OPAC Consulting Engineers

Three Mile Slough and Steamboat Slough Bridges

Sacramento Delta, California


California Department of Transportation


Design Completed 1995

Scope of work:

Seismic Retrofit Design

Construction cost:


Services Performed:

  • Seismic retrofit design studies


  • Seismic retrofit of lift bridge
  • Seismic retrofit of bascule bridge
  • Steel member capacity evaluation

OPAC was a subconsultant to DeLeuw, Cather & Company to perform seismic engineering services for the existing Three Mile Slough Vertical Lift Bridge and the Steamboat Slough Bascule Bridge for the California Department of Transportation (Caltrans). OPAC's work scope for the project included:

  • Modeling approach consultation.
  • Steel member capacity evaluation.
  • Seismic retrofit strategy development.

These two bridges were characterized by Caltrans as "difficult" seismic retrofits. OPAC provided guidance on modeling issues pertaining to these moveable bridge systems, determined the capacities of built-up, latticed, and rolled steel members, and assisted in retrofit strategy development.