October 9, 2007
Protecting a two-mile-long, 10-lane bridge from earthquake damage is not an easy task. Replacing the eastern span of the San Francisco-Oakland Bay Bridge requires the talent and time of numerous design firms and fabricators and tens of thousands of workers. One of the more interesting fabrication challenges came from the design of a new seismic protection device. Fabricator Trans Bay Steel Corp. relied on a unique partnership with its welding equipment distributor, Bayarc Welding Supplies & Solutions, and a welding product manufacturer, ESAB Welding & Cutting Products, to develop a process for fabricating this 6.5-foot-diameter hinge system.
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 Department of Transportation, determined that while the western side of the bridge could be retrofitted to withstand another quake, the 1930s-era eastern span needed to be replaced. The construction project was awarded to KFM, a joint-venture partnership of three construction firms: Kiewit Pacific of Vancouver, Wash.; FCI Constructors Northern Division of Benicia, Calif.; and Manson Construction Co. of Seattle, Wash.
Designed as a series of bridges, the 2.18-mile span includes some of the largest and heaviest components ever used in bridge construction and is expected to be able to withstand an 8.1-magnitude quake. One key component is a state-of-the-art hinge beam system installed at the expansion joints of the precast concrete deck segments. These hinge beams are designed to allow expansion and contraction caused by temperature changes and other minor motions. One end is fixed, and the other end has a sliding stainless steel-clad bearing surface. The bridge also has a replaceable "fuse" section, designed to shear in the event of an earthquake. This section of the hinge is similar to an electrical fuse—it is intended to absorb the stress and fail, thereby saving the bridge.
The hinge system was subcontracted to Trans Bay Steel Corp., a steel fabricator with a 250,000-square-foot facility in Napa, Calif. With a 225-ton overhead crane capacity and direct access to the San Francisco Bay, Trans Bay is accustomed to fabricating and transporting heavy materials. Fabricating this project—transforming 2.56- to 3.94-inch-thick A-709 HPS Gr. 70 steel plate into cylinders and cladding the sliding surface with a 316L stainless steel overlay—presented challenges even to this experienced fabricator. Caltrans specified tight tolerances for the job, and welding specifications had to meet AWS D1.5:1996 requirements for fracture-critical material.
"This was a completely new design, never built before," said Trans Bay President Bill Kavicky. "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. 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, Trans Bay turned to Bayarc Welding, its distributor for ESAB Welding & Cutting Products. Trans Bay has a long-standing relationship with Bayarc, which 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.
The fabrication process for the seismic hinge system began on an ESAB Avenger 2 cutting gantry. Kavicky worked closely with the engineers at ESAB Cutting Systems to design a special rotating, triple-torch oxyfuel bevel cutting head. The triple-torch head is designed to prepare cut surfaces for welding. The edge cut can be contoured to the exact shape needed for a good weld, eliminating the need for some secondary operations and reducing production time in the cutting of heavy plate. The vertical edge torch can be controlled automatically through the CNC. The head rotates 360 degrees to the direction of travel and can cut circles with a triple profile.
"We do our burning a little bit differently from other fabricators," Kavicky explained. "When cutting plate that needs to be shaped or rolled, most current applications will have the plate stripped on both sides at the same time and then cross-cut. With the Avenger and the rotating, triple-head torch, we can cut and bevel-prep the plate in a rectangular shape all in one cut. This machine has given us the accuracy to cut diagonals within 1⁄16 inch over a distance of 20 feet, and 1⁄32 inch or tighter on smaller elements. This means that down the road, when we're rolling the plate into a cylinder, the long seam makeup on the cylinder comes right up flush, and we get a good fit. This also means that when we go to fit the ends of the can together with the next cylinder to make a round seam, they butt squarely to each other and we are able to fit with very tight tolerances."
The Avenger provides cutting accuracies within the 1⁄32-in. to 1⁄8-in. range. "When the diagonal dimensions are this close, that means your width stretch-out and length stretch-out are almost perfect," Kavicky said.
After the plate is cut and inspected, it is formed on a Davi plate forming machine that was purchased specifically for this project. The machine is believed to be the largest forming roll on the West Coast, with capabilities to form A36 and A572 Gr. 50 steel 6 in. thick and 10 ft. wide. The specialized set of rolls can operate in both pinch and pyramid style and can also serve as a press brake. The machine must overcome 80,000 pounds per square inch (PSI) to form the plate into a cylinder.
Once the "can" is formed, the long seam is tack-welded and then moved to welding stations, where automated submerged arc welding (SAW) completes the seam. The steel first is heated to 140 degrees C using large natural gas torches. The internal long seam is welded; the can is then moved to an outside station where the seam is back-gouged to sound metal and ground smooth to remove any impurities and ensure a good-quality, full-penetration weld for the outer seam.
