The highway transportation network of the United States relies on the health and integrity of major infrastructure elements such as bridges. Frequently traveled parts of Oregon are within the seismically active Pacific Northwest and many of the bridges were designed and built to lateral demands that were assumed to be less than the current expectation, a deficiency caused by our growing awareness of seismic hazard and our enhanced understanding of the non-linear response of bridges. This vulnerability to damage from earthquakes can result in not only immediate damage, but also in potentially lingering economic impact caused by the disruption to traffic and freight mobility.

Using analytical methods, fragility curves were constructed assuming lognormal capacity and demand distributions. Probability of failure was determined for the four damage state conditions of slight, moderate, extensive, and complete levels of damage. These statistical values were compared to the median and dispersion values proposed by other researchers, in addition to those calculated using guidelines from the HAZUS Technical Manual. Older multiple-span Oregon bridges were found to be significantly more fragile than the HAZUS models.

As a result of this modeling and analysis effort, the relative fragility of the modeled typical 3-span and 5-span bridges was determined and quantified. Possible causes of the relatively high fragilities were also considered.