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Washington Bridge Narrows Gap Over Sound

Tue November 14, 2006 - West Edition
Rebecca Ragain



Four years after groundbreaking, the new Tacoma Narrows Bridge in western Washington has begun the fifth and final stage of construction.

The new bridge is scheduled to open in the summer of 2007. At 5,400 ft. (1,646 m) in length, it is the longest suspension bridge to be built in the United States since the Verrazano-Narrows Bridge in New York, which opened in 1964.

The new bridge will be the third suspension bridge to span the Tacoma Narrows, stretching across the Puget Sound between the cities of Gig Harbor and Tacoma. The first was the infamous “Galloping Gertie,” which both opened and collapsed in 1940.

The second bridge, built in 1950, still stands. In fact, the new Tacoma Narrows Bridge runs parallel to the existing Narrows Bridge. When the project is completed, westbound traffic will travel across the 1950s bridge in three lanes, while eastbound vehicles traverse the new bridge, which also has three lanes, plus a separate bike/pedestrian path.

Together, the two bridges will serve the estimated 90,000 commuters who cross each day, heading along State Route 16 toward Interstate 5 and Tacoma, Olympia, Seattle and other cities.

The $849-million project is the Washington State Department of Transportation’s first foray into design-build contracting, said Washington State Department of Transportation (WSDOT) Media Relations Manager, Claudia Cornish. The project contractor, Tacoma Narrows Constructors (TNC), is a joint venture between Kiewit Pacific and Bechtel Infrastructure.

The new Tacoma Narrows Bridge is publicly funded, with $800 million coming from municipal bonds; the state of Washington will retain ownership of the completed bridge. Tolls collected on eastbound traffic will repay the bondholders.

According to Cornish, the new bridge represents the largest single transportation project that WSDOT has ever undertaken. (Though there have been larger compilation projects, such as finishing Interstate 90.)

To get a sense of the project’s scale, consider the substructure alone: Each caisson is made of 6 million lbs. (2,722 metric tons) of steel and approximately 40,000 cu. yds. (30,584 cu m) of concrete. Each anchorage consists of 20,000 cu. yds. (15,292 cu m) of concrete and 1 million lbs. (454 metric tons) of reinforcing steel, and weighs in at 81 million lbs. (36,742 metric tons).

In addition to the superstructure and substructure, the design-build agreement specifies 2.5 mi. of roadway approach work. The work, which began in January 2003, has been performed with equipment including Cat 966 and 962 loaders, Cat D7 and D5 dozers, the Cat 345 excavator and the Case 1163 roller.

Additional changes also are being made to the SR 16 roadway, totaling 3.4 mi. of improvements. Next summer, crews will overlay the entire highway.

Though necessary, the road work is not the most impressive aspect of the project. More remarkable is the fact that both caissons were set into position on the first try.

“They were extremely accurate in their survey,” commented Cornish.

The concrete work also was unusual in a number of ways.

For one, the project required such enormous amounts of concrete that the contractor for that portion of the job, Glacier Northwest, purchased a portable batch plant in order to meet the demand.

“[The project] was such high volume and high profile … and we didn’t have one that fit for what we needed to do there,” said Mark Leatham, who was the general manager of concrete operations for Glacier Northwest’s Washington division during the Tacoma Narrows Bridge contract.

This portable plant was capable of producing between 195 and 250 cu. yds. (149 and 191 cu m) of concrete per hour, which it did day and night during peak construction. All told, Glacier Northwest produced more than 169,000 cu. yds. (130,000 cu m) over the course of its two-and-a-half year contract.

Leatham pointed out that the type of concrete used was not “garden variety,” but rather an extremely dense, corrosion-resistant material including microsilica.

Also notable was the process used to build the bridge’s towers, which reached a height of 510 ft. (155 m) above sea level.

TNC was able to use the existing 1950s bridge as a staging area, pumping the concrete horizontally through 1,205 ft. (365 m) of slick lines, down 21 flights of stairs to the caisson of the existing bridge, then across to two booms on a placing barge.

