NO. 5 BRIDGE: Selmon Extension

Dec. 2, 2021

Elevated bridge creates greater capacity for congested boulevard in Tampa

(This feature published as "Community Enhancement" in November/December 2021 issue)

The Selmon Extension provides a vital connection between Hillsborough and Pinellas counties in the Tampa Bay region.

It allows drivers to connect to the 16-mile Lee Roy Selmon Expressway-—which is owned by the Tampa Hillsborough Expressway Authority (THEA)—without hitting a stoplight between Brandon and Pinellas County.

The challenge was to build the elevated toll road in the median of Gandy Boulevard (S.R. 600/U.S. 92) without closing lanes during peak travel hours. The heavily trafficked boulevard, located in South Tampa, is an urban four-lane divided FDOT roadway connecting the Gandy Bridge over Tampa Bay to the eastern end of the Lee Roy Selmon Expressway. Utilized by regional traffic traveling to/from Pinellas County as well as local users, Gandy Boulevard was very congested, particularly during peak hours.

THEA worked with the community on a different type of solution—an elevated toll lane located in the median of Gandy Boulevard that took no businesses or homes.

The innovative design of the Selmon Extension, with 30 ft of vertical clearance (twice as high as a typical urban bridge), allowed easy access and visibility for businesses along the Gandy corridor. The sleek design takes the Extension bridge out of a driver’s line of sight—allowing the driver to see businesses and turn lanes from both sides of Gandy Boulevard.

THEA created a maintenance of traffic plan that minimized lane closures for the duration of the project. Business driveways were not affected and all turning movements that were there before the project will remain after completion.

THEA designed a precast segmental bridge where the bottom of the bridge will be 30 ft above the roadway. This brings the bridge out of the sight line of drivers and preserves cross-corridor visibility for the business on Gandy Boulevard. THEA also reduced the number of piers supporting the structure—again for increased visibility for the businesses.

The Kiewit team proposed an elegant extradosed concrete segmental viaduct for most of the length along Gandy Boulevard, transitioning to steel tub girders painted to match the segmental portion at the interchange on the east end of the project. The bridge foundation consists of 55 piers and caps supported by four redundant drilled shafts for each pier. The extradosed tendons (similar to a cable-stayed bridge with the cables encased in concrete finbacks) allow a shallow superstructure and longer spans between piers, which is particularly critical at the West Shore and Manhattan intersections. A total of 744 segments were precast off-site and transported to the project site for erection and post-tensioning into the viaduct structure.

The viaduct had to be constructed within a constrained urban corridor to minimize lane closures along the heavily traveled boulevard. After THEA performed a detailed safety analysis considering span length, sight distance, and roadway clear zone criteria and a commitment to maintaining access to existing cross streets and driveways, fixed pier locations were determined.

Coordination was also required with CSX Railroad on the east end of the project at the rail crossing. THEA also replaced and upgraded the railroad crossing gates and signals.

“The Selmon West Extension continued THEA’s legacy of providing innovative and unique solutions to meet the Tampa Bay area’s transportation needs,” THEA General Engineering Consultant Project Manager Jim Drapp told Roads & Bridges. “The project has not only exceeded the projections for usage by the traveling public, it has also enhanced the community for the local residents. Working with our design-build partners Kiewit and AECOM, THEA provided a one-of-a-kind, world-class solution to meet the transportation needs of the community with an aesthetic and technically superior bridge that will become a Tampa landmark.”

By removing the “pass-through” traffic coming to and from the Gandy Bridge, the Selmon Extension creates at least 35% greater capacity on Gandy Boulevard for neighborhood traffic and customers of Gandy Boulevard businesses.

Project: Selmon Extension

Location: Tampa, Florida

Owners: Tampa Hillsborough Expressway Authority (THEA)

Designer: AECOM

Contractor: Kiewit

Cost: $230 million

Length: 1.9-mile elevated bridge

Completion Date: April 19, 2021

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