What We Build

September/October 2007

A Tale of Two Bridges

New 5,400-ft-long suspension bridge joins existing span across Tacoma Narrows in Puget Sound

By Sheila Bacon

The new $849-million Tacoma Narrows Bridge joins an existing span built in 1950. It is the longest suspension bridge built in the United States since the Verrazano-Narrows Bridge opened in New York in 1964.

Suspension bridge design and construction has changed surprisingly little since the first such spans crossed rivers, straits and canyons. Even today, the design of foundations, construction of towers and techniques used to string suspension cable follow much the same course as earliest efforts.

Designers and builders of the new $849-million Tacoma Narrows Bridge in Tacoma, Wash., however, had the added benefit of computer technology. Global positioning systems helped crews place caissons and bridge decks with pinpoint accuracy in the fast-moving currents and dramatic tidal swings.

The new 5,400-ft-long, design-build toll bridge opened to traffic July 16. It is located parallel to and immediately south of a similarly sized existing bridge built in 1950. The bridges connect Tacoma with the city of Gig Harbor and the Kitsap Peninsula across the mile-wide Narrows of Puget Sound.

The original Tacoma Narrows Bridge will remain in use, bolstered by a 10-month renovation that will bring it up to current seismic codes. The existing bridge will accommodate westbound traffic while the new span will carry vehicles eastbound toward Tacoma. The Washington State Dept. of Transportation estimates that 85,000 to 90,000 vehicles cross the Narrows on a daily basis.

Swift currents and ever-changing tides challenged the construction of the bridge's caissons, which anchor the underwater foundation of the span’s two 510-ft towers..

Going with the Flow

The bridge was built by Tacoma Narrows Constructors, a joint venture between AGC of Washington member firm Kiewit Pacific, Vancouver, Wash., and Bechtel Infrastructure, San Francisco. It is the longest suspension bridge built in the U.S. since the Verrazano-Narrows Bridge opened in New York in 1964.

Placing the caissons, which form the underwater foundation of the new span’s two 510-ft-high towers, in the seabed was arguably the most difficult task in the bridge’s construction.

The concrete “X” design on the new bridge’s struts, similar to those on the existing bridge, is purely aesthetic. It is the only part of its design that resembles the existing bridge..

Bridge caissons are commonly built in rivers where crews know which direction the water is flowing, says Tacoma Narrows Constructors project manager Manuel Rondon. But in Puget Sound, crews had to adjust for daily changes in the tides as well as unexpected weather conditions that affect the intensity of the flow.

The close proximity of the second tower less than 100 ft away from the new caissons meant crews had to be continually on their toes. “In these tidal conditions, 60 ft is really like 5 mm,” Rondon says.

Three-story “birdcages” were used during concrete pours on the towers to enhance safety and hide workers from the traffic below..

Caisson placement was simplified with the use of laser surveying technology and GPS software that allowed precise positioning, says Jeff Carpenter, WSDOT’s Tacoma Narrows Bridge project director. Once each caisson was floated out from the Port of Tacoma to its final location, it was secured with 32 anchor lines while crews poured concrete and the caisson was sunk lower into the water.

Each line was attached to a sensor that was monitored 24 hours a day, seven days a week so crews could adjust the tension in response to the 7-knot currents and 15-ft tidal swings that cycled twice each day, says Claudia Cornish, WSDOT media relations manager.

Barges outfitted with GPS units also helped crews during bridge deck placement. Each of the 46 deck sections arrived via cargo ship from bridge prefabricator Nippon Steel/Kawada Bridge and were transferred to barges. Fitted with GPS sensors to monitor movement and location, barges were positioned beneath the span and the segments were lifted into place. Propellers on all four corners of the barge were attached to GPS units, which monitored the barge’s movement and kept it steady while the 450-ton, 120-ft-long sections were lifted.

Galloping Gertie

Locals and most engineering students are familiar with the dramatic collapse of the first Tacoma Narrows Bridge. Opened to traffic on July 1, 1940 after two years of construction, it collapsed just four months later during a 40-mph windstorm.

The bridge’s undulating behavior earned it the nickname “Galloping Gertie.” The bridge was the first of its type to employ a pair of deep I—beams to support the roadbed, which did not allow wind to pass through the truss. That caused the span to roll up and down even in mild winds, well before its eventual collapse. This rolling motion intensified during the fateful storm. Once the span began to twist, it was torn apart and fell into the Narrows.

The new bridge's towers are concrete, unlike the existing bridge's towers, which are steel.

