The South Sound’s wet, windy weather is forcing engineers to get creative in their efforts to wrap the giant suspension cables that support the new Tacoma Narrows bridge.
The two cables, each more than a mile long and 20.5 inches in diameter, need to be coated with a gluey zinc-based paste, then tightly wrapped in 948 miles of fine wire to protect them from corrosion, and then covered with three coats of rubberized paint.
But before the protective coatings can be applied, the surface of the cables must be dry, a condition that wouldn’t occur naturally in these parts for months.
Tacoma Narrows Constructors can’t wait that long, and over the past two weeks, its engineers have been experimenting with a variety of tents, plastic wraps, heaters and compressed air blowers to dry the cables enough to proceed with wrapping.
Last week, drivers on the parallel 1950 bridge noticed remains of tents flapping in the wind and then black plastic shrouds covering the cables from the tower tops to the Gig Harbor anchorage, giving them a Christo effect.
“This part of the process is always difficult,” said Dave Climie, TNC’s engineer in charge of the process. “It’s a bit more of an art than a science.”
Climie is one of the world’s foremost suspension bridge experts, a veteran of cable work on some of the longest suspension bridges in the world, including ones in Denmark, Hong Kong and mainland China.
The problems involved in pasting and wrapping the cables are not unique to the Northwest. “It rains everywhere,” Climie said. “If it’s not rain, it’s humidity, or wind or cold, all of which have their own complications.”
The wrapping would have been easier had TNC been able to stick to its original construction schedule, which called for the wrapping to take place last fall, ahead of the winter rains.
The bridge is running four to five months late, mostly because of an outbreak of corrosion, discovered in November 2005, in stockpiled wire used to build the cables. The cables are each made up of 8,816 individual wires, slightly smaller around than a pencil.
The delay put TNC in a crunch with its deadlines. The bridge originally was supposed to open to traffic April 2 – just over a month from now. TNC pushed that back 90 days to July 2, a date which most now agree will be next to impossible for the bridge builder to meet. For each day of delay after July 2, TNC will have to pay the state $125,000 in damages, to a maximum of $45 million.
A struggle to keep cables dry
A suspension bridge’s cables are considered the most vulnerable of its components. In the bridge-building industry, engineers engage in lively discussion over how best to protect them.
For decades, cables were coated with red lead paste. That process worked well but has not been used since the mid-1990s, when the lead was recognized as an environmental hazard.
Now bridge builders use a zinc paste, about the consistency of lumpy mayonnaise. The idea is to glop it on thick, Climie said, so when the wrapping goes on top, the stuff oozes around and between the wires and forms a solid coat.
The gray paste is made in Italy and takes a month to get. TNC ordered 1,710 five-gallon buckets of it, each of which weighs 66 pounds.
The final step in the cable-protection system is applying three coats of rubberized paint.
At the end of the day Tuesday, TNC crews had wrapped a total of 10 sections of cable, each about 40 feet long. The wrapping itself goes quickly, Climie said.
Machines designed by TNC’s Japanese partners, Nippon Steel and Kawada Industries, clamp onto the cables and, whirling like giant fishing reels, wrap the cable with 1/8-inch galvanized wire at a rate of 71/2 inches a minute.
The hard part is drying the cables and coating them with the goo. Pasting one 37-foot section between the suspenders takes four to five hours, Climie said, and that’s after it is sufficiently dry.
They’re not trying to get all the water out of the cables. That would be impossible and unnecessary, Climie said. Water is expected to work its way inside the cables and flow down them and drip out where the suspender cables are attached. After the bridge is finished, water will continue to flow through the interior of the cables, Climie said, though the more water you can keep out, the better.
“You want to make sure you’re giving the cables the best chance you can,” he said.
Rain, chill slow progress
TNC is not taking any chances with the wrapping wire, after its problems with the wire used to spin the main cables. Close to a third of 19,000 miles of wire corroded while it was in storage in Tacoma, rendering it unusable and forcing TNC to replace it, a process that set the construction schedule back three months.
The wrapping wire was manufactured at the Kiswire plant in Pusan, South Korea, the original source of the spinning wire. This time, Kiswire used different packaging and TNC is storing the wire differently.
“Of course, we’re taking precautions,” Climie said. “Once you’ve been bitten, you don’t stick your hand back in the dog’s mouth.”
The tents TNC is using were made to order by Yakima Tent & Awning, in Yakima. The heavy vinyl structures wrap completely around the cables to prevent spilled paste from dropping into Puget Sound.
The original 50-foot design of the tents presented too much surface to the prevailing south wind and needed to be shortened. TNC also has configured them so the sides can be rolled up like window blinds, to relieve the wind pressure.
Temperature is an issue, too. The surface of the cable needs to be at least 40 degrees before the paste will stick, and nighttime temperatures have been too cool for wrapping then. The paste must be stored in heated containers.
When the temperature climbs enough, TNC will go to two shifts, with six people on each of the four wrapping machines. That’s 24 people per shift, or about 50 people total.
How long is the operation likely to take? Climie rolls his eyes at the question. He can’t say for sure, but he thinks about two months.
“The less it rains the better,” he said. “And the less the wind blows.”
“We’ll fight our way, whatever it throws at us,” he said. “You have to do it and you have to fight your way through the conditions. There is no easy answer.”
Four steps to Protect cables from corrosion
1. Clean and dry outside surface of cable.
2. Coat cable with urethane/zinc paste.
3. Wrap tightly with 1/8-inch galvanized wire.
4. Apply three coats of paint.