The six-lane Hernando Desoto Bridge, built in the late 1960s, is 3.3 miles long with 164 spans, 160 piers and 10 abutments. It carries about 60,000 vehicles daily over the Mississippi River. The main bridge consists of two 900-ft-long steel tied arch spans over two shipping channels. Five 330-ft to 400-ft steel box girder sections complete the main crossing. Approaches and ramps are of precast concrete girders and plate girders spans.

Seismic. Structural steel, bearings and joints will help I-40 crossing hold up in a big earthquake.

The bridge lies about 60 miles southeast of the New Madrid Seismic Zone, the epicenter of three massive earthquakes in 1811 and 1812. To ensure the bridge can survive the next big one, work includes retrofitting footings, columns and pier caps. Structural steel will be braced to correct main span deficiencies such as overstressed truss members, insufficient resistance in the deck, excessive plastic hinges and inadequate reinforcing steel in footings.

Imbsen Associates Inc., Sacramento, Calif., the designer and owner’s representative, began preliminary work in 1992, says Fred H. Stephenson, Imbsen project engineer. Five phases worth about $90 million total are complete or under way. Phase 6, a retrofit of a 900-ft long precast concrete girder bridge at the St. Francis levee in Arkansas and crossing a floodplain, will be let this spring in a package with a 2,600-ft replacement steel plate girder bridge crossing a flood plain and railroad. “Our criteria is that the bridge can be shut down for up to two days for inspections but it cannot collapse,” says Stephenson.

“We are installing friction pendulum bearings on the main span and lead-rubber bearings on the approach spans,” he adds. “They will handle up to 22 in. of movement in any direction resulting from anything up to a 7.5 magnitude earthquake at the site. Special modular expansion joints help the structure absorb differential movement.”

Future contracts will finish the retrofits of both approaches and strengthen Memphis-side ramps. “The most vulnerable spans over water are done, so upcoming construction will all be on land,” says Robert A. Parrish, TDOT project coordinator.

W. L. Hailey & Co. Inc., Nashville, is working on a $23-million segment on the east approach. It will install six sheet pile cofferdams totaling over 1,700 linear ft with 482 driven steel pipe piles, and use 2.9 million lb of rebar cages, 2.3 million lb of steel casings and 20,700 cu yd of concrete. “We’re adding uplift and bearing piles,” says Andrew Swank, Hailey vice president. “Then we will expand the pier footprint to encompass the piles by adding rebar and concrete. The piers will receive a steel rebar cage and welded steel jacket, which is then filled with concrete. Caps will receive a cage and concrete.” Completion is slated for June 2007.

(Photos courtesy of Imbsen Associates)

eeking to avoid a potential seismic disruption to a key east-west highway, the Tennessee Dept. of Transportation and the Arkansas Highway and Transportation Dept. are halfway through a $175-million retrofit of an Interstate 40 bridge in Memphis. A contract worth up to $40 million in work will be let next spring, followed by up to five more segments for a possible 2010 completion.