HARRISBURG — Pennsylvania has more than double the national average rate of structurally deficient bridges, but they aren’t always the older ones.

In fact, more than 40 bridges that are more than a century old have strong ratings by the Pennsylvania Department of Transportation.

And more than 70 built since 1980 have poor ratings, according to The (Harrisburg) Patriot-News.

Officials say there are more than 5,000 structurally deficient bridges in the commonwealth despite expenditure of more than $2.7 billion since 2008 to repair and rebuild bridges. An advisory penal recently warned that more bridges are becoming structurally deficient each year than the state can afford to repair or replace.

PennDOT officials stress that structurally deficient bridges are safe to use, and bridges are closed once they are judged unsafe. But age is by no means the only factor in when that happens.

Take the 73-foot Frankford Avenue bridge over the Pennypack Creek in Holmesburg, northeast of Philadelphia. It’s the oldest bridge in continuous use in Pennsylvania, having been commissioned by William Penn in 1697.

Even though it carried members of the Continental Congress on their way to Philadelphia, the Frankford Avenue bridge isn’t one of the state’s structurally deficient bridges. The structure is rated “fair” by PennDOT inspectors, meaning the bridge is sound but showing minor cracks and signs of deterioration.

But then there’s the 161-foot bridge in Northampton County that was built in 2004 and is already structurally deficient — even though it carries on average 2,000 fewer cars than the oldest bridge in the state.

Transportation department spokeswoman Erin Waters calls it “an extreme case” due to a “unique geologic formation” below that makes the area prone to sinkholes.

Mother Nature, she said, plays a “huge” role in how well some bridges stand up.