When is an Historic Bridge No Longer an Historic Bridge?

Designers working on historic bridge rehabilitation projects deal with conflicting objectives: rehabilitate and update the structure to meet current design codes and functional requirements or uphold the historic form, nature, and function of the structure. However, the requirements to satisfy each objective may be mutually exclusive. One obstacle commonly encountered is that current seismic code provisions were not required or even understood for 19^th century bridges. In rehabilitating the bridge to meet current seismic requirements, it may not be possible to maintain the historic form of the structure. At a certain point, the question arises as to how much can be changed before the structure loses its overall historic nature. If a bridge is no longer truly "historic", is the added expense justified in comparison to replacement? In other words, does a shell of an historic bridge satisfy the requirements for maintaining an historic bridge with just its original form but not true function? Is that bridge still considered "historic"? When is an historic bridge no longer an historic bridge? This paper will discuss and introduce the problem of historic bridge rehabilitation in general terms, and then present some examples. The paper concludes with a discussion addressing the title question and the tradeoffs.