Downsizing Bridge Technology
Franklin County, Ohio's First Cable Stay Bridge

Mark D. Sherman, P.E., Franklin County Engineers Office
David W. Jones, P.E., Jones-Stuckey Ltd., Inc.

Can an owner ever justify building a bridge that costs 50% more than a standard bridge? As the environmental process becomes far too cumbersome to justify working against it, the use of longer span bridges may relieve the strain on the relationship between engineer and environmentalist. The bridge engineer must work outside his knowledge base to understand all ramifications of replacing bridges in sensitive environments. The bridge engineer needs to find solutions that shorten the project time, which includes the environmental process. In Franklin County, the Engineers Office used this theory to build its first cable stay bridge.

The bridge replacement for Beach Road over Big Darby Creek had several environmental issues to overcome:

• The existing bridge was an historic wrought iron truss built in 1888. The single truss spans 175 ft. across the Big Darby Creek. The State Historic Preservation Office requested that the replacement structure relate to the historic nature of the truss and be the same language in design as the truss we were replacing. That is to say, it should be a single-span structure with above-deck supports leaving the river channel open and clear.
  
  
• The Big Darby Creek is both a State Scenic River and National Scenic River. For its protection, a well organized local group looks into any and all proposed work along the stream and its tributary.
  
  
• Wetlands located near the river did not allow for a significant modification to the roadway profile.
  
  
• The Big Darby Creek has no flood-control devices upstream from the site, resulting in very high peak water levels during storm events. The roadway profile would need to be raised significantly in order to allow the 100-year discharge under a conventional bridge.

A single span bridge addressed all of these issues and was acceptable to all the opponents, but "Why a cable stay bridge at this location?" While the initial cost is more than an owner would normally spend on a bridge this size, several cost saving factors made this type of bridge an a attractive solution. First, we save the time and money that would be spent on a lengthy environmental battle. Second, the site had shallow bedrock that assisted in reducing the foundation cost. Third, there was no skew on the bridge, which made the superstructure geometry simple. Additionally, the overhead suspension system allowed the superstructure to clear the 100 year flood, and the precast nature of the girders, floor beams and deck panels made for a relatively maintenance free bridge with a service life in excess of 100 years. It became obvious to those involved with the project that the two-tower (three span) cable stay configuration met all of the criteria for this application.

The new bridge is scheduled for completion in the spring of 2001. It consists of drilled shaft foundations for the towers and rock anchors for the end span tie-downs. The superstructure consists of prestressed, segmented, post-tensioned concrete edge girders, with prestressed, post-tensioned concrete floor beams, supporting precast, stay-in-place deck panels. The deck is cast-in-place with high performance concrete. The cables of the bridge are actually high-strength steel rods that are grouted into a 4-inch diameter HDPE tube. These bar cables are anchored individually at the top of the towers and the bottom of the edge girders. The towers are cast-in-place concrete supporting a structural steel anchoring plate assembly, which is encased in concrete after the cables are installed. To provide for thermal movement of the deck and edge girders, the back-span edge girders have a vertical link assembly that allows movement in the girder while anchoring the cables vertically into the bedrock directly below the link assembly.

The bridge was engineered by Jones-Stuckey Ltd., Inc. of Columbus and is being constructed by C.J. Mahan Construction Company of Grove City, Ohio. It was a unique solution to a very environmentally sensitive site. With its beauty emanating from its structural design, the bridge will serve the citizens of Franklin and Madison Counties for a long time to come.