WSDOT seeing double with Tacoma Narrows Bridge

Nov 1, 2003 12:00 PM, by Steven Prokopy

The first incarnation of the Tacoma Narrows Bridge was constructed in 1940 to link the city of Tacoma, Washington, and the surrounding Puget Sound with the Peninsula area. The goal was not only to encourage growth but also to make a short cut to the Olympic Mountains. The bridge's now-legendary design flaws became a regular tourist attraction as people came from surrounding areas to ride the wind-induced rolling structure that came to be known as Galloping Gertie. At times the rolling was so severe, it was impossible to see the car in front of you. The 5,939-ft.-long bridge first collapsed only four months and seven days after its dedication during a 42-mph wind storm.

The second, 5,979-ft.-long Tacoma Narrows Bridge was built in the 1950s and was the first suspension span constructed in the United States after its predecessor's failure from torsional oscillations. This time around, aerodynamic testing was done as part of the suspension-span analysis.

Over the years, however, growing congestion has become a problem on the State Route 16 corridor as an estimated 85,000 to 90,000 vehicles per day cross a bridge that was designed to carry 60,000; use is estimated to increase to 120,000 vehicles per day in 2020. In addition, the existing bridge lanes (two in each direction) are narrow by today's standards and are separated by open grating. There are no shoulders, nor is there a physical separation of opposing traffic movements, and the bridge towers are located right at the outside lane edge.

The solution to this potentially paralyzing problem rests in the largest single construction contract in the Northwestern United States and the largest suspension bridge in the country within the last 40 years: an eastbound-only bridge (toward Tacoma) standing parallel, 185 ft. center-to-center from the current bridge, which will be turned into westbound-only lanes (toward Gig Harbor) upon completion of the new bridge. When completed, the $615 million design-build Tacoma Narrows Bridge project will also see improvements on the old bridge, roadwork on both sides of the structures, including new approach roads, 2.4 miles of SR 16 in and around Tacoma and Pierce County, a toll plaza and operations center, and a maintenance facility. The total project cost, including the cost to sell the bonds required to fund the project (to be paid back from toll revenue), will be $849 million.

SPEAKING VOLUMES

The new bridge will bear on 130- ∞ 80- ∞ 225-ft.-deep caissons, requiring about 34,000 yd. of concrete each and capable of sustaining 7-knot reversible currents, 15-ft. tidal swings, and 150-ft. water depth with 100-ft. scour potential. They will support towers rising 510 ft. and requiring 8,000 yd. of concrete each. Approximately 23,000 yd. of concrete will be needed for the conventional gravity anchorages. The total length of the bridge will be 5,400 ft. with a 2,800-ft. main span. Three 12-ft. traffic lanes, two 10-ft. shoulders will be separated from a 10-ft. bicycle/pedestrian path by a barrier. The main suspension cables will be 21 in. in diameter and made of 5,500 tons of wire.

Playing a major role in constructing the new bridge is Seattle-based Glacier Northwest, owned by Taiheiyo Cement USA, Inc., which has established a wet batch plant on the Gig Harbor side of the bridge on WSDOT-owned property. The 1¾-acre site houses a 250-yd.-per-hour Model 427M Con-E-Co plant, as well as a cement silo with 409-barrel capacity and an aggregate silo with four, 50-cu.-yd. compartments. Two additional overhead silos are used for silica fume and additional cement reserves in case additional storage is needed for longer, 20-hour pours. Initial estimates for volume of material needed from Glacier currently are 165,000 yd. of concrete. The company trucks in cement from its Seattle terminal and uncrushed glacial aggregate from Dupont. Glacier's permanent Tacoma plant (about seven miles away from the job site) is used as a backup and was tapped early in the job to supply the cutting edges of the caissons on the Tacoma side.

According to Mark Leatham, Glacier's general manager of concrete operations-Washington Division, dozens of mix designs have been created for the Tacoma Narrows project with more still being developed as the work progresses. “Issues concerning low permeability and corrosion are being addressed with mixes incorporating silica fume, particularly in the towers,” he says.

At present, concrete is being poured for the anchorages on the Tacoma side. On the Gig Harbor side, excavation for anchorages will continue through mid-December.

With essentially two job sites in such close proximity to the plant (150 ft. to the Gig Harbor site; 1.5 miles to the Tacoma area), staging areas have been set up for mixer trucks at each pour location where trucks park and allow for additional mixing. Since the plant is also relatively close to residential neighborhoods, truck traffic and noise concerns were addressed by Glacier. “The neighbors here are definitely vocal, but the plant is fairly isolated. There are special access roads to get under the bridge, but with traffic congestion, getting to the Tacoma side can take anywhere from five to 20 minutes. For noise abatement, all surfaces in the plant that touch aggregate have polyurethane liners to dampen sound.

Once WSDOT is satisfied that it can provide three lanes of traffic in the peak direction and two lanes in the off-peak direction, a $3.00 toll will be charged for east-bound traffic. The bridges are expected to be fully operational by early 2008.