This guide is compiled by staff at the Minnesota Legislative Reference Library on a topic of interest to state legislators. It introduces the topic and points to sources for further research. It is not intended to be exhaustive. Shortly after 6 pm on August 1st, , the Interstate 35W bridge over the Mississippi River near downtown Minneapolis was loaded with rush hour traffic creeping through an ongoing construction project. Without warning, the bridge collapsed, taking with it vehicles.

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Download the PDF. US bridges have gotten some attention for potential structural issues. Some structural engineering experts have even suggested that up to US bridges are in need of remodeling. However, the last truly fatal major US bridge collapse occurred in Three major bridges collapsed in the United States that year:. The I Bridge stands out in this group for the tremendous damage it caused.

It was the most expensive of the three collapses. More importantly, it was the only one of the three that was fatal. The bridge also had poor past assessments from US regulatory agencies, raising questions surrounding state upkeep and maintenance procedures.

A hindsight view of the collapse offers new knowledge of how we should maintain bridges. Our Cause Map, built through our own unique, rigorous method of Root Cause Analysis, aims to provide all important information and assess potential solutions by determining the acceptable level of risk. If you were facilitating a Cause Mapping session all of those who might have knowledge of the I Bridge collapse an asked for the reason it fell, you would hear:. And our Root Cause Analysis experts write all of these ideas down objectively, then analyze them later in our Root Cause Analysis exercise.

This includes focusing on the specific time, date, and location of the incident. Fortunately, security cameras and traffic monitoring allow us to pinpoint the exact time when the bridge began to fall: p.

Aside from the collapsing bridge, we can also captures several key differences at the time of this incident as opposed to all the other times the bridge remained standing. These key fundamental differences help us better assess solutions once we complete our Cause Map. The bridge collapsed during evening rush hour, and there was roadwork underway on the bridge. The incident took place while at Bridge in Minneapolis, Minnesota, which crossed the Mississippi River.

The Safety Goal is where we will start. The fatalities and injuries are our major overall concern. So we add a question mark to that part of the description at the top of our Cause Map. That way, we can continue to consider this impact while we or other researchers continue to gather research about this impact. This results in a major inconvenience for citizens.

The last piece of information we want to record is the frequency of the incident. Catastrophic interstate bridge failures in Minnesota are infrequent.

Writing down goals is a key starting point for any Root Cause Analysis. The lives of those injured and killed in this incident are the top priority in this situation, so we start with our safety goal and work directly backwards to injuries and fatalities. This process continues as we build our Cause Map. Why did the main span of the bridge collapse?

Because the gusset plate fractured. This image shows a fracture in a gusset plate that played a key role in the collapse of the Interstate 35W bridge. National Transportation Safety Board photo.

You have probably seen these on bridges before. They are essential to the structural integrity of the bridge. Gusset plates support the joints of the bridge. And why did the gusset plate fracture? Because of the insufficient load capacity of the gusset plate.

Why was there insufficient load capacity? Because the calculations that determined the load capacity were not sufficient. Since the purpose of Root Cause Analysis is to eventually pinpoint and select acceptable solutions, it may help to add detail to your steps as you are building your Cause Map.

An accurate map is essential, but not always enough to help solve the problem as it relates to your actual goals. Because of the fatalities associated with this bridge collapse, we are going to add all the detail we can, especially in an effort to address our Safety Goal. We can add more detail in between the causes. For example, 13 people were killed because they were in vehicles that fell into the water.

That helps us, because we now have a better understanding of the situation as well as more room for solutions. A real Cause Map is starting to form. We can still improve it, though. Sometimes, a cause is preceded by two required causes. For example, to break a window requires both something to strike it and the glass to be fragile enough to break. The same can be said of the fracturing of the gusset plate. While the load capacity was a problem, a heavy load also had to be placed on the bridge to provide enough pressure to fracture it.

We can take this a step farther, too. Sometimes, when looking at a past incident, a lack of evidence may leave things uncertain. To represent that on the Cause Map, we draw a vertical relationship between causes as we did in the graphic above, but use the word OR instead of AND.

If we name particular causes, but later dispute them and rule them out, we can cross them off the list so we remember what we considered early in the investigation. Early on in the investigation several possibilities that could have caused the fracture of the gusset plate were considered, and later refuted.

Possible causes for the fracture of the gusset plate that were considered, and then refuted, were corrosion damage, preexisting cracking, and temperature effects. Then we face an even trickier possibility that we may rule out a cause because we miss a piece of evidence, later discover that evidence and have to reconsider the cause.

