WASHINGTON – The bridge rating system used in Maryland was established by the Federal Highway Administration following the fatal 1967 collapse of the Silver Bridge between Ohio and West Virginia.
The system was designed to guide the distribution of federal highway funds and to ensure that all states would evaluate bridges by the same standards.
“It’s important that all states have the same ratings so that the money is equitably distributed,” said Joseph R. Miller, chief of bridge inspection for the Maryland State Highway Administration.
But several state highway officials said although federal standards have brought some uniformity, bridge ratings are still subjective. Inspectors must make relative judgments about the strength of a bridge’s parts based on what they see.
Inspectors examine state bridges once every two years unless a structure has problems that demand more frequent attention. They look for cracks, hollow areas, rust, misalignment of joints and other problems.
Three parts of the bridge receive separate condition ratings: the deck, or road of the bridge; the superstructure, or upper portion; and the substructure, or supporting bottom portion, which includes piers, footings, fenders and abutments.
Inspectors’ ratings of these three features are entered into a computer program and result in an appraisal rating on a scale from two to nine. Nine is the best, according to a handout from the Federal Highway Administration.
The rating takes into account a number of criteria, including the amount of traffic a bridge carries and the total tonnage it can hold. For example, a bridge that carries 5,000 vehicles a day and is able to hold an 18-ton truck receives a structural appraisal rating of four, the minimum acceptable rating for the structure to be left in place as is, Miller said.
Bridges with a rating of three have deficiencies requiring immediate repair.
Those with a two rating must be replaced entirely, or be reconstructed considerably, said Maryland State Highway Administration officials.
“People think deficient means unsafe,” Miller said. “It doesn’t.”
When a bridge receives a low rating, highway administrators may reduce the size and weight of vehicles allowed to cross the bridge, restrict traffic to certain lanes of the bridge, or close the bridge until necessary repairs are made, said Valerie Edgar, public affairs officer for the Maryland SHA.
“Any time we find a structure that cannot carry a 6,000- pound vehicle we close the bridge,” Miller said.
He added the highway administration rarely closes bridges because repairs are usually completed before this is necessary. Sometimes, after traffic accidents damage a bridge, state police will close it until inspectors arrive and verify its safety.
Edgar noted that some bridges are given low ratings not because they are in bad shape, but because they don’t meet modern standards. Some are in good condition, but have only one lane or were constructed from timber, she said.