Some Cable for Tacoma Narrows Bridge Badly Corroded

Sat December 10, 2005 - West Edition
Construction Equipment Guide

TACOMA, WA (AP) A problem with the wire being used to form cables on the new Tacoma Narrows bridge has some state officials breathing a sigh of relief that all cost overruns are the contractor’s problem under the “design-build” contract.

The state Department of Transportation said Nov. 29 that at least some of the 19,000 mi. of steel wire being used to form the cables is so badly corroded it is unusable.

Tacoma Narrows Constructors (TNC) is being paid a fixed price to design and build the bridge, so any costs to replace the wire will be paid by the company. The state is concerned, however, about the possibility of delays in construction as the company waits for replacement wire.

“It is bad news,” said DOT project Spokeswoman Claudia Cornish. “How bad is yet to be seen.”

Inspectors from Tacoma Narrows Constructors and the state are going through the thousands of 4-mi. coils of wire stockpiled for use in the cable spinning, looking for signs of the “white rust” discovered on some coils.

Cornish said it is too early to estimate how much of the wire is unusable, although approximately half appears affected to some degree.

The state and TNC have sent samples of the wire to labs for analysis and are awaiting results. The wire already used in the cable-spinning operation does not appear to be damaged, officials said.

Secretary of Transportation Doug MacDonald said only wire meeting contract specifications will be used on the bridge cables.

“We expect the contractor to obtain additional wire, if necessary, to assure the timely and correct assembly of the suspension cables,” he said.

Getting new cable may be a problem because the high-strength steel wire was made in Japan and South Korea. Thousands of strands of the wire, which is slightly smaller in diameter than a pencil, are wound into cables on the bridge.

The steel wire is coated with zinc for protection. When the zinc oxidizes as intended, it forms a durable coating that protects the steel. Under some circumstances, however, particularly when freshly galvanized products are stored wet or tightly packed, the zinc can form unstable oxides that eat at the coating.

“You want oxidation on zinc,” Cornish said. “This did not oxidize the way we wanted it to.”

Some of the wire has been stored outside, near the new bridge’s Tacoma anchorage. The rest has been stored at covered storage facilities. Cornish said there might be some correlation between how the wire was wrapped and the amount of corrosion. Two types of wrapping were used, one all plastic and the other plastic with a paper lining. She said the coils wrapped with the plastic and paper seem to be affected the worst.

Spinning crews have been working on the main cables for six weeks. So far they’ve gone through approximately a third of the 19,000 mi. of wire.