Narrows Bridge repairs, old and new
ROB CARSON
Designers of the new Tacoma Narrows Bridge got a chorus of boos from some quarters when they announced they wanted to build the bridge towers out of concrete instead of steel.
Alongside the more delicate and ornate steel towers of the 1950 bridge, the chunky concrete forms reminded some of Soviet-style apartment buildings.
Engineers praised the choice, however, and now, five years later, maintenance crews do, too.
The super-dense concrete in the towers and caissons is as strong as it was the day the bridge opened on July 15, 2007, engineers say, and there’s been no flaking or spalling. The sealant and paint on the 510-foot towers have needed no touch-up work whatsoever – and probably never will.
“It should be good for the life of the bridge,” said Chris Keegan, operations engineer for the state Department of Transportation’s Olympic Region. That saves taxpayers money, he said, and it means the $729 million bridge will be paid off sooner.
Tolls are used to pay for maintenance on the newer bridge. Maintenance on the older bridge comes from gas taxes. The new bridge cost $140,000 to maintain last year while the older span cost $620,000.
The more ornate steel towers on the 1950 bridge have been a maintenance problem since they were built, creating a constant chore of scraping and repainting to avoid corrosion.
Five years after the eastbound bridge opened, maintenance workers say they’re delighted with how the mile-long bridge is performing. Problems have been minor, they say, and the few that have surfaced were covered by the warranty agreed to by the Bechtel/Kiewit partnership that built the bridge.
The biggest problems with the bridge were caused by moisture trapped beneath paint and other protective coatings on the cables.
Pressed by a tight completion schedule, construction crews were forced to wrap and coat the main cables and suspenders in the rainy spring months of 2007 rather than the previous fall, when most of the moisture would have been cooked out by the heat.
The water trapped inside the coatings collected in blisters that needed to be popped, scraped and repainted.
“They looked kind of like water balloons,” Keegan said.
To remove water trapped inside the bridge’s suspender cables, crews removed paint on several inches of the bottom ends of the cables. The tops had to be resealed and repainted to make sure more water didn’t get back in.
As with most problems so far, the blistered surface coatings were covered by warranty and fixed by the contractor without charge.
Crews on the 1950 bridge have spent decades scraping and repainting suspender cables, originally coated with lead-based paint.
In the past five years, they’ve stripped the old paint off the cables and replaced it with the new, more flexible coating that is far more weather-resistant, Keegan said. When finished, the suspenders on the old bridge should be just as weather resistant as those on the new bridge.
The main cables on the new bridge, the 201/2-inch diameter strands that support the suspenders and deck, also had some initial problems. Moisture and gases trapped by tough outer coating made it necessary to scrape and repaint sections.
The asphalt coating on the bridge deck has begun to degrade, but that was expected after this amount of time, said Keegan. Only minor pavement repairs have been made so far, and those were covered under the warranty. In a year or two, he said, the Transportation Department will lay down a new topcoat.
“We figure every six to eight years, we’ll skim off the top three-quarters of an inch of existing asphalt and pave back over it,” Keegan said. Estimated cost of the repaving is $2.9 million.
The biggest ongoing maintenance chore on the new bridge has to do with its drainage system.
On the old bridge, the open grates in the deck let rainwater (plus accumulated oil, gasoline, brake-pad dust and assorted garbage) drop into the Narrows.
On the new bridge, rainwater is captured by road-level scuppers and directed by drains into above-ground cisterns on both ends of the bridge. Steel grates catch the larger bits of debris – Styrofoam, cans, paper and other garbage – as the water flows into the cisterns.
In months of heavy rain, so much debris accumulates on the cistern grates that crews need to shovel if off every couple of days.
Noise from the bridge’s expansion joints – flexible connectors that accommodate the bridge’s end-to-end movements and contraction and expansion from temperature change – was a sore point with nearby residents. Transportation Department crews put up noise barriers to partially contain the sound. Money for the noise walls came from tolls.
Preventive maintenance on the new bridge is made easier by electric maintenance platforms called travelers that run beneath the deck. The 1950 bridge has travelers, too, but large portions of the older bridge’s understructure are inaccessible from them. Also, the old travelers are powered by temperamental diesel motors.
The new traveler lost wheels at one point, Keegan said, and elevators in the bridge towers needed minor adjustment.
“It was nothing major,” he said. “It was like the elevators in any new building. They just needed some fine-tuning.”
Underwater, the giant caissons that support the towers have had no apparent problems, but they’ve not had detailed inspections either.
“We’re due for an underwater inspection next year,” Keegan said.
rob.carson@thenewstribune.com
253-597-8693
