By: Roads & Bridges
Welding plays a major role in constructing the east span, called the skyway section, of the San Franciso-Oakland Bay bridge, which consists of roadway sections supported by 28 concrete and steel piers.
The substructure of the skyway consists of three major components: large tubular steel piles that are driven deep through the Bay mud, footings (the steel and concrete foundations through which the piles are driven) and concrete piers built atop the piles to support the roadway.
The piles themselves consist of large pipes formed of steel 2 to 3 in. thick. They are driven in two sections, and as each section is added, a circumferential weld joins it to the pile section already driven. Later, the piles are joined to the foundation box with perpendicular pile head connection plates that extend from slots in the sleeves through slots cut in the piles and are welded in place. These plates are installed at eight points around the circumference of the pile.
Welding the pile sections together while they are being placed posed several challenges. Because of the material thickness, more than 50 passes per joint were required, and these must meet stringent California Department of Transportation (Caltrans) bridge-welding specifications. Modular automated welding systems were selected to achieve the necessary weld quality and productivity within these and other constraints.
Each of the 12 Bug-O systems used for this part of the project imparts a programmed motion to the welding torch that replicates the hand motion of a skilled welder. The system is built around a modular drive system that carries the welding torch. Together with a carriage and control module, the unit is moved along a rail by a rack-and-pinion drive. For the pile welds, the rail is curved to match the outside diameter of the pile to which it is held magnetically.
A major key to the success of this application is a Pendulum Weaver and Weaver Control Module that adds a weaving motion to the modular drive system. A high-torque, low-inertia motor provides precise starts and stops, and the unit’s high-speed pendulum motion includes independent control of right and left dwell times. Together, the main and pendulum weaver motors drive the automated welding head through a combination of X and Y coordinates in a complex motion that replicates the movement of a skilled welder’s hand. In effect, the pendulum weaver wiggles back and forth as it moves along the joint. The added motion is combined with an intermittent drive operation to successfully weld consistently high-quality joints with the Dual Shield 1?16-in. wire.
Two complete drive systems are mounted on each circular rail, one welding counterclockwise from 6 o’clock to 12 o’clock, while the other welds clockwise between the same points. Operation is staggered, with one unit starting about 18 in. ahead of the other. When the first unit reaches the end of its travel, the operator puts it into a free-wheeling mode and rolls it back to the starting position, allowing the second unit to travel beyond 180°. Because the cycles are phased, the welds overlap to provide the necessary continuity.
While based on standard components, the Bug-O systems used for welding the piles were adapted to the special requirements of the job. Normally, the units are not directional, so they had to be converted to right-hand and left-hand configurations. This also entailed moving the control panel mountings so they would be on top of both units.
Another change was the addition of two switches that allow the operator to turn the travel on and off and to control the weld contact from the pendant. These override the switches on the main control panel so the operator can control the process while watching it closely without having to reach up to the control panel.
Weld prep for the circumferential welds includes a 45° bevel on the upper edge of the joint and a backup bar, rolled to shape and welded inside the pile. The lower edge of the joint (upper edge of the pile) is left flat.
Welds are full-penetration circumferential FCAW butt welds, made with Dual Shield wire, using Lincoln DC600 welders with LN 25 wire feeders.
The actual welding is done inside kiosk-type shelters that provide wind and weather protection to help control weld quality and maintain productivity. The shelters are moved from one piling to another as the job progresses.
Once the piles are in place, Bug-O programmable shape cutters are used to cut slots for splice plates. Slots approximately 70 in. high and 23?4 in. wide are cut at eight locations around the pile’s circumference to accommodate plates that are welded to connect the foundation box and pile together.
The Bug-O programmable shape cutter is a two-axis machine that runs on a track similar to the welding units and carries a torch on a motorized cross arm. Six of the compact, lightweight machines are used in this operation. They are easy to move from location to location, and their memory can be programmed to cut up to 10 different shapes.
As with the automated welding systems, the shape cutters also required some special modifications for this application. The rail magnets were modified to fit on the radius of the piling so that the rail and shape cutter were parallel to the cut, and the cross arm was shortened to fit in a confined space. All shapes are preprogrammed in the office, and the operator simply uses a dummy pendant to call up the shape needed at the time.
