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Construction Close-Up: The Skyway Pile Head Welding Turning Up the Heat in Close Quarters
The process is called pile head welding, and it's a critical part of building a strong, resilient bridge foundation. For the welders, it's a challenging task conducted in blazing temperatures and close confines deep within the cofferdams. For the set-up crews working in the footing boxes in advance of the welders, the work is less sweltry but no less demanding.
Here's how it's done:
Before welding begins, the driven piles are cut off about 12 inches above the top of the footing box. The boxes are chambered steel structures the size of a small house that have been pinned to the Bay bottom by the 8-foot-diameter piles driven 250 feet or more through sleeves in the box. Most Skyway piers call for six piles; four piers will require only four.
Working in advance of day and night welding teams, the 10-man set-up crew enters the box and cuts slots into each pile where a 2-inch thick, 5 1/2-feet-tall, 2-feet-wide steel "shear plate" is vertically inserted. A counterpart slot has already been cut in the foundation box by its manufacturer. Once the pile slots are cut, the set-up crew uses a pulley system to move the 900-pound plate into position, bridging a 1-foot gap between the pile and its footing box sleeve. The top of the plate rests five and a half feet below the top of the box.
Enter the pile head welders tasked with welding the shear plates to lock each pile to its footing box sleeve. They have their work cut out for them. In all, the Skyway requires 1,280 plates and 5,120 plate welds. Each weld requires 18 pounds of metal.
Although light, power, ventilation and the surrounding cofferdam create a safe, serviceable working area dozens of feet below the water line, welders must ply their craft in temperatures reaching 150 degrees. The roasting environment is caused by heating strips used to raise the temperature of the pile and foundation box surface to 200 degrees in order to reduce metal stress.
In this cramped, hot setting, welders must wear heavy clothing to guard against burns. They also wear sunscreen to protect exposed skin from the UV rays emitted by the intense welding light. But stocked with enough fluids, trained in safety, hardened by countless hours in the trade, the welders are accustomed to tough conditions.
Two shifts of 12 welders work day and night on the pile head welds. Each pile requires eight plates; each plate requires four welds - two inside the pile and two on the foundation box. The inside pile welds require the welder to work on a platform suspended from a protective gazebo placed atop each pile. The platform stands about three feet below the bottom of the shear plate and about two feet above the water inside the pile.
The water is pumped into the pile to avert a 300-foot void between the work area and the bottom of the pile, which is plugged with concrete.
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Unlike the horizontal pile splicing welds, the head welds are vertical. The first few passes are done by hand; the rest are performed with the aid of a machine that runs on a track to lay down precise beads. Twenty-five passes are required for each weld. On any pile, most of the sleeve welds are done first to accommodate any slight movement of the pile; the welds on the pile side of the plate then follow.
The finished welds must pass close inspection to ensure they meet specifications. When all the welds at any one pier are complete, the gap between the pile and the footing box sleeve is filled with a special concrete grout. The entire foundation box is then filled and topped with concrete in preparation for the next stage of the work-building the pier columns.