Rt 34B over Salmon Creek Bridge Replacement

A two-lane highway bridge replacement project in Central New York became a banner structure for the state.

The new State Route 34B bridge over Salmon Creek in Lansing, N.Y., keeps its predecessor’s aesthetics but is the longest and tallest slant-leg rigid frame structure in New York State. The 500-ft bridge is made of modern 50-ksi weathering steel, giving it a 100-year expected service life.

The old bridge was a three-barrel arch structure with a rustic appearance, and weathering steel was the perfect option for the replacement to mimic its look and feel as defined by the State Historic Preservation Office (SHPO). Steel provided the flexibility to design and construct a unique bridge that gives the community something worthy of replacing a register of historic places-eligible structure. It also lent itself to a rapid erecting process that limited closures and detours for a central roadway in and out of the Ithaca, N.Y., region and to Cayuga Lake attractions.

By selecting a slant-leg rigid frame structure, the SHPO invited several unique design challenges. The knuckle region has a large, unbraced height and a large axial and flexural demand. The supporting legs of the structure are slanted and spring from thrust blocks embedded in the bedrock below. These legs have high axial, shear, and flexural forces, uncommon in typical steel structures. The intersection of the leg members with the roadway girders contained the highest axial and flexural forces within the structure and an extremely high web height.

Because of the axial and flexural forces within the main span and legs, the splice location could not be optimized at the points of dead load contraflexure and was analyzed for these axial and flexural forces. Erection was done with both knuckles attached to the main span and lifted as one piece, fitting on the previously installed legs. The legs were supported during construction with a concrete pad and counterweights on the approach above. Post-construction, they’re supported on pinned shoe bearings containing seven steel-finger plates.

The depth of the gorge the bridge crosses suited the slant-leg frame, because bedrock was relatively high and offered a sound foundation for the legs. The visual appeal when looking up from the gorge is striking.

Project Team

• Owner/structural engineer: New York State Department of Transportation, Albany, N.Y.

• General contractor: Tioga Construction Company, Herkimer, N.Y.

• Steel team:

  • Fabricator: Canam-Bridges, Claremont, N.H. *AISC full member; AISC-Certified fabricator*

  • Detailer: DBM Vircon, Phoenix *AISC associate member*