Both Beltway bridges over the Potomac River will be widened from six to eight lanes and completely redecked beginning next year. The projects may take as long as two to three years, but are not expected to cause major traffic jams for motorists.

The Woodrow Wilson Bridge and the Cabin John Bridge (its official name is the American Legion Bridge) now cause huge traffic tie-ups daily when the eight lanes of Virginia and Maryland Beltway traffic funnel onto the six-lane bridges.

The Maryland State Highway Administration, which will oversee the reconstruction on both bridges, foresees few traffic jams because the bridge medians and narrow sidewalks will be removed to add extra lanes during the construction and some of the work will be done at night. This will permit six lanes of traffic to be maintained on the Wilson Bridge during the day and at least during rush hours on the Cabin John Bridge, said Maryland's district engineer William L. Shook.

The decks of the two bridges, built in the early 1960s, are disintegrating because of the current traffic, which is much greater that they were disigned to carry. About 120,000 vehicles use the Wilson Bridge each day, Shook said, and the Cabin John Bridge carries an average 99,000 vehicles a day.

Shook said road salt, which is used to melt winter ice and snow, also has contributed to disintegration of the bridges. The salt seeps through cracks in the concrete and corrodes the bridge's steel reinforcing rods, he said. The new bridge decks will be built with stronger and more impermeable concrete.

Design studies are now being completed for the projects and the only other thing to be determined, said Shook, is where the money will come from.

The Cabin John project will cost about $22 million, with the federal government paying 90 percent of the costs, as it does on almost all interstate highways. Maryland and Virginia own and jointly maintain the bridge.

The Wilson project is estimated to cost $40 million to $60 million, and it has not been determined who will fund the project. The federal government built and owns the bridge, although Virginia, Maryland and the District agreed 15 years ago to equally split its maintenance costs because the bridge crosses all three jurisdictions. Only a few hundreds yards of the 5,897-foot bridge are considered to be in the District, all of it over Potomac waters where the 1791 District boundary line angled across to Jones Point in Alexandria.

The U.S. Department of Transportation is proposing to pay 90 percent of the Wilson Bridge costs, as it did in 1976 when $800,000 was spent to repair the drawbridge's draw, with Virginia, Maryland and the District paying the remaining 10 percent.

However, local officials have objected to providing any funds for the current proposal, arguing that this is not normal maintenance and that the federal government should pay for a major widening and reconstruction to its own bridge.

District officials have said flatly the city does not have the money to pay for any of the bridge costs. Rep. Herbert E. Harris (D-Va.) introduced legislation last week to have the federal government pay all of the costs, saying "Local taxpayers have already spent millions for temporary repairs on the bridge . . . which is now more patched pothole than pavement. But (now) more than patching is needed."