Structural damage assessment and management

Failure analysis report: The I-35W Mississippi River bridge collapse

Given the existence of safety codes within the United States as compared with the developing world, it might be assumed that a structural failure would be a relatively rare event. However, a remarkable failure occurred within the nation's heartland, in the form of the I-35W Mississippi River bridge collapse in 2007. As noted in an editorial in the LA Times: "the collapse of an interstate highway bridge over the Mississippi River in Minneapolis took everyone, including engineers, by surprise. We do not expect our bridges to drop out from beneath us, and their designers take great pains to ensure that they do not" (Petroski 2007). The bridge was well-traveled every day and no previous reservations had been noted regarding its structural soundless. The sudden and catastrophic nature of the collapse was completely unexpected and unanticipated.

The usual method of failure prevention is to anticipate failure, which, in the case of a bridge, is likely to be a steel member becoming unable to carry its load. "The intended load on a bridge consists of two distinct parts. The so-called dead load is the weight of the steel and concrete that makes up the structure itself. This can be on the order of 80% or more of the total weight that a bridge is expected to bear" (Petroski 2007). This weight includes traffic, ice, snow, wind, earthquakes, and the pressure that can incur from other natural disasters. "The size and configuration of structural elements, like beams and girders, are chosen to give the bridge more strength than it needs under a worst-case scenario, a concept known as a 'factor of safety'" (Petroski 2007).

Bridges with a cantilever design had failed in the past, including 1967 collapse of the 41-year-old Silver Bridge across the Ohio River, which could have potentially suggested problems with this type of bridge in hindsight. However no bridge design is fail-safe (Petroski 2007). "No matter how carefully bridge designers anticipate failure on the drawing board (or computer screen), their structures will only be as reliable as how carefully built, maintained and inspected they are. Just because a bridge has given decades of successful service under adverse conditions of increasingly heavy traffic and neglect does not mean that it will continue to do so. It is the function of regular and careful inspections to catch what designers might not have anticipated" (Petroski 2007). The I-35W Mississippi River Bridge was built in 1967 and when it was last inspected in 2006 it was found to have "no major structural defects or deficits…We were told the deck would have to be replaced in 2020" according to Minnesota Gov. Tim Pawlenty ("9 thought dead as Minneapolis bridge collapses," MSNBC 2007).

The reasons for the I-35W Mississippi River Bridge failure were cited as a fundamentally faulty design that could not be detected during ordinary inspections designed to detect normal wear and tear. "The designers had specified a metal plate that was too thin to serve as a junction of several girders, investigators say. The bridge was designed in the 1960s and lasted 40 years. But like most other bridges, it gradually gained weight during that period, as workers installed concrete structures to separate eastbound and westbound lanes and made other changes, adding strain to the weak spot" (Wald 2008). In other words, the bridge was quite literally an 'accident waiting to happen.' Inspections could detect deterioration but inspectors did not fundamentally evaluate the bridge's structural soundness. Only such a far-reaching evaluation could have predicted the catastrophic collapse which occurred. According to the final safety commission report on the bridge "undersized gusset plates were found at eight of the 112 nodes (joints) on the main trusses of the bridge. These 16 gusset plates (2 at each node) were roughly half the thickness required and too thin to provide the margin of safety expected in a properly designed bridge" (Butcher 2008).

The nature of the disaster also highlights how the risks of a faulty design are not always immediately evident upon its construction. The bridge gained weight over time, adding to the strain in a manner that was not dangerous during its first years of existence. However, the designers should have anticipated this fact when making the final drafts for the construction of the bridge. The bridge ultimately only lasted forty years, a relatively short span of time for a major work of infrastructure. The collapse happened at the worst time possible: not only was it a high-traffic time, but a nearby baseball game was drawing tourists. "There were two lanes of traffic, bumper-to-bumper, at the point of the collapse. Those cars did go into the river" ("9 thought dead as Minneapolis bridge collapses," MSNBC 2007). Ironically, there was also some minor structural repair taking place at the time of the collapse, further adding to the lives lost and endangered (as well as the weight on the unstable bridge). "Repairs, which closed at least two of the eight lanes, involved guardrail replacement on the joints and concrete work" ("9 thought dead as Minneapolis bridge collapses," MSNBC 2007).

The fact that the bridge's collapse was due to faulty design stimulated a fundamental reevaluation of the bridge inspection program in the United States. Warm-weather inspection and repair programs usually only inspected for "corrosion and age-related cracking, but that was not the problem in the Minneapolis collapse," and the failure to evaluate structural soundness could have prevented the incident (Wald 2008). It is important for inspectors to note how safety codes have been evolving over time, and that in the past bridges may have been built with design flaws that would not 'pass muster' now. It is also very difficult to anticipate how well a structure will 'age,' as was also evident in the Minneapolis bridge collapse. It was advised highway departments "re-analyze the design of bridges before carrying out major work on them. Previous practice has been to assume the design was sound, but to inspect for age-related deterioration" (Wald 2008). The necessary "calculating loads and looking at designs" that could have identified the flaw was not part of the inspection program's standard operating procedures (Wald 2008).