Illinois Department of Transportation

I-255 over the Mississippi River

St. Louis, MO

The Illinois Department of Transportation needed to replace the I-255 over the Mississippi bridge because the existing undersized steel truss bridge had become obsolete and was prone to frequent closure due to flooding.

Benesch designed a new twin, tied arch structure consists of 15-span structures, each with 14 piers and two abutments. The superstructures are 52 feet wide, each consisting of an eight-inch reinforced concrete deck supported by 3,093 linear feet of welded plate girder approach spans and 910 linear feet of tied arch span over the navigation channel. Although box section arch tie girders were typical on bridges of this type, these structures utilized an I-shaped section. The replacement design also greatly simplified connecting details and fabrication while reducing the potential for harmful secondary stresses.

The I-255 structures received national attention, garnering various awards (including the Innovative Structure Award from SEAOI and the ACEC IL Eminent Conceptor Award) for their design, which enhanced the structures’ aesthetics while providing structural security.

The new structures were also embraced on a local level. The St. Louis Post-Dispatch described the bridge’s singular engineering qualities, stating that “Once in a while engineering and architecture, function, and form, utility, and beauty, meet as equals in a triumph of design. Everything in sight works and looks good at the same time.”

Project highlights

  • Twin tied-arch design utilizing I-shaped girder
  • Streamlined design that simplified connection details and fabrication while reducing secondary stress potential