) thick. There were five weld geometries including transition butt joints and tees. Each weld took about four and one half hours to make.

Construction is continuing on a new bridge that crosses the San Francisco Bay between Oakland and Yerba Buena Island. The new bridge is located just north of the existing bridge, which was built in 1936. The old bridge will be removed after the new bridge opens in 2013. The new bridge will consist of the skyway, a self anchored suspension (SAS) and transitions on the east and west end to Oakland and Yerba Buena Island respectively. The SAS part of the bridge is located on the east side of Yerba Buena Island and the supporting tower is approximately 50 m from the island’s east edge. The cables of a self-anchored suspension bridge are not anchored on adjoining land masses, but to the roadway itself. The roadway of the SAS will be 1870 feet (570 m) long.1

Though currently supported by temporary ironwork, the SAS roadway will be supported by a single, four-legged tower. The tower legs were erected in a series of strand jack lifts that positioned five segments on each of the four legs. The total height of the tower will be 525 feet. This welding project involved welding the first ten meters of the tower legs into a single unit after being positioned on the footing. A gang of ironworkers made these welds over about two months in June and July of 2011 using the narrow gap improved electroslag process. These welds are longer than any other known electroslag welds.

Electroslag Welding Process
The electroslag welding (ESW) process is a resistance (non-arcing) welding process that uses the electrical resistance of the slag pool to generate heat that melts the welding electrode and the plates to be joined. (Fig. 2) The welds are single pass welds regardless of plate thickness, with documented welds greater than a foot thick being made in one pass. The molten slag and metal are contained by water-cooled copper shoes that are moved up the joint as the weld progresses from the start at the bottom to the finish at the top. Small amounts of welding flux are added to replace slag that solidifies against the copper shoes. Welding electrode (wire) is fed into the top of the joint and prevented from contacting the plates by a consumable guide that also melts and also becomes part of the weld along with the electrode. Power is supplied by standard DC welding power supplies and specially designed welder controllers and instrumentation.

The root gap was nominally ¾” (19 mm for every plate thickness and joint type and all of the welds were 10 meters high.

The ESW technology (equipment and training) was provided by Electroslag Systems (EST&D) in Portland, Oregon as part of a development project with Portland State University and American Bridge Company.

The American Bridge/Fluor Ironworker gang consisting of Dan Ieraci (welding superintendent), Rory Hogan (foreman), Alex Blanco, Devan Murphy, Jeremy Dolman, Rich Garcia, Jeff Stone and Jeff Souza made the 20 ESW welds. The gang performed both the set-up and actual running of all the welds. (Fig. 3)

The training for the ESW welds was conducted while making the Procedure Qualification Record welds (PQRs). The gang practiced the operation and made a total of six PQR welds 8 foot tall to qualify the men and procedures and to hone skills on set up, cooling shoe manipulation and flux feeder operation. Slag leak drills were conducted to familiarize the gang with causes and corrective action. Operation of all the equipment including welders, controllers, data recorders, water chillers, cooling water flow control panel, wire electrode feeders and cooling shoe movement was reviewed. In addition to the eight foot high PQRs, one full-sized mock up weld (80 — 100 mm transition weld, 10 m high) was made in the American Bridge/Fluor yard in Oakland after the welding equipment had been shipped to the site.

Weld Set-up
Setting up to make the weld included the installation of access equipment such as ladders and fall protection welded to the side of the tower along the entire length of the weld. Cooling shoe backing beams against which the cooling shoe clamps jacked had to be precisely placed at correct angle and distance from the weld. The weld joint had to be cleaned of as much dirt and rust as possible, ideally down to shiny steel. The start of each weld required a specially fitted start sump assembly, which was different for each weld. The steel for the sump had to be custom fitted and installed with enough precision to prevent the leakage of molten slag metal during welding. Access to many of the welds was difficult due to limited space between massive steel sections in the tower base. (Fig. 4)

Due to the length of the welds, there was an inordinate amount of welding lead and water hoses. The welding lead delivered the welding power to the consumable guide and grounding attachments. Water hoses moved cooling water from a water chiller to the copper cooling shoes and back to the chiller. All of this equipment had to be moved and repositioned for each weld.
Other equipment that had to be positioned were a weld control station, two water chillers, an electrical distribution panel, welding electrode drive station and three direct current welding power supplies. Each weld required a single consumable guide. Consumable guides are ¼” thick, the appropriate width for the joint and 35 feet long. They were stored in a sealed container attached to the side of the bridge tower and were extracted from the container as one of the last steps in the set-up. They were then precisely positioned in the weld joint. The ironworkers installed insulators every six inches for the entire length of the consumable guide to prevent it from contacting the base plates and shorting the welding power.

