The Francis Scott Key Bridge, a majestic, arched structure spanning the Potomac River between Georgetown and Rosslyn, will undergo an extensive $14 million overhaul starting tomorrow, raising prospects of traffic backups and delays for thousands of commuters during the next eight months.

The long-debated project, which has drawn widespread praise from historic preservation and other groups, is designed to repair long-term deterioration, improve safety and restore the dilapidated appearance of the 63-year-old landmark, named for the author of "The Star-Spangled Banner."

The District government plans to embellish the bridge with globe-topped light posts, similar to the original fixtures, to create "a necklace of lights" across the river. "At night, the new lighting will help draw attention to the bridge, which is a centerpiece for views up and down the Potomac," said a city report.

The renovation is expected to disrupt rush-hour traffic for commuters as city officials seek to divert cars from Key Bridge to other Potomac crossings. Two of Key Bridge's six lanes will be closed, along with a ramp to the bridge from the Whitehurst Freeway.

"What we will see is longer delays on the bridges," said George W. Schoene, the city's traffic services chief. "It's going to be tight."

Key Bridge is the last Potomac span scheduled for major reconstruction until the early 1990s, when the Theodore Roosevelt Bridge is to be renovated. In recent years, extensive repairs have been carried out on the Woodrow Wilson, Cabin John, Chain, Arlington Memorial and 14th Street bridges.

Key Bridge, which was last resurfaced in 1970, has deteriorated because of the long-term effects of heavy traffic, wet weather and use of salt in winter. These factors have caused cracks and decay in the concrete undergirding of the bridge, which is crossed by 72,000 vehicles a day.

In 1983, the American Automobile Association ranked Key Bridge as the second-worst span in the Washington area, terming it more defective than all bridges except for the now-renovated Cabin John crossing. The AAA cited Key Bridge's unusually low federal "sufficiency" rating -- 6.4 on a 0-to-100 scale.

Still, city officials have repeatedly described the 1,792-foot bridge as soundly built and safe, saying it has never posed any risk of collapse. The renovation, largely financed with federal funds, is designed to correct its main shortcomings.

"We won't have those structural defects," said Gary A. Burch, chief of design and engineering for the D.C. Department of Public Works.

The Romanesque structure, which opened Jan. 17, 1923, was designed by Nathan C. Wyeth, a Washington architect, and is regarded as a historical example of concrete arch work. The bridge is listed by the District as a historic site, and officials say it is eligible for the National Register.

The city's initial plans for renovating the span were revised to counter objections from preservation and environmental groups. Don L. Klima, who reviewed the project for the federal Advisory Council on Historic Preservation, said, "It's a good job, and we're overall pleased with it."

The District plans to preserve the bridge's weathered facade, install curved railings and decorative barriers, and clean the dingy underside of the structure near the C&O Canal. Officials agreed to replace the light posts, which were installed in 1952 and are considered an eyesore.

One aspect of the plan has drawn criticism. The city plans to widen walkways used by hundreds of bicyclists and pedestrians at both edges of the bridge in an attempt to reduce crowding and improve safety. Preservation officials say the extended walkways will cause shadows and mar the bridge's appearance.

Much of the work centers on reconstructing the bridge's deteriorating deck, including the road surface and a layer of reinforced concrete beneath the surface. Other parts of the bridge also will be repaired, including portions of arches and columns where concrete has decayed.

Traffic is expected to be disrupted during the first eight months of the 15-month project, which will be carried out by Kiewit Eastern Co., a subsidiary of the Omaha-based Peter Kiewit Sons' Inc. The company won the contract by submitting a $14.1 million bid, $2.4 million less than the city's estimate.

The contract provides for a bonus of $9,000 a day for completing this part of the work in less than eight months and a similar penalty for taking longer. However, Michael Ramsey, Kiewit's job superintendent, termed the schedule "very tight" and said it appears unlikely that the work will be finished much sooner.

The city plans frequent shifts in traffic rules on the bridge during the first eight months. From 6 to 10 a.m. on weekdays, three lanes will be open for cars and trucks heading into the District. Traffic going to Virginia will be restricted to one lane during those hours.

At most other times, officials said, traffic will be limited to two lanes in each direction. After 9 p.m., they said, the work may be expanded to leave only one lane open in each direction. Construction will start on the middle lanes and shift to the outer lanes in about two months, officials said.

In an attempt to divert cars to the Roosevelt Bridge during afternoon rush hours, the city has removed a median barrier and installed a traffic signal at 27th and K streets NW. Westbound traffic on K Street can turn left at the signal to reach the I Street entrance to the Roosevelt Bridge and Rte. I-66.

Commuters who normally take the westbound Whitehurst Freeway to the bridge will be affected by the work. The ramp from the freeway to the bridge will be closed, and westbound freeway traffic will be prohibited from turning right on M Street to reach the bridge from 4 to 6:30 p.m.

During afternoon rush hours, westbound traffic on M Street in Georgetown will be limited to three lanes instead of four, and all westbound traffic on the Whitehurst Freeway will have to continue on to Canal Road.

Schoene urged commuters who normally use Key Bridge to switch to the Roosevelt Bridge, and he said drivers who usually cross the Roosevelt Bridge should consider taking Memorial Bridge. Schoene also recommended shifting from Memorial to the 14th Street bridges. "We would like to see that occur," he said.