Contractors building the new east span of the Bay Bridge reached a milestone Friday morning when a huge crane lifted into place the last deck segment for the suspension span -- the elegant cable-supported section of the $6.3 billion project.

Workers in hard hats cheered, snapped photos and displayed a Christmas tree in keeping with a construction tradition -- called a topping -- to celebrate the end of a key stage in bridge building.

Their short celebration occurred minutes after the bridge deck settled into place at 11 a.m. A news conference for reporters and politicians was held slightly earlier.

"This is a major milestone in replacing the San Francisco-Oakland Bay Bridge," acting Caltrans Director Malcolm Dougherty said as the crane slowly lowered the deck piece the last few inches.

The 1,049-ton steel box gives the east span a nearly continuous surface from Oakland to Yerba Buena Island, even though the bridge won't officially open for another two years.

Installing the 28th and last piece of the 2,007-foot bridge deck allows contractors to start stringing cable up and over a 525-foot tall bridge tower; that work is expected to begin in December.

The deck piece installed Friday is key.

"It's the place where the cable actually comes in and locks into the bridge," Caltrans spokesman Bart Ney said.

"It's self-anchored. That means the cable doesn't go into the ground, but into this piece. There's a big hole in the deck where will the cable will go down and anchor into this section."

Unlike the Golden Gate Bridge, where cables were strung before the deck was installed, the new Bay Bridge suspension span requires that the deck be installed before the cable. Temporary supports hold up the deck until the cable is tensioned.

A big crane took five hours to lift the last deck piece 150 feet up and into place.

This last deck segment joins the suspension span and the already completed skyway that will carry motorists toward Oakland.

For the 280,000 daily Bay Bridge drivers, the next visible construction drama will be preparation for installing cable.

On metal catwalks sloping up the bridge tower, crews are installing a tramway system similar to a ski lift. It will be used to pull the cable over the bridge tower and back again.

"Motorists will see a lot of work on the catwalks," Ney said. "You can already see this ski lift being put into place."

During the short ceremony, Oakland Mayor Jean Quan reminded the audience that replacing the east span is an earthquake safety project.

"We felt those tremors last week," Quan said. "We know we all live in earthquake country."

The 1989 Loma Prieta earthquake collapsed a section of Bay bridge deck and closed the structure for a month.

Contact Denis Cuff at 925-943-8267.

Follow him at Twitter.com/deniscuff. Read the Capricious Commuter at www.ibabuzz- .com/transportation.