After the guide was in place, the cooling shoes, three on either side of the joint, were clamped against the each side of the joint.

Actual welding is initiated after placing a measured amount of starting flux in the joint. The initial flux charge is followed by applying voltage to the consumable guide and starting the wire electrode. After the electrode contacts the starting sump base and electrical current is flowing, an Ironworker adds the remaining starting flux at a carefully controlled rate to reach steady state welding conditions as rapidly as possible without over whelming the power supplies. Constant communication is required between the control station (at the top of the weld) and the weld start (at the bottom of the weld) which were separated by the approximately 3-story height difference.

After the weld and slag pool have been established, the wire electrode, which is continuously fed into the guide, is melted and fills the joint. As the weld pool progresses up the joint, the bottom copper cooling shoe is removed and placed above the highest shoe, thereby leap-frogging the shoes ahead of the weld pool. (Figs. 5a, 5b, 5c.) Again, access to handle and place the up to 40-pound shoes in exact position was a challenge. The ironworkers needed to constantly organize cooling hoses and leads to avoid weld-stopping tangles.

As could be expected in such a massive structure, there was the occasional occurrence of less than ideal joint fit. Adjustments had to be made by the ironworkers to minimize slag pool consumption and adjust for occasional slag leaks. These adjustments required split-second decisions on cooling shoe placement, shoe movement, leak mitigation and slag pool replenishment all while hanging up to 30 feet above the tower base. Of the 20 individual weld and 200 meters of total weld length, there was only one instance of weld that stopped between start and completion. That weld was subsequently set up and completed without incident the same day.

The ironworker at the control station had to monitor and perform several different tasks simultaneously. During weld start-up, the ironworker had to energize the welding circuit, start and control electrode speeds, report welding current to the ironworker adding flux and communicate to the whole gang appropriate data so that each ironworker could respond accordingly.

After the weld was running in a steady-state mode, the ironworker managing the weld had to constantly monitor the data recorder. Adjustments in wire speed and flux addition rate were necessary to retain ideal welding parameters and therefore weld properties. If corrective action was required, for example compensating for a slag leak, the ironworker had to make adjustments at the control station and coordinate additional corrective action at the weld. Important information that was constantly exchanged with radios included welding speed, audible weld characteristics, joint geometry (gap and alignment) and cooling shoe movement.

All twenty welds were successfully completed in approximately two months. A typical weld after slag removal (#11, an 80 to 100 mm transition weld) and corresponding cross section are shown in Figs. 7 & 8. The welds require 100% Ultrasonic Inspection (UT) and 10% X-ray (RT) where possible. Many of the welds have been inspected with UT; there is some minor surface (visual) repair work. However, compared to the time and repair work that would be required if the welds were made by another process, the ESWs made by the gang saved untold time and effort.

The twenty 10 meter high welds made by the ironworkers in the new San Francisco/Oakland Bay Bridge Tower required extraordinary professionalism and skill. Welds of this magnitude had never been attempted prior to this project and the gang was able to safely complete the welds with a minimum of repair work.

EST&D would like to express its gratitude to the ironworkers for their excellent skills and dedication. Appreciation is also gratefully acknowledged to the rest of the American Bridge/Fluor engineering and support groups, especially American Bridge/Fluor employees John Callaghan, PE project manager, Jim Bowers, welding quality control manager, and Daniel Hester, senior field engineer. Finally, personnel from CalTrans and Smith Emery Inspection Service were also invaluable in the successful completion of the project.

1. baybridgeinfo.org
2. Zeyher, A., All the State’s Horses, Roads and Bridges, Vol. 45, No. 5, May 2007, pp. 26–30.