Fred Conway, Alabama Department of Transportation
On the morning of January 5, 2002, a fully loaded fuel tanker, northbound on I-65 through Birmingham, AL, crashed into a bridge pier. The bridge did not collapse but the resulting fire that raged for several hours caused the steel bridge to sag about 8 ft (2.44 m). The accident occurred where the northbound and southbound lanes of I-65 cross over each other as part of a braided route-to-route interchange with I-59. The pier hit by the truck supported the southbound lanes of I-65 as they cross over the northbound lanes at a skew of 60 degrees. Since I-65 is the main north-south route through Alabama and carries an estimated 100,000 vehicles per day, a rapid replacement was needed.
After checking with local suppliers, it was decided to replace the bridge using precast, prestressed concrete girders and a cast-in-place concrete deck. The precast girders could be cast and delivered to the site before steel fabricators could procure materials and start fabrication of the steel girders. All concrete for the construction utilized high performance concrete (HPC) capable of attaining high early strengths to accelerate construction.
While redesigning the bridge, the Alabama Department of Transportation (ALDOT) decided to widen the bridge to allow for a future southbound lane and to lengthen the center span by 20 ft (6.1 m) to allow for a future northbound lane. As a result, the replacement bridge has span lengths of 75, 140, and 75 ft (22.9, 42.7, and 22.9 m) made continuous for live loads. To accommodate the longer main span length while maintaining the existing profile, the ALDOT design for the replacement bridge required 15 AASHTO Type IV girders spaced at 5.25 ft (1.6 m) centers. The specified concrete compressive strength was 9000 psi (62 MPa) at 28 days.
The contractor proposed a value engineering redesign of the girders for the main span utilizing modified BT-54 bulb-tee girders at the same spacing. The modification to the bulb-tee was to increase the width of the standard section by 2 in. (51 mm) to efficiently accommodate 48 0.6-in. (15.2-mm) diameter prestressing strands. The specified concrete strength at release of the strands was 7000 psi (48 MPa). Fifteen modified bulb-tee girders were used in the main span and eight standard width bulb-tee girders in each side span.
Speed of construction was a major factor in selecting the use of high strength precast, prestressed concrete girders for the bridge. The precaster was able to fabricate them offsite within 15 days and achieve the specified strengths.
For the deck, formwork consisted of stay-in-place corrugated metal forms. Specified compressive strength for the 7.25-in. (185-mm) thick cast-in-place concrete deck was 4000 psi (28 MPa) at 14 days. The continuous deck was constructed in five placements and cured with a curing compound. The last deck placement was 7 days old when the bridge was opened to traffic.
Specified compressive strength for the substructure was 3000 psi (21 MPa) at 28 days. This strength was achieved in 7 days. The bridge was completed on February 26, 2002, only 52 days after the accident occurred. The total construction time was 37 days. This construction period is the shortest construction time for a complete bridge project in the history of bridge building in Alabama.
The low bid for the project was $2,096,421 with an incentive or penalty of $25,000 for each day of early or late completion, respectively. As a result of finishing 53 days earlier than the 90-day completion time, the contractor received a $1,325,000 incentive payment – the largest cash incentive ever paid by ALDOT.
Further Information
For further information, see “Precast Helps Rebuild Bridge in Record Time,” ASCENT®, Summer 2002, pp. 32-34 or contact the author at 334-242-6007 or [email protected].