Andy Warhol (Seventh Street) Bridge

The Andy Warhol (Seventh Street) Bridge, an eyebar-chain, self-anchored suspension bridge, carries Seventh Street over the Allegheny River, the Tenth Street Bypass, and the Three Rivers Heritage Trail in downtown Pittsburgh.

Named for the famed artist who hailed from Steel City, it is one of the “Three Sisters” bridges constructed from 1924 to 1928—the only trio of identical, side-by-side bridges in the world—and is the first self-anchored suspension span constructed in the United States.

The bridge required rehabilitation due to accelerating age-related deterioration. The project involved replacing the bridge deck, totally repainting the superstructure, performing structural steel substructure repairs, and applying scour protection. The Allegheny County Department of Public Works chose Michael Baker International to perform analysis and design of the rehabilitation. The design team combined recognition of historical significance with modern engineering practices to complete a structurally superior, sustainable rehabilitation that was also aesthetically relevant and pleasing.

The bridge was analyzed for the first time using a fully 3D finite element model to examine the effects of unbalanced loading and modern vehicles on the structure. Completing the rehabilitation required numerous materials that are not normally used in new bridge construction, like post-tensioned tie-down anchorages, forged steel bridge pins and nuts, permanently lubricated bronze bushings and washers, and bronze dedication plaques cast to replace missing plaques. Workers used electric shear wrenches to install thousands of ASTM F3125 Grade F1852 high-strength bolts with button heads, to mimic the look of rivets, thus improving structural capacity while being sensitive to appearance. New bridge lighting on sidewalks and pylon rooms replicates the style of the original lighting fixtures. The new roadway curb boxes are designed to be as unobtrusive as possible while still allowing water to drain and prevent salt and debris from sitting on and corroding the stiffening girders.

The complex rehabilitation was performed as a conventional design-bid-build construction project and concurrent with road work on I-279/HOV lanes/North Shore Expressway. This necessitated well-organized traffic control for nearby PNC Park and Heinz Field (homes to the Pittsburgh Pirates and Steelers, respectively) events, maintenance of pedestrian crossings at the adjacent streets, and sustained access to riverside trails and adjacent businesses.

The bridge also had to act as its own lay-down yard, resulting in tight site conditions. Temporary underdeck shielding and coordination with the U.S. Coast Guard and local river users allowed safe river access. Notice was broadcast daily to mariners, and a monitored phone number and radio channels were established for large vessels. Temporary Duquesne Light (electrical) conduit enabled work on sidewalk brackets and replacement of electric conduits and supports. Temporary conduit in plastic corrugated pipe was placed on the sidewalk to maintain safe working conditions around energized lines, as well as to maintain a major power supply for downtown Pittsburgh.

The team used a variety of other construction innovations, including vibro-screed (air screed) and pump trucks to place the concrete deck, over-pouring the deck by ¼ in., subsequent grinding to provide correct cross slopes and longitudinal smoothness, and employing a temporary hold-down system using permanent post-tensioning rods. The new reinforced concrete deck is fully structural, using channel-type shear connectors to make the deck composite. The existing buckle plates, once the structural part of the deck, now remain as stay-in-place forms.

Project Team

• Steel fabricator and erector: Advantage Steel and Construction, Saxonburg, Pa. *AISC CERTIFIED*

• Engineer: Michael Baker International, Moon Township, Pa.

• General contractor: Brayman Construction, Saxonburg, Pa. *AISC MEMBER*

• Owner: Allegheny County Department of Public Works, Pittsburgh