Special Reports

Deck pieces in for the fit of their lives

When the 46 deck sections for the new Tacoma Narrows bridge left the Samsung plant in South Korea, they already were nearly perfectly matched.

Samsung pre-fit them by setting them up on blocks at the Koje Island shipyard, making tiny adjustments until the sheet of steel on top of each section met adjoining ones with no more than a 4-millimeter gap.

In the Narrows, welders are making the sections fit even better. Design specifications allowed the fabricators a one-sixteenth of an inch variance in height, but that’s not close enough to suit welding supervisor Bill Madron.

“We do not take that 16th,” he said. “We try to put it on zero.”

Even small discrepancies in how joints fit can make big differences in their strength, or, as welding engineers put it, their “fracture toughness.”

If welders discover that part of one section is higher than the adjoining one, they climb below the deck and loosen the appropriate bolts in underlying U-ribs.

The holes in the U-ribs are slightly larger than elsewhere: a full inch for a seven-eighths-inch bolt instead of the standard 15/16th of an inch. That way, welders can loosen them and pry the top plate up or down slightly to bring it into perfect alignment.

To coax the high side down, they tap steel wedges under L-shaped frames called strongbacks, then retighten the bolts.

Welders then crawl along the joints with hand-held electric grinders, working them until they are perfect V-shaped grooves, so clean and straight they look as if they might be parts of surgical equipment instead of a bridge deck.

The welders use 200-amp generators to run between 24 and 26 volts of power to their equipment, enough to snap a powerful electric arc between the deck and their welding gun.

The arc is so bright workers must wear hoods with darkened glass to avoid burning their retinas. The intense heat from the arc (about 2,700 degrees Fahrenheit) melts filler metal and the steel in the deck, fusing them.

The tops of the steel decks are five-eighths of -an inch thick, about the width of a pack of playing cards. Welders make two passes over the gap by hand, adding one-eighth of an inch of filler material per pass.

Then they hook up mechanized welders that roll along the joint like robots to lay down four more layers to make up the remaining three-eighths of an inch.

Thin layers minimize the possibility of voids, which would be discovered by quality-control inspectors who examine the welds with X-rays.

Tacoma Narrows Constructors currently has two six-person welding crews working one shift a day. It eventually will have four welding teams going at once, two in the midspan and one on each of the side spans. By the time they’re finished, they will have welded 45 joints. Each is 80 feet long, for a total of 0.68 of a mile.

How strong will the welds be?

“There’s not going to be any breaks in the welds,” Madron said flatly. “Not in this day and time. Our fill materials are stronger than the base material. It is unbelievable how good they are.”