Thomas A. Andres, Florida Department of Transportation*
Hurricane Ivan made landfall as a 130 mph (210 km/h) hurricane (Category 3) in the early morning of September 16, 2004, just west of Gulf Shores, Alabama. The east wall of the hurricane made a direct hit on the I-10 bridge over Escambia Bay located east of Pensacola, Florida. The 90 mph (145 km/h) sustained winds produced a 12.9 ft (3.93 m) storm surge, which topped the approach spans and created buoyant forces on the structure. As a result, the bridge suffered extensive damage with multiple spans and piers falling into the bay. Approximately 58 spans were completely destroyed and 66 spans were misaligned with 35 pile bents missing or destroyed.
Because the bridge serves as a vital east-west link for interstate commerce, immediate repairs were necessary. On September 17, 2004, an emergency contract was awarded by the Florida Department of Transportation (FDOT) to repair the existing bridges, so that the I-10 bridges could open to traffic. The repair consisted of constructing new pile bents and moving existing eastbound spans to the westbound bridge. This allowed the westbound bridge to be opened in just 17 days with one lane of traffic in each direction. The existing eastbound bridge was then retrofitted with 3,720 ft (1.13 km) of temporary steel trusses to replace the spans lost during the hurricane and the spans salvaged for the westbound bridge. On November 20, 2004, two I-10 westbound lanes and one eastbound lane were opened 27 days ahead of schedule.
Congress approved emergency funds to replace the existing retrofitted bridges and the FDOT issued a Notice-to-Proceed on April 20, 2005, for the $242.8 million Design-Build Project for the I-10 Bridges over Escambia Bay. Both the eastbound and westbound structures will be 13,868 ft (4.23 km) long with a 59-ft (18-m) wide deck consisting of simple span approaches and a three-span, post-tensioned unit over the channel. While the vertical clearance for the existing bridge is 10 ft (3.1 m), the new approach spans will have a minimum vertical clearance of 25 ft (7.6 m) to allow for storm surge and wave height from future hurricanes.
The superstructure of the typical approach spans consists of 78-in. (1.98-m) deep Florida bulb-tee girders at 12 ft 6 in. (3.81 m) centers with a span length of 136 ft (41.5 m). Specified concrete compressive strengths are 6000 psi (41 MPa) at transfer and 8500 psi (58.6 MPa) at 28 days. The deck thickness is 8.5 in. (215 mm) including a 1/2-in. (13-mm) thick sacrificial layer.
The three-span post-tensioned unit has span lengths of 166, 250, and 200 ft (51, 76, and 61 m) and utilizes a drop-in unit in the main span. The depth of the girders on either side of the main span taper from 9 ft 6 in. (2.90 m) over the piers to 6 ft 6 in. (1.98 m) at the drop-in unit. The specified concrete compressive strength is also 8500 psi (58.6 MPa).
The project makes extensive use of precast components for the pier footings, bent caps, pier caps, prestressed concrete piling, and Florida bulb-tee girders. Due to the extremely aggressive coastal conditions and in order to achieve a service life of 75 years, the high performance concrete (HPC) provisions of the FDOT’s Standard Specifications were used. The HPC requirements include high-early strength, low permeability, workability through admixtures, low heat of hydration, and minimal shrinkage and thermal expansion. These attributes were achieved through the use of additives such as fly ash, silica fume, water reducers, and air-entraining agents. In Florida, durability is achieved through reduced concrete permeability and appropriate reinforcing steel cover. Low permeability concrete was achieved by using fly ash (18-22 percent by weight of cementitious materials). In addition, silica fume (7-9 percent by weight of cementitious materials) was added to the substructure concrete within 14 ft (4.3 m) of the mean high water elevation. In general, the FDOT combines the use of HPC with good detailing practices, crack control designs, and the employment of advanced concrete curing specifications to achieve durable structures.
The use of HPC in the superstructure combined with 0.6-in. (15.2-mm) diameter, Grade 270, prestressing strands at 2-in. (51-mm) centers allowed for longer approach span lengths with shallower members. Similar benefits were achieved in the post-tensioned, continuous, dropin channel span.
The I-10 Escambia Bay project encountered a setback on August 29, 2005, when the 28-ft (8.5-m) high storm surge of Hurricane Katrina swept through the plant of one of the precast concrete suppliers damaging offices, production equipment, and the batch plant. Production has now resumed for the Escambia Bay Project. The first bridge is scheduled to be opened by December 2006 and the project completed by December 2007.
*Contributions to this article were made by Jonathan Van Hook and Rafiq Darji.