Author(s): L. W. Zevenbergen; D. W. Robinson; P. F. Lagasse
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Abstract: On August 1,2007, the I-35W bridge over the Mississippi River in Minneapolis, Minnesota, failed catastrophically, killing 13 people and severing a vital link in the Minneapolis-St. Paul transportation network. As cleanup was underway and investigators searched for the cause of the collapse, the Minnesota Department of Transportation (Mn/DOT) began to focus on rebuilding, knowing how critical it was that traffic flowed smoothly as soon as possible. Ayres Associates performed the hydraulic and scour analyses for the replacement bridge as part of the design team led by Figg Bridge Engineers for Flatiron-Manson, on this design-build project. Even though the extremely aggressive construction schedule required the hydraulic analysis be completed in less than three weeks, Mn/DOT required two-dimensional (2-D) numerical modeling in addition to the standard HEC-RAS modeling for “NoRise” certification. Several factors dictated the use of 2-D modeling. The I-35W bridge is immediately downstream of a Mississippi River lock and dam. During extreme floods the distribution of flow may vary depending on the operation of the lock chamber and gates. As a result the flow distribution at the bridge also varies. Mn/DOT also required that there would be no adverse hydraulic or scour impact on the 10t h Avenue bridge located immediately downstream of the I-35W bridge. A downstream impact can only be assessed through 2-D modeling. Other potential impacts were also evaluated at two major storm tunnel outlets located at the crossing. This paper highlights the results of the two-dimensional modeling that provided detailed information on hydraulic conditions throughout the bridge reach and demonstrated that the new bridge would not adversely impact other structures. The 2-D modeling was also used to determine the scour potential for the new bridge design.
Year: 2009