The men building the Tacoma Narrows Bridge know they are working with history looking over their shoulder. On the bottom of the section of the Puget Sound called the Tacoma Narrows lies the remains of a bridge that became known as Galloping Gertie. Gertie was famous for swaying like a roller coaster in the high winds that whip through the canyon-like sound. On Nov. 7, 1940 (only four months after the bridge opened) the winds became too much for Gertie; harmonic resonance caused the bridge to buckle wildly, snap apart and a large section plunge into the water below.
It took another 10 years for a second bridge to span the water separating Tacoma and the Olympic Peninsula. Now, another bridge is being built to help deal with growing congestion within the Washington State Route 16 corridors. Existing rush hour traffic is substantially greater than the current roadways' estimated capacity with between 85,000 and 90,000 vehicles each day using the corridor, and it has been estimated that the traffic volume will increase to approximately 120,000 vehicles a day by 2020.
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The steel frames on the right and left, welded with Miller's Big Blue engine drive, will split and guide the main bridge cable into 16 small strands.
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"Right now this is a bottleneck," says Frank Moritz, Tacoma Narrows Bridge Project field procurement manager. "The Peninsula is all of sudden being discovered, and rush hour traffic can get to be a stand still. In addition, the lanes on the existing bridge are narrower than the state requirements, further adding to the congestion."
Tacoma Narrows Constructors (TNC) is a joint venture between Bechtel Corporation and Kiewit Pacific. TNC hired Parsons and HNTB to design the $849 million dollar project, with TNC heading up construction. When completed, the Tacoma Narrows Bridge will be 5,400 feet long with two eastbound general-purpose lanes, one high-occupancy vehicle (HOV) lane, two 10 ft shoulders, and a bicycle/pedestrian lane. In addition, the new Tacoma Narrows Bridge will have the structural capacity to carry a light rail line or an extra highway deck underneath.
With a center length span of 2,800 feet between two towers, the Tacoma Narrows Bridge will be the longest suspension bridge built in the United States since the Verrazano Narrows Bridge was opened in 1964 in New York City. Because building a bridge of this size is a massive undertaking, Moritz knew the project needed welding equipment that could meet the demands of the largest bridge project undertaken in the last 40 years.
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A TNC welder uses a Big Blue enine drive to weld the steel frames, a component of the 23,000 ton anchorage for the new Tacoma Narrows Bridge.
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"We lean on Ray Francisco from Pacific Welding Supply, a distributor of Miller welding machines," says Moritz. "This project is like 10 different job sites all rolled into one. You start in the morning by welding 1/4-inch plate and by noon you are welding on a beam that's got a 4-inch flange. This is not like any other welding job where you know ahead of time exactly what you are going to need because not everybody is doing the same type of welding."
Versatile, Flexible and Reliable
TNC needs versatility, flexibility and reliability from the welding machines used on the Tacoma Narrows project. Multi-process diesel engines drives, such as Miller's Trailblazer® 301 D, provide 20 to 300 amps of DC output for Stick, TIG, MIG, and flux Cored welding (100 percent duty cycle at 300 amps, 25 volts for MIG and flux cored). Contractors favor this machine because of its excellent arc characteristics, especially for flux cored welding when paired with a portable wire feeder such as the SuitCase® 12VS. TNC uses the flux cored process for almost all of its welding because it improves productivity by 20 to 45 percent over Stick welding, which helps keep the project on schedule.
In addition to welding output, TNC benefits from the Trailblazer's 10,000 watts of generator power and a unique design that provides separate windings for generator power and welding power. When working on a caisson (the steel and concrete foundations for the towers) the generator power of the Trailblazer 301 D is a necessity.
"There are some days when you are on the water and you do not have the luxury of running a 400 foot extension cord. If you have the Trailblazer 301 D, you can just plug into it and do your work. There is a lot of versatility with that machine, and that is extremely important for this project," says Moritz.
Pacific Welding's Francisco adds that, "The Trailblazer provides strong generator power while welding. Further, because tools don't interfere with the welding arc, the Trailblazer maintains productivity because a fitter can cut or grind while a second person welds."
Sinking The Caissons
TNC used the Trailblazer 301 D to weld the I-beam structure that supports the rebar cages on the two caissons that will support the bridge's two 510-feet high concrete towers. The caissons themselves measure 131 ft. long x 81 ft. wide x 18 ft. high. The caissons have a hollo
w wall and "honeycomb" structure (see illustration) because they are divided into 15 "dredge wells" that are 22 ft. square. Todd Pacific Shipyard of Seattle used 50,000 lbs. of flux cored wired to weld the caissons and thousands of feet of steel bracing and rebar that reinforce its hollow walls. The walls themselves are about 4-1/2 ft. wide. The bottom edges of the external walls taper to an 8-in edge to create a "cutting edge" to cut through the mud at the bottom of the sound.
