The construction of Interstate 579 (I-579) through Pittsburgh, Pa., more than 60 years ago separated the Hill District from the downtown. With the help of federal grant money from the Transportation Investment Generating Economic Recovery program, the Urban Open Space Cap project creates a new modern park with room for outdoor events and reconnects the disenfranchised district to the city’s economic core.
The I-579 project functions as a “cap” over the interstate that provides a walkable link from the Hill District to the downtown. The new park includes art installations, story walls, outdoor classroom space, and an amphitheater, as well as bike and pedestrian pathways.
After evaluating several options, the design team selected adjacent precast concrete box beams for several reasons: The beams are strong enough to support the unique park loading, including up to 5 ft of soil. Locally available bridge elements were economical. And precast concrete box beams were ideal for the urban site because they could be delivered with minimal site disturbance and lifted from adjacent parcels to minimize the impact of the construction on the highway below.
Because the structure is located over the interstate, minimizing future bridge maintenance was a high priority. The precast concrete beams do not need to be painted and will be durable. The beams also provide a smooth soffit for aesthetics under the bridge, and they offer a consistent surface for attachment of the under-bridge lighting system.
The highway spanned by the new structure varies in width due to the presence of four ramps merging and diverging from the main roadway. Several of these ramps include retaining walls on spread footings, and it was important to ensure that the project would not have an adverse impact on these components. Very little gore area was available between the roadways to accommodate piers for the new structure. The new structure and park also had to tie into the existing sidewalks, which involved 20 ft of elevation change.
The design team for the bridge structure and park addressed these challenges, and others, through innovative structural and landscape design solutions. New abutments were constructed behind the existing retaining walls, which were trimmed to accommodate the new beams. These abutments were supported by over 200 drilled micropiles, which were used to minimize disturbance to the existing walls. Adjacent prestressed concrete box beams were used for the superstructure, which was divided into three units to allow transverse post-tensioning of the beams, and better match the span configurations of the adjacent structures. An 8-in.-thick cast-in-place concrete deck slab was placed on the beams to act compositely and achieve full load-sharing among the beams. The deck contained two layers of reinforcement steel in each direction to provide a robust system. The cast-in-place concrete deck included redundant waterproofing features to protect the entire superstructure. New piers were founded on drilled shafts that could be installed in narrow gore areas and transition directly into slender multicolumn bents, to limit impact to the roadway template. Compatibility with requirements of the Americans with Disability Act was established for the significant grade change across the park through walkway switchbacks into the tiered raingardens at the northwest corner.
Bridging the Gap
“Of all the challenges this project faced, developing a viable structural solution for the bridge was the greatest hurdle. The new bridge had to ‘fill the gap’ between the two existing vehicular bridges, while maintaining the required vertical clearance above the interstate and carrying sufficient soil depth to sustain plantings on the surface,” says Nicholas Burdette, PE, Northeast Region bridge lead, HDR. The project team developed the preferred precast concrete adjacent box-beam solution to minimize initial costs, limit the bridge’s impact on existing structures and interstate traffic, and use low-maintenance components.
The complexity of the constrained urban site was exacerbated because there are no standard design codes for bridges carrying a park. The project team developed specific criteria to meet the demands associated with the intended use of the park area. They evaluated both the final in-service condition of the bridge and the construction loadings associated with placing the fill and amenities on the bridge.
Working within the congested urban core was a major challenge and required careful planning. This project included 14 phases of traffic for demolition, substructure construction, and superstructure construction work, including installation of more than 1 million lb of reinforcing bars; 126 box beams, each weighing 140,000 lb;1800 ft of curved architectural walls on top of the deck structure; 50,000 ft² of reinforced architectural sidewalks; and 14,000 ft of micropiles to support the abutments.