Weight limit is lowered on Black River bridge

LITTLE ROCK — An Arkansas bridge of the same design as a bridge that collapsed four years ago in Minnesota and that has been posted with lower weight limits until it can be replaced doesn’t have the problems that led to the 2007 tragedy, according to the Arkansas Highway and Transportation Department.

Last Monday, the weightlimit on the U.S. 63 bridge over the Black River in Lawrence County was posted at 33 tons for five-axle trucks. The weight limit on unposted bridges is 40 tons, but an evaluation team from the Highway Department recommended the new posted limit based on an engineering analysis of the structure and an on-site investigation.

Replacement of the 2,608-foot-long bridge, which carries 10,000 vehicles a day, is scheduled to begin in 2013. Until then, state highway officials said, they would continue to monitor and evaluate the existing bridge.

Effects on travel in the area should be limited because other nearby bridges already are posted at lower weight limits, the department said. Four small bridges west of the bridge over the Black River are posted between 26and 36 tons.

But the Arkansas Motor Carriers Association said the posted limit still would inconvenience truckers, especially ones who aren’t from the area.

“It’s a big cross-over route for over-the-road drivers from Memphis to Springfield, Mo.,” said Lane Kidd, the association’s president. “It’s a major corridor. They should’ve made the repairs necessary towhere we didn’t have to reach this conclusion.”

The U.S. 63 bridge over the Black River is a deck-truss design, which is a structure supported by steel beams in the shape of an arch below the driving surface.

The bridge in Minnesota was an eight-lane, steel truss arch bridge that carried Interstate 35W across the Mississippi River in Minneapolis.It collapsed suddenly during the afternoon rush hour on Aug. 1, 2007, killing 13 people and injuring another 145.

But Arkansas highway officials point out significant differences exist between the Minnesota bridge and the Arkansas bridge.

“They are similar in design, but you can’t compare them,” said Randy Ort, a department spokesman.

For one, the bridge over the Black River has just two lanes compared to the eight lanes on the I-35W bridge, which was built in 1967.

For another, a 2008 report from the National Transportation Safety Board said weight on the bridge was a major factor in its collapse. Construction was ongoing at the time of the accident.

They found that 24 “under-designed” gusset plates went undiscovered in reviews during the bridge’s design and construction. Gussets are used to tie steel beams together and are attached using rivets or bolts. Significant increases in the bridge’s weight because of improvements through the years, along with increased weight on the bridge due to construction equipment and rush-hour traffic the day of the collapse, likely contributed to the failure of the faulty plates, the report said.

The U.S. 63 bridge overthe Black River is one of five deck-truss bridges that remain on Arkansas highways and the only one posted with a weight limit. The department hasn’t used the decktruss design in decades.

Its superstructure is rated structurally deficient. All three components on a bridge - the deck and substructure are the other two - are rated on a zero-to-9 scale used in the department’s visual bridge inspection rating system. Zero is the worst and 9 is the best.

Any component rated below 5 classifies a bridge as structurally deficient, which doesn’t make the bridge unsafe but rather means that deterioration, cracks or other flaws can be seen in the bridge components to the point that they need to be monitored or repaired. The superstructure is rated at a 4, which meansit’s in poor condition, said Glenn Bolick, a department spokesman. The deck is rated at 5, which means it is in fair condition, while the substructure is at 6, which means it has a satisfactory rating, he said.

When bridge superstructures reach a 4 rating, they typically are posted with lower weight limits. Bridges are designed to accommodate some lateral movement, Bolick said. The concern is that when bridges age, the lateral movements created by heavy trucks “don’t match up anymore,” he said. “You have to have less weight so the lateral movement is all in sync.”

Arkansas has four other deck-truss bridges on state highways. Only one is considered structurally deficient - the bridge on Arkansas 5 at Norfolk in Baxter County,which was built in 1936.

The newest Arkansas deck-truss bridge, taking U.S. 62 across Table Rock Lake in Carroll County, was built in 1952. The remaining two deck-truss bridges on statemaintained roads are at U.S. 62 at Imboden in Lawrence County, built in 1937, and at U.S 62 at Pyatt in Marion County, built in 1948.

A sixth deck-truss bridge - which carries U.S. 65 across the South Fork of the Little Red River at Clinton in Van Buren County - is being replaced as part of a project to widen U.S. 65 to five lanes between U.S. 65B and Arkansas 336. An $11.7 million contract was awarded in January 2009. The project is scheduled to be completed in early 2012, said Bolick. The bridge was the oldest of the decktruss designs in Arkansas. It opened to traffic in 1931.