Ed Wasserman, Tennessee Department of Transportation
On July 6, 2004, bridge inspectors advised the Metropolitan Government of Nashville and Davidson County to close a 60-year old viaduct crossing the rail yard of the CSX Railroad; thereby eliminating one of the three viaducts leading into downtown Nashville from the west. The new viaduct was opened to traffic 2 years and 7 months later, having started from the initial planning stage. High performance concrete (HPC) played a vital part in the execution of this successful project.
Design Considerations
The viaduct to be replaced was 773 ft (236 m) long and 47 ft (14.3 m) wide and consisted of 13 spans of rolled steel beams supported on steel bents. The two maximum spans over the railroad were each 50 ft (15.2 m) long. As the railroad prohibited any at-grade crossing of the four tracks by construction equipment, it was decided that the replacement span over the tracks would be 130 ft (39.6 m) long. This provided for piers set at a clear distance of 25 ft (7.6 m) from the centerline of the outside tracks. The beams spanning the rail yard would have to be launched over the flanking piers. Because of the existing tie-ins of the viaduct deck with existing buildings at two critical locations, the proximity of the ends of the viaduct to intersecting streets, and the need to provide 23 ft 6 in. (7.2 m) of vertical clearance over the tracks, the substructure depth of the new bridge was critical.
Design Implementation
The final configuration chosen for design was a seven-span continuous bridge 773 ft 7 in. (236 m) in length, jointless from back-to-back of the full height abutments. The superstructure uses six lines of 48-in. (1.22-m) wide by 54-in. (1.37-m) deep box beams with a center-to-center spacing of 9 ft 8 in. (2.95 m) and a 58-ft (17.7-m) wide, 8-1/4-in. (210-mm) thick composite slab. The bridge is designed to be continuous for all loads applied to the composite section. The foundations for the integral abutment walls and the intermediate bents are composed of 42-in. (1.07-m) diameter drilled shafts, set in two pair clusters for the bents and three shafts for each abutment.
Depth restrictions necessitated the use of precast, prestressed concrete box beams 54 in. (1.37 m) deep over the rail yards for the 130-ft (39.6-m) long spans. The span-to-depth ratio of about 29 and the beam spacing required 10,000 psi (69 MPa) compressive strength HPC in combination with forty-six 0.6-in. (15.2-m) diameter, 270 ksi (1.86 GPa) low-relaxation strands with an initial jacking force of 2.02 million pounds (8.99 MN). The 54-in. (1.37-m) beam depth was used throughout the bridge.
The beam lengths varied from 73 ft 0 in. to 128 ft 3 in. (22.3 to 39.1 m) in the seven spans. Spans 1, 5, 6, and 7 required concrete with a compressive strength of 6000 psi (41 MPa) at strand release and a range from 7000 to 7750 psi (48 to 53 MPa) at 28 days. Span 2 with a beam length of 122 ft 3 in. (37.3 m) and spans 3 and 4 with beam lengths of 128 ft 3 in. (39.1 m) required concrete compressive strengths of 8000 psi (55 MPa) at strand release and 10,000 psi (69 MPa) at 28 days.
Concrete Mix Proportions
The following mix proportions were used by the fabricator to obtain the required 10,000 psi (69 MPa) compressive strengths:
Retarder was required when the ambient temperature was 85°F (29°C) or higher and the maximum slump was not to exceed 8 in. (200 mm) after the addition of the high-range water-reducing admixture.
Beam Production
Beams were produced on twin beds, on alternate days. The beams on the first bed were cast and the beams steam cured for 18 to 20 hours. During the curing period on the first bed, the beams on the second bed were prepared and cast.
Twelve beams 128 ft 3 in. (39.1 m) long and six beams 122 ft 3 in. (37.3 m) long were cast with the 10,000 psi (69 MPa) mix. The highest measured release strength was 12,510 psi (86.3 MPa), the lowest was 8590 psi (59.2 MPa), and the average was 9950 psi (68.6 MPa). The 28-day strengths were 12,540, 11,350, and 11,860 psi (86.5, 78.3, and 81.8 MPa) for the highest, lowest, and average, respectively.
Because the bridge was designed to be continuous for dead and live loads applied to the composite section, the negative moment over the interior supports, combined with the effective prestressing force required that 21 of the 46 total strands be debonded, in order not to exceed the 0.6 f’c stress limit on beam compression. This led to concerns that the number of debonded strands might compromise the shear capacity at the beam ends. A check of the shear capacity using the disturbed region by strut-and-tie methods as well as the sectional method led to two decisions. To accommodate the tension in the strut, the prestressing strands were extended 30 in. (760 mm) outside the beam end and bent up to be anchored in the cast-in-place diaphragm that acted as the closure pour between beams at the supports. No supplemental reinforcement was used in the strut. Additionally, the end diaphragm of the beams was increased from the normal 18 in. (460 mm) thickness to 48 in. (1.22 mm) and the shear reinforcement for the end region was designed as if the beams were not prestressed.
Closing Remarks
Bids for the new viaduct were taken on October 14, 2005, and the bridge completed on October 14, 2006—5 months ahead of the predicted date and within the predicted budget. Costs for the bridge were $111/ft2 ($1195/m2) for the superstructure and $31/ft2 ($333/m2) for the substructure. The ability to accomplish the rapid construction within the specified budget and satisfy context sensitive issues led to the decision to construct the bridge from concrete components. The precast, prestressed concrete box beams met the critical clearance requirements and allowed for timely delivery that the schedule demanded.
Further Information
For further information about this bridge, please contact the author at [email protected].