John A. Heath, Heath & Lineback Engineers, Inc.
Hartsfield-Jackson Atlanta International Airport is in the midst of a major upgrade and facilities enhancement. The work includes a new parallel runway designed to accommodate the largest airplanes in service, design, or development. The new runway is located to the south of the existing four runways and requires a skewed crossing of I-285. The crossing includes bridges to carry the main runway, parallel taxiway, and a parallel non-licensed vehicle roadway.
The bridges are now nearing completion. They utilize a special prestressed concrete bulb-tee beam as one component of a unique design to support the large wheel loads anticipated for the next generation of wide-body passenger aircraft.
The bridges, which cross the existing eight lanes of traffic, are designed for a future expansion of the interstate to 18 lanes. The bridges were originally conceived as tunnels because the runway is as high as 70 ft (21.3 m) above the interstate elevation. The city of Atlanta elected to procure the project through a design-build process. The successful contractor, Archer Western Contractors, submitted a bridge design developed by Heath & Lineback Engineers, Inc.
The performance criteria for the design were based on loads assumed for the Airbus A380-900, which is currently under development. This airplane will have a total weight of 1,340,800 lb (5.96 MN) with a maximum load of 389,000 lb (1.73 MN) on each of six sets of wheels. Impact loads of 40 percent and longitudinal braking loads of 80 percent of live load were specified by the owner. The project includes a 500-ft (152-m) wide main runway structure and a 250-ft (76-m) wide taxiway structure. The designbuild team elected to span the interstate using a four-span arrangement with beams set normal to I-285. This results in the sawtooth pattern of the deck due to the skew of the runway as it crosses the interstate.
The design-build team selected precast, prestressed concrete beams as the primary superstructure members because of constructability and economy relative to other options. One consideration was that the design met the required four-hour fire rating without additional fire protection coatings or shields.
In order to carry such large live loads, the design used special 81- and 83-in. (2.06-and 2.11-m) deep bulb-tee beams with a top flange width of 93 in. (2.36 m) set on a beam spacing of 8 ft 0 in. (2.44 m) to achieve the maximum span length of 133 ft 0 in. (40.54 m) normal to the interstate. The beams contain as many as seventy eight 0.6-in. (152-mm) diameter strands and are based on design concrete compressive strengths up to 10,000 psi (69 MPa) at 56 days with release strengths up to 7,000 psi (48 MPa). The subcontractor, Standard Concrete Products (SCP), produced a total of 788 beams over a two year period. Their mix design was capable of achieving the required 7,000 psi (48 MPa) strength in 18 hours. The mix used Type III cement, and a low water-cement ratio achieved by using high-range water-reducing and retarding admixtures. Steam curing was typically used during the winter months. SCP used new stressing beds, forms, and handling equipment for the project.
Elastomeric bearings are used throughout and the beams are framed together with diaphragms at the ends and midspan. The large live loads led to the diaphragms being designed to include post tensioning for load distribution in the transverse direction. Cast-in-place concrete with a 28-day compressive strength of 6000 psi (41 MPa) was used for the 30-in. (760-mm) wide diaphragms that contain up to six tendons composed of twenty-seven 0.6-in. (152-mm) diameter strands. The deck used reinforced concrete with a 5000 psi (34.5 MPa) design compressive strength and included shear reinforcement for load distribution from the wheel loads. The combination of wide flanged beams placed side by side and cast-in-place diaphragms required the flanges to be blocked out for constructibility purposes in the region of the diaphragms, leading to the unusual shape shown in the photograph.
During the construction effort, the airport authority requested the addition of a maintenance vehicle bridge adjacent to the runway and taxiway structures. This bridge required spans of almost 170 ft (51.8 m) to cross the interstate. It was quickly determined that the beams used for aircraft loads with span lengths of 133 ft (40.5 m) were capable of carrying vehicular loads for the 170-ft (51.8-m) span lengths and a new design was developed for this configuration.
