Nearly 1,800 of Virginia's highway bridges are too old or too weak and shough be repaired or replaced, a transportation, lobbying group reported yesterday.

The Road Information Program (TRIP) cited examples of bridges in the state, including several in Northern Virginia, that have been found "structurally deficient" or "functionally obsolete" -- although not necessarily unsafe, according the TRIP's report.

"Virginia's road agencies are being forced to postponed these urgently needed repairs because of their tight budgets," said R. M. (Buck) Buchanan, a representative of the Virginia Road and Transportation Builders Association that commissioned TRIP's bridge study.

The state Department of Highways and Transportation, whose engineers provided much of the data for the survey, concurred in the findings but called the group's estimate of problem bridges conservative.

"We already have nearly 4,000 bridges on which we have had to lower the posted load limit," said Albert Coates, the department's spokesman.

TRIP, a national organization of highway contractors, construction equipment manufacturers, insurance companies and others with a financial interest in the condition of bridges, estimated it would cost $91.3 million a year over the next 15 years to put Virgini's bridges into acceptable condition.

During the 1980 General Assembly session, the department told lawmakers it lacks funds to maintain the highway system, let alone the state's bridges.

The TRIP report said an estimated 758 bridges are "structurally deficient" and cannot handle the maximum vehicle weights now authorized on the roads leading to the bridges. Another 1,006 bridges are "functionally obsolete" and are hampered by narrow clearances, sharp roadway approaches and too few lanes for existing traffic, the report said.

"Virginia's bridges are wearing out faster than they can be replaced with existing funds." Buchanan said. "Bridge repair is expensive, but because of inflation it will cost us far more later to catch up."

Among the Northern Virginia bridges said to need repairs are the Jefferson Davis Highway bridge over the RF&P railway yard in Alexandria; the Telegraph Road bridge over Route 236 (Duke Street) in Alexandria and the Payne Street bridge over Accotinik Creek in Fairfax.

TRIP, which compiled its information from local state and federal reports already submitted, emphasized that there is little danger of bridge collapse since most spans were constructed to handle "reasonable overloads."