Charleston County has the highest percentage of substandard bridges in the state, with the worst of the bunch being the twin drawbridges over the Ashley River, according to AAA Carolinas.

Some 42 percent of county bridges are either functionally obsolete or structurally deficient, according to the organization.

However, the public need not worry, said Lee Floyd, state bridge maintenance engineer.

“It doesn’t mean that the bridge is unsafe. Anything unsafe would be restricted to a safe (weight) level or closed. We have a good bridge-inspection program,” he said.

Floyd said functionally obsolete and structurally deficient are U.S. Federal Highway Administration terms that raise a red flag.

The only bridges in Charleston County identified in the AAA Carolinas report as having both issues are the double drawbridges over the Ashley.

The drawbridges are in good shape, Floyd said. The state Department of Transportation is monitoring erosion around part of their foundation caused by tidal currents, he said.

“It seems to be holding its own. It’s not getting any worse. We know about it and we’re keeping an eye on it. It’s nothing to worry about,” he said.

Highway engineers have also identified an erosion issue affecting the foundation of the Interstate 526 bridge over the Wando River, which is not on the substandard bridge list. Sheet piling will be driven around the affected pilings, which will be filled with an underwater concrete, he said.

Structurally deficient is defined as being in relatively poor physical condition and/or inadequate to handle truck weight. Functionally obsolete means a bridge has an inadequate design for current traffic.

Floyd criticized the AAA Carolina rankings for creating worry among motorists. The organization manipulates data to get the ratings, he said.

“It’s a just a formula that they invented themselves. It has no application in the real world. They create a lot of unnecessary angst,” he said.

AAA Carolinas has been rating the state’s bridges since 1998, giving extra emphasis to the amount of traffic a bridge carries because of the effect on commuters and travelers. The rankings are based on sufficiency ratings and traffic counts provided by the state DOT, the organization said.

David E. Parsons, president and CEO of AAA Carolinas, could not be reached for comment.

In a prepared statement, Parsons said, “Inadequate funding for road and bridge maintenance over the past decade means we still have a significant number of substandard bridges in South Carolina. We need to find new sources of funding for our state’s Department of Transportation.”

South Carolina needs $2.9 billion for its bridge system in the next 20 years, Floyd said.

“That’s not going to be easy to come by,” he said.

The AAA Carolinas report lists six bridges on Interstate 26 as substandard, including two in North Charleston. One of them crosses S.C. Highway 7 and the other spans Midland Park Road.

Seven Charleston County bridges are listed in the 2013 South Carolina Substandard Bridge Ranking Top 20. The bridges are given a sufficiency rating that is based on structural adequacy and safety, serviceability and functional relevancy based on current traffic volume and importance for public use.

The Ashley River drawbridges have the lowest sufficiency rating in the county at 47 percent. The S.C. 171 Bridge over Wappoo Creek is next, with a sufficiency rating of 50 percent, followed by the S.C. 7 bridge over the Ashley River, with a rating of 65 percent.

Spartanburg County has 137 substandard bridges, the most in the state.

Charleston County has 115 bridges in that category, the third-highest in South Carolina.

The average age of the top 20 substandard bridges in South Carolina is 53 years, and the bridges carry an average of 53,645 vehicles daily and 7.5 million vehicles each week, according to AAA.

South Carolina has the lowest highway funding per mile in the country and the fourth-largest state-maintained highway system, covering more than 41,000 miles. Some 22 percent of 8,157 bridges in the state are substandard, according to AAA.

In 2007, the state DOT began shifting more funding toward bridge replacement and rehabilitation, from about $65 million a year to the current level of $119 million.

The Legislature is searching for additional revenue and new taxation sources to help improve the inadequate annual funding for the state’s transportation infrastructure, but efforts to reach a passable transportation budget have proven difficult so far.

Reach Prentiss Findlay at 937-5711.