Six bridges on secondary roads in Prince William County are in such poor condition that replacing them should be considered, according to a report by the Virginia Department of Highways and Transportation.

Because of funding problems, however, only one of the six will be recommended for replacement in the county's proposed six-year plan for secondary roads that goes before the county Board of Supervisors next week for final action. Two of the other bridges are being improved without being replaced, while no funds have been proposed for work on the remaining three, officials said.

"It's a matter of setting priorities for the limited funds," said David E. Ogle, resident engineer in VDHT's Manassas Residency. "I consider a roadway that gets choked with traffic every day a higher priority than a bridge that can carry less than the standard 20 tons but has been posted properly."

The six bridges are built of timber decks and steel beams, and three are single-lane. They have been posted for weights ranging from five to 12 tons. Some of the bridges date to the late 1800s. In most cases, the overall structure has deteriorated, with rust having eaten through the steel and the abutments that support the bridges have begun to crumble.

Ogle said the bridges are safe, if their weight limits are observed. Posted bridges, he said, receive twice-a-year inspections.

"If a bridge weren't safe, we'd repair it or close it," he said, noting maintenance funds always are available for short-term repairs.

"By law, maintenance is our first priority," he said.

Prince William County has 110 secondary bridges and about 600 miles of secondary roads. Its 1982-83 budget for secondary road improvements, under which bridge replacement falls, totals $2.2 million and drops to $1.9 million for 1983-84. The maintenance budget, meanwhile, will rise from $2.9 million for 1982-83 to $3.5 million for 1983-84, in keeping with a new law that requires that maintenance be given highest priority.

Recommended for replacement is Bristow Road Bridge (Rte. 619) over Cedar Run. Last year, it served a daily average of 1,694 vehicles, was posted for 12 tons and had a sufficiency rating of 2 out of a possible 100 points, making it one of the two worst bridges in the county.

A major factor in that rating, Ogle said, was that the bridge provides an important link in Rte. 619 as a cross-county connector and no other road can serve as an adequate detour.

Even though the bridge's 1983 sufficiency rating was revised to 17.3 out of a possible 100, making it the county's fourth worst bridge, Ogle said, it is unlikely that the project will be dropped from the proposed six-year plan. He said the bridge's importance, coupled with the fact that $50,617 already has been spent on surveys and engineering to prepare for the project, make it top priority.

The estimated cost of the project is $850,000, making it the most expensive of the six, according to Ogle. The other five bridges are:

* Spriggs Road Bridge (Rte. 643) across Powells Creek, currently one lane. Last year, it had a rating of 2 out of 100 points, with a daily traffic count of 3,092 vehicles. Spriggs Road is not a major connector and much of its traffic comes from Montclair residents whose use should decrease when another road opens later this year, Ogle said, so no funds have been allocated for its repair.

* The one-lane Ingram Drive Bridge (Rte. 686) over Chestnut Lick. It is posted for five tons and averages 54 vehicles a day. No funds have been allocated for its repair.

Lightner Road Bridge (Rte. 685) over Catharpin Creek. The bridge, providing the only access to one part of the community, has been posted for 12 tons and has a daily traffic average of 114 vehicles. No funds have been allocated for its repair.

* Fleetwood Drive Bridge (Rte. 611) over Cedar Run, which served 324 vehicles a day last year. Under the revised ratings it now ranks as the county's worst bridge, with a rating of 3 out of 100 points. State highway officials say that because it has been repainted and partially redecked it should not require replacement.

* Logmill Road Bridge (Rte. 701) over Chestnut Lick, which carried 54 vehicles a day last year. Its major deficiency is the roadway's approach alignment. Funds have been set aside to improve the bridge without replacing it.

While the Spriggs Road Bridge could be replaced for about $70,000, replacements for the Ingram, Lightner, Fleetwood and Logmill bridges would run about $100,000 each, according to Ogle.

Ogle expressed hope that Prince William County will be able to use revenue-sharing funds to pay for the bridge improvements that it otherwise would be unable to fund.

"The county's road needs are far outstripping funds available through the state, primarily from gas taxes," planning director Roger W. Snyder said.

Snyder said the county can handle deficient bridges several ways: The highway department can reduce the bridges' weight limits; the county can add them to the six-year plan at the annual review, despite its funding shortfalls; the county can repair them with revenue-sharing funds; or county supervisors can follow Fairfax County's example and sell road improvement bonds, an approach that would require a referendum.

Financing the repairs or replacements with local funds, however, flies in the face of the board's long-held position that the state is responsible for maintaining county roads and bridges.

Snyder said he could not predict whether the Board of Supervisors would decide to use revenue-sharing funds to replace the Spriggs Road, Ingram Drive and Lightner Road bridges, the three for which funds have not been allocated.

"When you don't have enough money to go around," he said, "you have to set priorities."