Unless repairs are made promptly on the deteriorating Woodrow Wilson Bridge, it may have to be closed to heavy trucks, the District of Columbia's transportation director has warned. If that is done, as many as 12,000 trucks a day may have to find detours around -- or, even worse, through, Washington.

Thomas Downs, the transportation chief, told the D.C. City Council's Transportation Committee that the concrete deck of the Potomac River span at Alexandria has become so fragile that the Maryland Highway Administration may have to cut the present 40-ton truck weight limit in half in the next several months.

That would eliminate all of the 18-wheel tractor trailers that shuttle up and down the Eastern Seaboard on Interstate Rte. 95, and sever one of the nation's premier freight arterials.

Although maintained jointly by Maryland, Virginia and the District -- with Maryland responsible for the deck -- the bridge is owned by the federal government. Congress has not provided the $65 million necessary for total redecking. Downs said prospects are dim for obtaining it earlier than next year.

If tractor trailers are barred, alternate routes would have to be found, Downs said. Going around the western and northern side of the Capital Beltway is currently blocked by a ban on trucks through the section of highway between North Chevy Chase and North Bethesda. And putting them on Washington streets would cause intolerable safety and enviromental hazards, Downs said.