Special Reports

Fastening bridge deck sections takes plenty of patience, precision

In some ways, putting the deck of the new Tacoma Narrows bridge together is like putting your kid’s bike together on Christmas: The parts were made in Asia and there’s “some assembly required.”

In this case, though, it’s not a simple matter of slipping tab A into slot B.

Fastening the deck sections will take more than 115,000 high-strength steel bolts, and the task is complicated by the fact that most sections are as big as 12-story buildings and hang 180 feet or more above the water.

Because portions of the deck will flex and strain differently with the different forces exerted on it, the connections between deck sections require a great number of strengths.

The resistance of the deck has been calculated so precisely that designers established an inventory of every single bolt, all of which are referenced in an inch-thick dossier of single-spaced pages.

Most of the bolts are seven-eighths of an inch in diameter and 3 to 4 inches long. Some are as long as 8 inches.

Ironworkers apply them in the top and bottom chords, the diagonal bracing in the truss work and at the U-shaped ribs that support the steel top plates.

The holes were placed and drilled by computer-controlled equipment, then fitted together on blocks at the Samsung shipyard on South Korea’s Koje Island.

“This bridge has already been put together one time,” said Tim Moore a state bridge engineer who helped monitor the process.

Even so, workers in the Narrows must tease the sections into their precise alignment by lifting with gantry cranes and pushing and pulling with hydraulic jacks.

When the alignment is nearly perfect, they fit flat, half-inch-thick steel splice plates with corresponding holes over the bridge members and drive gradually tapered steel shafts, called drift pins, into 15 percent of the holes to bring them into alignment.

At that point, the ironworkers start “stuffing” bolts. They work in pairs, feeding the bolts in one side and topping them with nuts and washers on the other.

One ironworker holds a long, closed-end spud wrench on the end of the bolt to keep it from turning while another “snugs up” the nut on the other side. Snug is not a precise term, but is defined as “full effort by an ironworker.”

The precision comes later. A pneumatic wrench calibrated with a Skidmore Bolt Tension Indicating Device is used to apply the tension specified by designers: in most cases, a minimum of 41,000 pounds of tension for a seven-eighths-inch-in-diameter bolt.

Once the pneumatic wrench is properly calibrated, two ironworkers work together, one holding the bolt from turning, the other zapping it with the wrench.

Tightening one nut takes less than 10 seconds.

Sounds fast, but consider this: Even if that rate could be sustained continuously, which it cannot, it would take more than seven hours to tighten all 2,542 bolts in just one typical joint.

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