The look of a colossal bridge started to take shape yesterday morning when three cranes slowly and painstakingly lifted a 172-foot-long, 57-ton steel beam onto a V-shaped pier, where a handful of hard-hatted men standing aslant bolted it into place.

The arching beam, one of 368 that will be placed on the new Woodrow Wilson Bridge, represents a seminal moment in the building of the two-span, 12-lane structure. After four years of dredging and foundation work in and around the Potomac River, the steel beams are the first parts of the bridge that will connect pier to pier and, after several months, shore to shore.

"It's nice to see a gap spanned. It's what bridging is all about," said David Tackoor, an engineer for Potomac Crossing Consultants, which is managing the Virginia side of the project, where the beam was laid. "It's like when you break through a tunnel."

The new bridge, the biggest and most expensive public works project in the Washington region, will replace an aging span that is crumbling under the strain of carrying 200,000 vehicles a day, considerably more than the 75,000 it was built to handle. Those cars and trucks include hordes of commuters who sit in traffic daily on both shores as well as commercial haulers and travelers moving up and down the coast on Interstate 95.

Traffic engineers hope the new bridge will ease the daily crush for all users, and project officials said yesterday that the first span remains on schedule for a 2006 opening. Once that happens, the existing bridge will be torn down and a second six-lane span will be built in its place, opening to traffic in 2008.

Together the structures will have eight general-use lanes to match the number feeding in from the Capital Beltway, plus one carpool lane and one merge lane on each side. They will also have pedestrian and bicycle access.

In addition to being significantly wider, the new bridge will be 20 feet taller than the approximately 50-foot-high bridge that stands today. That will reduce the number of times the drawbridge will open, from 260 times a year to 60, project officials said.

Upgrades are being made to several interchanges in Virginia and Maryland to increase capacity on the Beltway and to accommodate the additional lanes on the bridge. Most of those projects will be finished by the time the second span opens, though improvements to the Route 1 interchange will not be completed until the middle of 2009, and the Telegraph Road interchange will not be finished until late 2011. A new ramp from the Beltway to Indian Head Highway in Maryland opened in September.

Jim Ruddell, the project's construction manager, said that 30 percent of the total work has been completed and that it is $147 million under budget, lowering the bridge's total expected cost to $2.43 billion.

"It's a good sign," Ruddell said. "We're real pleased with our progress to date."

Over the next several months, beams will be laid across each set of piers, where they will sit on rubber bearings with lead cores and connect to each other with smaller steel beams. Each of the connector beams is actually composed of two smaller ones, held together by about 250 bolts.

Project officials said that about two beams would be delivered each night to minimize disruption to residents and drivers. The beam construction will cause some temporary closures of the Mount Vernon trail between 8 a.m. and 6 p.m., they said.

After the beams are in place, workers will start laying the bridge's concrete deck, the final phase of the first span. Ruddell said the next major milestone will come in the spring, when a drawbridge made of 7,000 tons of steel is scheduled to arrive by ship. Workers will then lift it onto the piers, linking the steel beams stretching from both shores and making it a permanent part of the region's skyline.

The bridge project, the biggest and most expensive public works effort in the region, dominates the scene as vehicles pass on the old bridge. A worker stands atop one of the 368 steel beams that are being placed on concrete piers and eventually will reach from shore to shore.A view of the project from the Virginia side shows the concrete approach at left and the piers to which the beams are being attached.