MEMPHIS, TN (AP) The Hernando DeSoto Bridge may be one of the safest places to be if the “big one” hits, thanks to a $170-million overhaul to help it withstand an earthquake.
The 30-year-old bridge, which opened to traffic in 1973, is in the middle of a seismic reconditioning program that will provide it the strength and flexibility to handle earthquakes resulting from the New Madrid fault line.
The redesign includes new bearings that will allow its deck to slide — under control — 22 in. in any direction the shock waves send it, and an additional 2,400 cu. yds. (1,835 cu m) of concrete to strengthen the underwater footings of each of the massive piers holding up the bridge.
“The bridge is going to be the safest place in Memphis,” said Fred Stephenson, structural engineer and project consultant.
The bridge, a tied-arch span that carries Interstate 40 traffic over the Mississippi River, was built years ago to withstand strong winds but without the benefit of more recent projections about the damage Memphis could suffer from a large New Madrid earthquake.
The retrofitting costs much less than the $300 million to $400 million estimated price of building a new span over the Mississippi, Stephenson said.
Work will extend from the bridge’s Memphis side to the river levee in Arkansas. The Tennessee Department of Transportation began the project in January 2000, has spent about $72 million so far and is currently on its fourth retrofitting contract. Nine more contracts are scheduled to be awarded over the next six or seven years.
So far, new bearings have been installed on all eight bridge piers in or near the water. The bearings are located between the top of the piers and the bottom of the superstructure that includes the highway decking.
Included in the work are “friction pendulum” bearings, some of which weigh 22 tons (19.8 t). They resemble a stainless steel dog dish, with a half-globe slider resting in the bottom of the bowl. The bowl is connected to the pier underneath, the slider to the superstructure above.
The slider could move 22 in. in any direction during an earthquake, but because of gravity, would return to rest in the bottom of the bowl.
The work going on now involves Kansas City-based Massman Construction, which built the piers in the late 1960s and has returned to strengthen the piers’ footings.
The current contract, which will complete the retrofit for the portion of the bridge that’s over the water, will be finished in about 18 months, said Bob Parrish, assistant to TDOT’s regional director.
Next, workers will extend the project to a bridge section over the parking lot of the Pyramid arena, home to the NBA’s Memphis Grizzlies.
“They are very tall piers in the parking lot. They’re vulnerable,” Stephenson said. Steel shells will be placed around those bridge piers.