Safety's Sleuths: The NTSB Investigation of the Minneapolis Bridge Collapse

Bridges are not supposed to fall down. But last August 1, the 1,900-foot-long bridge that carried I-35W over the Mississippi in Minneapolis came apart and landed in the river, carrying thirteen people to their highly unexpected deaths. We fear dying from a lot of things, but it is safe to say that nobody on that bridge that day spent a lot of time worrying about whether they would die as an interstate-highway bridge fell out from under them.

That very fact attests to the rarity, in this country anyway, of major structural failures in transportation-related public works such as bridges. One reason they are so rare is that for the last four decades, the National Transportation Safety Board has investigated accidents involving the nation's transportation infrastructure. In so doing, it performs a critical task that historian of technology Henry Petroski says is essential to the continued safety of engineered structures. In "Design Paradigms," a book of engineering case studies that includes three famous bridge failures, Petroski writes, "The surest way to obviate new failures is to understand fully the facts and circumstances surrounding failures that have already occurred." That is what the NTSB is doing now with regard to the Minneapolis bridge collapse.

The same day of the collapse, the NTSB dispatched a nineteen-member "Go Team" from its headquarters in Washington, D. C. to Minneapolis. As rescue and recovery work allowed, members of this team collected many kinds of information. They used an FBI-provided three-dimensional laser scanning device and gyro-stabilized high-resolution cameras to establish exactly where the parts of the bridge came to rest after the collapse. They collected names and recollections from dozens of eyewitnesses and secured the original of the famous security-camera recording that showed part of the bridge in the act of collapsing. By the third week of August, NTSB officials had interviewed over 300 people, including over a hundred who called in to a specially arranged witness hotline. In the following month or so, critical pieces of the bridge were removed to a nearby secure site for detailed inspection and investigation.

One of the most important tools currently available to the NTSB is powerful computer software called "finite-element analysis." This is a way to solve the fundamental materials-science equations that describe how steel (or any other solid) behaves under complicated conditions of stress. While it can't predict exactly where cracks will occur in an overstressed beam, it can reveal locations in a complex structure such as a bridge, where the local stresses exceed the tensile strength of steel. It is in such locations that cracks and failures are most likely to occur.

But as with any computer program, finite-element analysis software is only as good as the data you put into it. This is why the NTSB has spent the last three months gathering as much information as they can on not only the details of the bridge structure, including core samples showing how thick the deck was, but also other factors such as loading. You may recall that at the time of the collapse, a construction crew with heavy equipment was working on a portion of the bridge. The NTSB has concluded that a total of 287 tons of construction equipment and materials were on the bridge at the time of the accident. The exact location and weight of this extra loading is critical input to the computer analysis. The NTSB has made good progress in procuring such information by talking with eyewitnesses and viewing an aerial photograph taken by an airline passenger from a plane that passed over the bridge shortly before its collapse. Although the NTSB turned the disaster site over to the Minnesota Department of Transportation on Oct. 12, some thirty NTSB staffers are still working full-time on the investigation, which is not expected to be wrapped up for over a year.

Well-run operations are often taken for granted. Things could be very different. In places where there is nothing like the NTSB, disasters like this can be much more frequent, and citizens trying to affix blame have little if any recourse if something terrible happens to them or their loved ones. The NTSB could be corrupt, for example, or subject to bribes or falsification of its reports in response to political pressures. To my knowledge, however, its reputation for probity and "just-the-facts" scientific integrity is essentially spotless. This is no minor achievement, and the engineers who work for the Board have accomplished great things in the service of informing the both the technical public and the general public about the reasons for tragedies such as the Minneapolis bridge collapse.

Every major engineering failure marks the start of a detective story. Accident investigation is one of the few lines of work where engineers can spend their professional lives in the role of detectives. Now and then the culprit is a true criminal, but most of the time, accidents are due to inattention, bad communications, or inadvertent mistakes rather than any active will to do harm. Nevertheless, harm is done.

We will have to wait a while longer before we have the full story of how a part of I-35W suddenly lost altitude that hot August day. But it will be a story worth waiting for, because we can learn from it how to keep accidents like it from happening again.

Sources: The NTSB posts updates periodically on its accident investigations at its website. The latest such release about the Minneapolis bridge collapse was posted on Oct. 23, 2007 at http://www.ntsb.gov/pressrel/2007/071023c.html.