Another Narrows Bridge Milestone

Editing by Carl Molesworth | September 28, 2010

Tacoma Narrows Constructors was approaching another milestone in its five-year, $849-million project to build the new Tacoma Narrows Bridge as 2006 ended. In fact, by the time you read this, TNC may already have finished lifting the 46 sections of the bridge deck into place.

The Washington State Department of Transportation and its design-build contractor, TNC, announced in May that the new bridge will open to traffic in early July 2007. Groundbreaking for the project took place on Oct. 5, 2002.

The bridge is being built parallel to and south of the existing 1950s Narrows Bridge on State Route 16. The original construction schedule in TNC's contract anticipated that the project would take about 55 months to complete. But that proved overoptimistic as some of the bridge's technical challenges were encountered (see PB&E, Sept. 1, 2003, and June 6, 2005). According to the new schedule, the construction duration would be 58 months and the hoped-for opening in April 2007 moved back to July.

Bridge progress last spring included the completion of cable spinning and compaction on the north cable, including the placement of the "suspender" cables that will hold the 450-ton bridge deck sections. Most of the adjoining road work was completed and the toll plaza finished.

One at a Time

The bridge deck placement, which is one of the most challenging technical aspects of the project, represents the fifth and final phase of the project. The first 16 sections of the mile-long roadway arrived in Puget Sound in early June 2006 from South Korea.

The project utilized two semi-submersible ships, the TEAL and the SWAN, to deliver the deck sections to the bridge site. The first deck lift took place in August.

The deck sections, which average 120 feet by 78 feet and weigh 450 tons, were lifted one at a time with gantry cranes that straddle the cables.

Two different types of gantry cranes were used to hoist the bridge deck sections into place. On the side spans, the gantries' lifting mechanisms are winches located on the caissons. In the mid-span area, the gantry cranes lift the sections using strand jacks located on the gantries' main girders. Each section will be attached to vertical suspender cables, and then connected to the main cables.

Tacoma Narrows Constructors has been using Lifting Gear Hire Corp. electric winches and come-a-longs in the erection of the new suspension bridge.

According to Tony Fiscelli, general manager of Lifting Gear Hire's USA Corporate Headquarters, located in Bridgeview, Ill., the electric winches are being used to inch large gantries along giant spun cables that support a 710,000-pound I-beam on the new bridge.

"A giant barge carries all of the deck sections to the place where they will be erected on the bridge," Fiscelli explained. "Strand jacks are being used to lift the bridge deck pieces from these barges into their final location on the main cable. Our come-a-longs are then used to inch the deck plates closer before they are settled into their final positions."

The first deck section was placed in the center span location. The main suspension cables deflected 12 feet as a result of the first deck section being lifted into place. This caused a substantial change in the cables' geometry. The weight of the first deck section shaped the cable into a "V" at midspan. It appeared as a "V" until crews installed sidespan deck sections to act as counterweights.

The remaining 45 blocks were lifted in an order that, to the casual observer, might have appeared random. That was not the case. Rather, the non-linear order of lifting sections was designed to maintain equal stresses on the bridge's two towers. The precise lifting sequence ultimately pulled the towers back to a plumb, vertical position.

During tower construction, the bridge builder used large cables to pull the tower tops about 2 feet toward shore. That was done so that during the deck installation phase, the weight of the sections would pull the towers toward one another and back to a perfectly vertical alignment. The weight of all 46 sections will pull and lower the main cables about 25 feet below its non-weighted profile. This stretching process will place the new bridge deck at the same level as the 1950 bridge deck.

As deck construction got under way in July, crews simultaneously were building the deck out from the east and west anchorages. In the initial assembly of the sections, the deck pieces were joined temporarily with shear lugs and bolting fixtures. When the majority of deck sections were in place and the cables near their final profile, the crews could weld the deck plates and bolt all other truss sections together.

Different Deck Surface

Like the new towers, the new bridge deck is different from the 1950 span. Instead of a lightweight concrete deck surface, the new deck is composed of lighter steel plates with beams running lengthwise and crosswise in an orthotropic pattern. It's a type of deck that enables a suspension bridge to perform better under different loads, temperatures, winds, and earthquake conditions.

The new bridge deck will have only two expansion joints, one on the east side and one on the west side. Each will allow up to 30 inches of bridge deck movement. That movement can be caused by thermal (temperature) changes, seismic events, wind, and even forces generated by vehicles braking and accelerating on the bridge. That configuration is in contrast to the existing bridge deck, which has large expansion joints at each tower and smaller expansion joints every 120 feet along the length of the deck.

Deck lifting and assembly of all sections is scheduled to be complete in early 2007. Simultaneously, crews will continue the bolting and welding process to permanently join deck sections that are already in place. Bolting and welding activities will continue through March. Once the deck has been joined together to form a seamless roadway, the surface will be topped by two inches of asphalt and final details completed.

When the new bridge opens next summer, TNC will close the old span for renovation, which is expected to take about a year.

Meanwhile, The Tacoma Narrows Bridge Citizen Advisory Committee is formulating recommendations to present to the Washington Transportation Commission on toll rates for the new Tacoma Narrows Bridge. The commission is expected to set toll rates early next year, and tolls are scheduled to begin with the opening of the new bridge next summer.

Erin Hunter, of Tacoma Narrows Constructors, said the new bridge will give eastbound commuters two general-purpose lanes, an HOV lane and a "drop" lane by summer 2007. In summer 2008, both new and existing bridges will be open in their final form and will allow the existing bridge to handle westbound traffic on two general purpose lanes with one carpool lane, and the new bridge to handle eastbound traffic with two general purpose lanes, one HOV lane and a fourth "drop" lane, beginning at the 24th Street eastbound on-ramp in Gig Harbor and ending at the Jackson Street off-ramp in Tacoma.

The very first Tacoma Narrows Bridge opened on July 1st, 1940, only to collapse in a windstorm several months later on Nov. 7, 1940. "Galloping Gertie," as the bridge was known, became famous as "the most dramatic failure in bridge engineering history." A new and much safer Tacoma Narrows Bridge opened on Oct. 14, 1950, and is the fifth-longest suspension bridge in the United States.

The new Tacoma Narrows Bridge will help alleviate congestion, improve driver conformability and increase safety for drivers as they travel along the State Route 16 corridor. Currently, 85,000 to 90,000 vehicles use the corridor on a daily basis, but the 1950 bridge is only designed to handle approximately 60,000 per day. Officials estimate that the use of the freeway will increase to 120,000 vehicles per day by 2020, which prompted the building of the additional bridge.