1Assistant Professor, University of Massachusetts, Amherst
In its latest report, the National Bridge Inventory (FHWA 2000) revealed that 691,060 bridges currently exist in the United States. The Federal Highway Administration rated nearly 30% of these bridges as substandard. Despite this discouraging figure, detailed information on the number of U.S. bridges that have failed or were in a severe condition is not readily available elsewhere. The New York Department of Transportation is thus far the only agency that is attempting to collect information and develop a database on bridge failure cases in the United States .
The problem of structural redundancy focuses on the phenomenon of a local failure mushrooming to a global failure. For buildings, the problem was identified around 1969 (Ronan Point) and the engineering community started from scratch to treat it. The equivalence for bridges started the same time, around 1967 (Silver Bridge Collapse due to fatigue) and keeps on getting the media attention all the way until few years ago (Skagit River Collapse, 2013) due to collision of a truck to a truss member. Although equivalent research work on buildings has advanced significantly during the last years, for bridges there are still big knowledge gaps.
An interesting, and often forgotten, aspect in assessing the safety of bridges is that the structural scheme of a bridge is changing over time, either slowly (e.g. because of deterioration) or relatively fast (e.g. because of scouring of the foundation) or very fast (e.g. after a damaging event). With changes in the structural scheme, the collapse mechanism, can also change over time introducing uncertainties in the assessing the safety of the bridge.
The question which rises from all the above is: Is the current state-of-practice and research adequate to deal with the problem of bridge redundancy and robustness? There are many bridges which are considered extremely redundant according to today’s standards, BUT ARE NOT simply because they are not generally progressive collapse safe. Unfortunately, the number of bridge collapse failures vastly out number and out cost the expectations set by the design by a wide margin.
The outcome of research in the field of bridge redundancy can be of extreme value to bridge engineers in assessing the current safety of bridges and possible future rehabilitation/maintenance. The study can become a groundbreaking research effort with major impact on the efficiency, safety and reliability of highway bridges. It could cover a broad area of research fields from progressive collapse to structural health monitoring. In that sense the results can be of major interest to every structural engineer. Especially structural engineers or agencies who are involved in bridge maintenance/analysis/design will be very interested in using the new methodologies.
 Federal Highway Administration (FHWA). (2000). ‘‘National bridge inventory record count 1998–2000.’’ <http://www.fhwa.dot.gov/bridge/allbridg.htm> (Dec. 5, 2002).
 Kumalasari, W., Hadipriono, F.C. (2003). Analysis of Recent Bridge Failures in the United States. ASCE Journal of Performance of Constructed Facilities: 17; 144-150.