If school grades worked like the bridge rating system does, students would be much happier.

"(A) 70 is quite good; 77 is in really good shape," said Teri Pope, spokeswoman for the Georgia Department of Transportation's District 1 office in Gainesville.

"We don't start considering bridges deficient until they get into their 30s."

The "sufficiency rating formula" is a method of evaluating bridges by calculating several factors to derive a numeric value between 0 and 100.

The factors are:

• Structural adequacy and safety.

• Serviceability and whether it's functioning efficiently, taking into account such items as number of lanes, average daily traffic and structure type.

• How essential it is for public use, considering detour length and average daily traffic.

• Special considerations, such as traffic safety needs and if additional maintenance is needed, such as straightening the trusses on Boling Bridge at Ga. 53/Dawsonville Highway.

The 100-point scale is where the similarity to the schoolhouse stops.

"The sufficiency rating is not a pass/fail proposition," Pope said.

For example, a grade of 60 doesn't mean the bridge is failing and "absolutely does not mean a bridge needs replacing," she said. "A rating of 50 simply means the structure becomes eligible for limited federal funding for (rehabilitation) or replacement."

At 50, a bridge is deemed "structurally deficient."

The structure gets pegged for replacement as it drops to about 40.

For example, Boling Bridge, which is at Chestatee River on the Hall-Forsyth County line, has a 39.45 rating.

The bridge is slated for replacement in state fiscal year 2014, which begins July 1, 2013.

In the fall of 2010, the DOT spent $586,000 to repair the overhead truss system — the large green beams — on the bridge. The beams were severely bent because of vehicles such as bucket trucks and cranes hitting them again and again, year after year, Pope said at the time.

That expense has drawn some criticism from residents, who have said the DOT shouldn't have spent money on a bridge it intended to replace anyway.

Pope's response: "That work was essential to keep the bridge in safe working order until 2017 or so when the new bridge is open to traffic."

She said the DOT does have "the ability to extend the life of the bridges as they age and the ratings go down, by posting them for lower allowable weight loads."

In terms of the structural integrity, the three criteria most important in a sufficiency report are the ratings for the substructure, superstructure and deck conditions.

"A sufficiency report, however, will also give consideration to things like lane width, the presence of sidewalks and how long a detour might be if the bridge were closed — criteria that having nothing at all to do with the bridge's structural integrity," Pope said.

Also, the rating "has nothing to do with capacity or number of vehicles using the bridge," something noteworthy in Hall County, where traffic streams across bridges every day.