One way to remember why we may have done this in the first place is to add the evidence we used to rule out the cause below the box. Why was there an increased load on the gusset plate? Because of the concentrated weight over the gusset plate and increased load on the bridge. Why was there concentrated weight over the gusset plate?

Because there were construction equipment and materials on the bridge, concentrated over the gusset plate. The increased weight on the bridge was a result of an increased dead load on the bridge and the high volume of traffic on the bridge. Why was there a high volume of traffic on the bridge? As we will show on the Cause Map, it was rush hour, half of the lanes were closed, and there was an increase in use of the bridge from the time it was built.

Why were the lanes closed? A repaving job was underway. We use the most specific possible numbers as evidence for the increase in use, showing the exact number of vehicles it was designed to handle and how many crossed it on a regular basis at the time it collapsed.

The increase in dead load was a result of two previous repairs. Cause Maps are excellent at demonstrating not only potential causes and solutions, but also the shortcomings of previous solutions and unintended results that may come from that decision.

That way, organizations can keep that possibility in mind as they determine future solutions. One of the causes in our Root Cause Analysis is an unintended result from a previous decision. In , it was determined that there was an impact to the material and labor cost goal due to decreased life of the bridge. The decreased life was due to rebar corrosion from the rebar interacting with road chemicals.

The solution was to increase the concrete overlay to protect the rebar. From here, diving deeper into these causes and building out a more detailed Cause Map will give us a better handle on the issue of weight on the bridge vs. Why was the load capacity of the gusset plate insufficient? Three of these causes have their own immediate causes we can also draw out on the Cause Map.

The necessary calculations were not performed because the design was meant to be preliminary. The design firm review process was not followed because there was no procedure for ensuring all calculations had been performed.

As for the error going unnoticed, the State of Minnesota may be able to find really solid solutions in this part of the Cause Map. If an error goes unnoticed for 40 years, it would seem there are some basic checks that could be put in place to resolve the issue.

Why did the error go unnoticed? Even at this point, we can Cause Map useful, solution-focused steps to determine how the error went unnoticed in the load rating calculations. Gusset plates are not generally considered essential to the load rating and they were excluded to simplify overall analysis. The reason gusset plates are not considered important to the load rating is that there is a general assumption that gusset plates the key joints on the bridge are stronger than the parts they connect.

And generally, this assumption is correct. However, the gusset plates in this case were not properly sized, thus compromising the strength of the bridge.

As for the second cause of the error going unnoticed inadequate inspection of the gusset plates , we can extend our Cause Map and focus on three main reasons. The first is that the bowing displacement of the gusset plates went unmentioned. As you can imagine, displaced gusset plates can be detrimental to the structure of the bridge. Remember, gusset plates are joints that support other parts.

The bowing of the plates is believed to have occurred during construction. In addition to the bowing of the gusset plates, gusset plates were not listed as separate inspection elements and there was a lack of training on gusset plate inspection. These events occurred because the gusset plates were not considered an important part of the load rating. Does this mean that discounting the importance of gusset plates to the load rating is THE root cause in our Root Cause Analysis?

But it does show that when you are building a Cause Map to perform extensive Root Cause Analysis of an incident, you may see a cause in multiple places.


Root Cause Analysis of the I-35 Bridge Collapse

Infrastructure included. Months before the fifth busiest bridge in Minnesota tumbled into the Mississippi River, I saw the fragile and duct-tape useless condition of dozens of bridges in the Twin Cities. The tricky part: Getting my television bosses to listen. No one cares about bad bridges, aging sewer systems, neglected roads, sunken pot holes, until the boom happens.



The bridge opened in and was Minnesota's third busiest, [4] [5] carrying , vehicles daily. The NTSB cited a design flaw as the likely cause of the collapse, noting that a too-thin gusset plate ripped along a line of rivets, and additional weight on the bridge at the time contributed to the catastrophic failure. Help came immediately from mutual aid in the seven-county Minneapolis—Saint Paul metropolitan area and emergency response personnel, charities, and volunteers. Construction on the replacement bridge completed quickly, opening on September 18, The bridge was located in Minneapolis, Minnesota's largest city and connected the neighborhoods of Downtown East and Marcy-Holmes. The south abutment was northeast of the Hubert H. Humphrey Metrodome , and the north abutment was northwest of the University of Minnesota East Bank campus.

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