As the towers rose, the booms did not reach the required height so cranes were used to lift buckets of concrete to the top. The cranes — 610 ft. (186 m) tall Tower Cranes Liebherr 550 and Potain — were raised three times to accommodate the height of the towers.

By June 2005, the towers were complete. In July, the fourth phase of the project began as workers hauled the first cable wire, the 0.625-in. (15.9 mm) pilot line, from the anchorages to the top of the towers.

The cable crews then spun 19 steel bundles, with 464 wires per bundle, to make up the new bridge’s suspension cables. They also installed 264 pairs of suspender cables to each main cable in preparation for deck assembly.

While watching the cable spinning, Tacoma Narrows Bridge project engineer Dennis Engel, of WSDOT, was struck by the realization that this mechanical process has essentially stayed the same since the Golden Gate Bridge was built.

“We like to think technology has changed considerably, but it really hasn’t … Our [spinning wheels] are more tightly controlled [by computers] but it is the same basic concept,” said Engel.

Cable spinning continued through April 2006. In May, the cargo ship Swan set out from South Korea with the first 16 of 46 total deck sections. The Swan anchored under the west-side span on June 29. On Aug. 7, the first deck section — 116 ft. (35 m) long and weighing 488 tons — was lifted to deck height.

Most of the bridge decks were lifted twice. The first lift used trans-ship gantries to lift the section from the cargo ship and lower it onto a barge.

The barge then ferried the section to its proper place along the main suspension cable, where another gantry system lifted the deck section to bridge elevation using vertical suspender cables. Then the section was connected to the main cables of the bridge.

Gantry cranes, designed by Nippon Steel Kawada Bridge and built by TNC, performed the lifts, which Cornish described this system as “fairly complex.”

The side-span gantries’ lifting mechanisms were winches located in the caissons. They lifted at the rate of approximately 10 ft. (3 m) per minute. On the other hand, the mid-span gantries operated by using strand jacks located on the gantries’ main girders. Their lifting rate was approximately .8 ft. (.2 m) per minute.

By the end of September, all the deck sections had been lifted from the Swan, and the ship was returning to South Korea to take on the final 15 sections. In the meantime, the Swan’s sister ship, the Teal, arrived with the second batch of sections.

Before the second batch of sections could be unloaded, a broken winch line forced the return of the Teal to Commencement Bay in Tacoma, where a replacement line was installed.

This was just one of many unforeseen circumstances that occurred during this years-long project.

“It [the broken winch line] was just one of the things that happened that we just have to solve,” said Cornish. “On a job this size there are tremendous challenges. Our engineers are solving problems every day.”

On the whole, the biggest hurdles to the construction process have been environmental, not mechanical. For instance, the Narrows experiences occasionally tidal swings up to 17 ft. and has currents running approximately 7.5 knots.

Cornish pointed out: “Most places you build a bridge you have water flowing in one direction. Here you have tidal swings four times a day.”

“The great challenges have been environmental: wind, currents, rain,” agreed Erin Hunter, TNC’s Public Affairs Manager, replying by e-mail.

Indeed, weather posed challenges during many of the project’s stages to-date, from setting the caissons to lifting the deck sections. The same will be true for the remaining steps.

Once deck assembly is finished in early 2007, the crews will work on the deck surfaces, a process that also depends on the weather. The final work on the cables — horizontal wrapping to bind the bundles of wires that make up the main suspension cables — is also sensitive to the elements. Finally, everything will need to be painted, which is a difficult task in pouring rain.

The opening of the new bridge this summer will be followed by 11 months of work on the 1950s bridge, in order to bring it up to current earthquake code. By early 2008, the entire project should be complete.

As of the first week of October, the new bridge was running approximately three months behind the original schedule included in the contract.

“We feel like that’s pretty darn good for a project this size,” said Cornish.

Engel added: “It’s exciting to see it all come together. Every day something changes out there.” CEG