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This TNC operator, using a Big Blue engine drive paired with a SuitCase 12 VS feeder, fine tunes his flux cored welding arc on scrap plate before welding on the real thing.
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To move the caissons in position, TNC welded aerodomes on the top of each cell so that the caisson resembled an upside down egg carton of giant proportions. The aerodomes contained sufficient air, which enabled the caissons to be floated into position at the Tacoma (east) and Gig Harbor (west) sides of the Narrows. While the caissons were being readied, TNC prepared to hold the caissons in place during construction by burying two sets of 32 anchors (steel plates that are 5 ft. x 8 ft. x 3 in. thick) into the sound's bottom. The anchors and their cables radiate out from the caissons like spokes on a wheel, with a greater number of anchors on the upstream and downstream sides to compensate for 7-knot currents that rip through the Narrows.
TNC tugboats maneuvered the caissons into position and workers then connected the anchor cables and began pouring concrete in 10 ft lifts, welding more I-beams and securing rebar (repeated about 21 times per caisson). By the time the cutting edge touched the sea bottom, the Tacoma caisson was 151 ft tall and the Gig Harbor caisson was 130.5 ft tall and TNC hit its target with the precision of laser guided bomb. "Building the caissons was like erecting the Empire State Building, only you're building the basement last and you have to do it blind," says Moritz.
Once they touched the sea bottom, TNC removed the aerodomes on the caissons' top so that they could begin cutting through about 60 feet of mud until they touched glaciel till. Giant clam shell buckets removed mud from each cell, allowing the caisson to slowly settle. TNC "steered" the caisson by strategically removing mud from specific dredge wells.
Small Footprint
As noted earlier, the walls to the dredge cells are 4-1/2 ft. wide at the base, and the towers narrow as height increases. Space is at premium on the caissons, as well as on the two support barges that flank each caisson. Because of the confined space, which is typical in bridge construction, TNC also benefits from the Trailblazer's compact design; it measures 33-in. x 18.75-in. and is 48-in. in diameter, weighing 560 lbs. (about 130 lbs. less than competitive diesel engine drives). To increase portability, the Trailblazer 301 D comes with an eye lift and optional running gear.
"Because the caissons do not allow for a big machine, the Trailblazer 301 Ds worked perfectly," says Moritz. "You can't drag a big welder behind you on the caissons, it's just impossible."
The Anchors
Work on the shores, where TNC is building the anchorages, is less constricting. For welding in these areas, such as on the anchor frames, Pacific Welding rented Big Blue® engine drives with a 500-amp output at 100 percent duty cycle and 20 kW of generator power available when not welding. While all of this output wasn't necessary for the bridge project, Pacific Welding wanted to introduce Moritz to this product for future jobs that need a higher welding or generator output.
The Tacoma Narrows' conventional gravity anchorages consist of 23,000 cubic yards of concrete each, with each welded anchor frame guiding a portion of the 21-in. diameter main cable, which has been split into 19 separate strands.
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TNC used up to eight Trailblazer 301 D engine drives for welding and generator power on the caissons for the Tacoma Narrows Bridge.
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"Those 19 strands will go through the 8-in. thick steel of the anchor frames and that's where we will get the total tension for the entire bridge, some 25 million pounds per cable. Once they have those 19 strands of cable at the correct tension they'll come in and they'll grout and seal it. Each anchor frame will be sealed in 23,000 cubic yards of concrete, so the fancy welding we do is buried and will never be seen again," says Moritz.
Local Quality and Environmentally Sound
"One of our main goals was to use local vendors whenever possible, without sacrificing quality," says Moritz. "Pacific Welding is a perfect fit with our needs because they are just a phone call away." Responsiveness to the projects' needs is a must and Pacific Welding provides quality support although short lead times and "just-in-time" requests are the norm. Another goal of the project is to be environmentally sound in their bridge building practices. "We have neighbors right next to the construction site and we actually border their backyard. It is absolutely imperative that we are environmentally friendly at all times," says Moritz. With the Sound being one of the most beautiful areas in the United States, all members of the team are aware of the environmental repercussions that can befall the project.
To this day, the old Galloping Gertie is still underwater as a constant reminder to the crew of the consequences involved in bridge building. The crew goes to great lengths not to disturb it while they are below the surface working. Galloping Gertie was declared a National Historic Site in 1992 to prevent scavengers from diving down and taking the metal used to make the bridge. There is no doubt that with the quality care involved in the construction of the new Tacoma Narrows Bridge (to be complete in 2007 with upgrades to the existing bridge to than take nine months and the project completed in 2008) Gertie will have a friend standing watch and protecting her for decades to come.