Its replacement was built in 1950 with open trusses and stiffening struts to let the wind through.

While the size and basic components of the new suspension bridge mirror those of the existing 1950 bridge, the two spans differ in many ways. The existing span’s towers were built of steel at a time when the building material was relatively affordable. The new bridge’s towers are concrete structures.

The decision to use concrete was made mainly for economic reasons, but the use of the different building material also placated the area’s historical community, Cornish says. Because the original span is eligible for placement on the National Register of Historic Places, supporters felt that a replicated span would minimize the significance of the existing bridge.

The differences in the two bridges are clear. The steel towers are lean and parallel, while the concrete towers are thicker and incline toward each other. The existing bridge’s sidewalks are located outside the towers, while the new span’s entire bridge deck is within the tower legs. Even the shade of green paint used is different.

The only nod given by the new bridge’s design to its existing cousin is the concrete X’s on the struts, a purely aesthetic element.

Anchorages Aweigh

Work associated with the new span began just six months after the state transportation department and Tacoma Narrows Constructors signed the design-build agreement. In January 2003, road construction began on State Highway 16 to accommodate the toll collection facilities and new approaches. The following July, the first of the two caissons was put in place.

The community celebrated the opening of the new Tacoma Narrows Bridge in July. Festivities included a 5K fun run..

As construction continued on the caissons, crews began pouring concrete for the eastern anchorage. Two anchorages—one at each end of the bridge weighing 81 million lb—support the weight of the bridge cables.

In August 2004, concrete pours for the two towers began in earnest. Crews began stringing suspension cable across the Narrows in October 2004, finishing in April 2006. A few months later, vessels bearing the first of the 46 bridge deck sections began arriving and deck erection proceeded. Expansion joints were installed and suspension cables were wrapped.

Construction wound down with the painting of the cables in April 2007, followed by deck paving in June and the removal of tower cranes in early July.

Design-Build

WSDOT took a bold step by using the design-build project delivery method on the new bridge’s construction. The new Narrows bridge project represents only the second time the state agency has used design-build; the first was a $40-million highway interchange in Vancouver, Wash., seven years ago. By starting construction well before final designs were complete, Cornish estimates that approximately two years were shaved from the schedule.

“Design-build allowed us to take advantage of a larger and broader base of expertise,” says Linea Laird, WSDOT construction engineer.

As the towers were poured, crews utilized an innovative jump-form system, Carpenter says. Instead of using tie-offs while suspended from the towers, workers and equipment were enclosed in the equivalent of a three-story box, which workers dubbed the “birdcage.” The benefits of this enclosure were twofold: Safety was enhanced because crews were not suspended high in the air, and by keeping workers shielded, they were not a visual distraction to the motorists driving on the neighboring bridge.

The project team commissioned studies that modeled the effects of winds, currents, tides and the existing bridge on the new span, Cornish says. Consultants built scale models of the new bridge and evaluated their strength under the forces of both wind and water.

“There was a tremendous amount of planning behind every event,” Cornish adds. “We didn’t want any surprises, we didn’t want any fantastic feats and we didn’t want to have to rescue anything.”

Down to the Wire

Despite careful planning, the construction of the new Tacoma Narrows Bridge was not without setbacks. A 3.5-month delay in the bridge’s grand opening was due largely to the delivery of a batch of corroded, 1.96-in.-dia wire from a supplier in South Korea.

The wire was scheduled to be used in the bridge’s suspension cables. Crews had to engage in a lengthy process to separate the damaged wire, categorize and test the undamaged product and procure new supplies. In all, a third of the wire delivered was unusable, Cornish says.

Project team members scrambled to order new wire from companies in the United Kingdom, China and South Korea before continuing with suspension cable installation.

Although the dedication ceremony in July marked the opening of the new bridge to traffic, additional work still remains. Construction continues on a 10-ft-wide, barrier-separated bike path scheduled to open to the public later this year, and crews will be seismically upgrading the existing span and resurfacing its bridge deck into 2008.

Tolls on the Go The new Tacoma Narrows Bridge is Washington state’s first project to feature the “Good To Go!” electronic toll collection system. The system allows commuters to pay tolls without stopping at a tollbooth. A small transponder placed inside a vehicle’s windshield interacts with toll-recording equipment located alongside the road. Each time a vehicle passes through the toll collection area, an antenna mounted above the roadway links the transponder to the driver’s prepaid “Good To Go!” account, and the system automatically debits the toll. Tolls for vehicles using the system are $1.75 each way. Drivers can also pay a $3 toll manually at a staffed tollbooth.