Developments in the engineering design and construction of truss bridges illustrate a history of engineering innovations. While historic bridges present these developments in a forum readily accessible for public viewing, only a small fraction of this engineering heritage remains. Historic truss bridges from the late 19th and early 20th centuries are vanishing rapidly. 60% of Colorado through-truss bridges in existence 20 years ago have been removed (Rutz 2004a) and it is estimated that 50% of the nation’s truss bridges have been removed over the same time period (DeLony 2005). At this rate of attrition, the American engineering legacy of the truss bridge may soon be relegated to the history books.
One avenue for preservation is rehabilitation of such bridges for pedestrian use. Conversion to pedestrian use permits ready public access to historic structures and has the added advantage of providing incentives for continued maintenance. Unfortunately, the engineer for today’s historic bridge preservation project often finds the bridge has insufficient lateral strength to satisfy modern requirements (Rutz and Rens 2004). This is due to two circumstances that combine together, hampering preservation projects:
- today’s design wind load is significantly higher than that used for the original design a century ago and
- the use of traditional structural analysis can lead to an incorrect conclusion that
wind load results in structure overstress.
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