This project is being sought to alleviate traffic on existing bridges linking Illinois and Missouri in downtown St. Louis. The new bridge will be a cable-stayed bridge with three planes of cables and two single-pylon towers inclined nine degrees from the river and soaring 435 feet above the roadway. The structure will carry relocated Interstate 70 and Interstate 64 and will provide four traffic lanes plus two full shoulders in each direction, making it one of the widest bridges ever built across the Mississippi River at 222 feet. The overall length of the structure will be 3,150 feet and the main channel span, which will completely span the river and eliminate obstructions to river traffic, will be 2,000 feet long.

Modjeski and Masters, working with the Federal Highway Administration, the Illinois and Missouri DOTs, and nationally and regionally recognized experts such as Drs. Ian G. Buckle and Douglas Foutch, developed a consensus procedure for the seismic design of this structure. This included:

• Establishing seismic criteria including acceleration coefficients for several probabilities of exceedence

• Developing site-specific response spectra using guidance available in NEHRP publication 97, FEMA publications 302 and 303, and synthetic ground motion time histories from the MAE Center.

• Developing soil-spring constants to be used in the analysis

• Performing multi-modal seismic analyses using the site-specific spectra and spring constants

As the analysis progressed through final design, the following items were addressed: verifying previous soil data with new boring results; developing soil response models; interfacing with lateral spread mitigation efforts; developing ductile detailing requirements for the tower, superstructure and side span piers; and evaluating joint displacements and expected damage.

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