Saturday, February 14, 2009

Arch Bridges: Hernando de Soto Bridge

We'll continue our study of tied arch structures with the Hernando de Soto Bridge (named after the first European to see the Mississippi River).  This bridge carries I-40 over the Mississippi in Memphis, Tennessee.  It's different from the previous tied arch bridge we studied (the I-80 POC) because it's supported by two steel truss arches.  
The Mississippi River is so wide at this location that the bridge piers were built in the water. There are several steel girder approach spans and two 900' arch spans going over the river.  To cross in a single span would have required a suspension bridge, which would still have been inadequate when the Mississippi floods it's banks.
This bridge was opened to the public in 1972 and is currently getting a seismic retrofit due to it's proximity to the New Madrid Seismic Zone. The retrofit (designed by Roy Imbsen) is similar to that used for many long span bridges.  The superstructure is isolated from the substructure, in this case by large, friction pendulum bearings.  Isolation is a good solution for long span bridges because otherwise the large forces generated by the heavy superstructure are difficult to resist.  
Just as in yesterday's I-80 bridge, the axial forces in the arches are resisted in tension by the superstructure. The isolation devices are placed below the superstructure and reduce the lateral force being transfered to the substructure during an earthquake.
Creative Commons License
Arch Bridges: Hernando de Soto Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

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