The SAW procedure includes dozens of 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 per minute, the welding parameters are set at 625-650 amps and 34 volts, and the machine travel is 30.31 IPM to achieve 3.3 J per min. of heat input. This process uses a DC lead arc with an AC trail arc. Trans Bay currently uses ESAB SpoolArc® 95, 5⁄32-in.-dia. welding wire and ESAB OK 10.62 flux for this process. SpoolArc 95 is designed for single- or multipass welding in applications that require strength and impact toughness.
After the cylinders are welded, they are rerolled to meet the 0.04-in. to 0.12-in. tolerance in roundness. Diaphragm plates are 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 are also performed with the SAW process. The final specifications require 80,000-PSI yield strength, 90,000-PSI tensile strength, and minimum Charpy impact value of 30 foot-pounds at -30 degrees C.
Throughout the fabrication process, every weld is inspected visually and with a nondestructive test method, such as magnetic particle or ultrasonic. Inspectors from Trans Bay and Caltrans also frequently check the wire, flux, and the welding process to ensure that all elements are in place to meet the exacting specifications of the welding codes.
After the postweld treatment, the cylinders are moved to a large lathe to be squared off to ensure that they are perfectly perpendicular to the center axis. The largest lathe on the West Coast, the Betts lathe is 90 ft. long, has a 120-in. swing, a capacity of 200 tons between centers, and a 200-HP motor.
Each cylinder is 8 to 9 ft. long. Four to eight cylinders are welded together in two separate sections to form each side of the hinge system. The round seams are processed with the SAW method similar to the long seam. A fuse then is added between the two sections and the closure seam is made. The fuse is made from A572-Gr. 50 material with a thinner wall and is meant to fail during a serious seismic event to relieve the strain on the bridge structure.
One of the bigger challenges of the project was applying a stainless steel overlay to the sliding section to facilitate movement and prevent corrosion. ESAB suggested converting some SAW equipment for cladding. An automated A6 welding head was converted to deliver a strip of cladding material 2.56 in. wide. The modular system was converted from wire welding to cladding by changing the drive feed mechanism and reworking the programming on the controller. Instead of welding wire, the cladding head welds a strip electrode 1⁄16 in. thick and 11⁄2 in. wide to the metal surface. This is done with an ESAB LAF 1250 power source, designed to work with the A6 head to produce good arc stability at high or low voltages. The variable-voltage control permits precise adjustment of the welding parameters.
Trans Bay uses ESAB stainless steel strip band with a carbon component of 2 percent, with 20 percent chromium and 23 percent nickel additives. It also uses 10.05 flux. Trans Bay purchased 120,000 lbs. of strip and 100,000 lbs. of flux for this project.
First, the cylinders are machined and ground to establish the starting dimension, and the metal is heated. Although Caltrans requested a 316 stainless steel overlay, ESAB recommended adding a 309 stainless steel underlay first. This bonds directly to the underlying steel. A layer of 316 stainless steel then is applied to the 309 stainless steel. Trans Bay operates the system at 27 V and 770-780 amps to create each 0.014-in.-thick layer. After the cladding, the cylinders are machined down to a total thickness of 0.20 in. of stainless steel and sanded to a final surface finish of 8 µm. The cladding operation runs continuously 24 hours a day for five days to apply the two layers, each 126 in. long at two separate and simultaneous locations.
At the end of the fabrication process, all welds are examined by nondestructive methods, ultrasound and X-ray. All but the overlay section is sandblasted and painted. The entire system then receives a final visual and dimensional check. The tolerances of the pipe hinge beam are measured from the centerline as it is passed through the entire length of the beam. In reference to this centerline, each cylinder must be within 0.20 in. of roundness and the centerline within 0.04 in. of each stainless steel overlay. Furthermore, the overlay surfaces must be within 0.04 in. of each other and the fixed section within 0.12 in. of the centerline where the restraint brackets mount.
Because of these tight tolerances, Trans Bay was concerned that vibration during transport by truck or rail could change the dimensions. Fortunately, Trans Bay has water access, so it chose to transport them by barge to the bridge, ensuring smooth, easy handling and accurate dimensions on delivery.
Like most of the components of the new bridge, the final component is massive; a complete hinge system weighs from 75 to 127 tons. When complete, it will be a hallmark in California bridge construction. When finished in 2012, the new eastern span of the San Francisco-Oakland Bay Bridge will be more than a safer-than-ever bridge. It will represent an unprecedented combination of American ingenuity and capabilities, a testament to the achievements of the fabrication industry.
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