By ThinkReliability Staff
On the evening of July 4th, after watching fireworks, revelers at a park in Merrillville, Indiana headed back to their cars over a pedestrian bridge. The bridge became overloaded and collapsed when two suspension cables snapped. Somewhere between 50 and 120 people fell into the lake. Although 25 were treated for injuries, nobody was killed, thanks to quick action by nearby lifeguards, police officers, firefighters and other rescuers who formed a human chain to help get everyone safely out of the water. We’ll use this as an root cause analysis example. A thorough root cause analysis built as a Cause Map can capture all of the causes in a simple, intuitive format that fits on one page.First we complete the outline. The problem is a bridge collapse. It happened at 10:00 p.m. on the 4th of July, while there were large numbers of people on the bridge. It was a pedestrian bridge in Merrillville, IN, and people were crossing it to return home after a party.
Once we have defined the problem we list the impacts to the goals. People being injured is an impact to the safety goal, as is the potential for drowning. People fell into the lake, which was an impact to the customer service goal. Additionally, the loss of the bridge is an impact to the material and labor goal.
We begin our Cause Map by listing the impacted goals and asking “why” questions to fill out the Cause Map to the right. Begin with 5 “why” questions to start the Cause Map. This is known as the “5-whys” technique. For example, the safety goal was impacted. Why? The safety goal was impacted because people were injured. Why? People were injured because they fell into the lake. Why? They fell into the lake because the bridge collapsed. Why? The bridge collapsed because the suspension cables broke. Why? The cables broke because the weight on the bridge exceeded the bridge capacity.
Even more detail can be added to this Cause Map as the analysis continues. As with any investigation the level of detail in the analysis is based on the impact of the incident on the organization’s overall goals. For this investigation, we can add some more detail to the “5-why” Cause Map to help our investigation. For example, pedestrians fell into the lake because the bridge collapsed AND because pedestrians were on the bridge, returning to their cars after the 4th of July party.
There may have been additional stress on the bridge due to pedestrians jumping up and down, as reported by witnesses. Additionally, we can add more detail after the “weight exceeded capacity” on the bridge. The bridge was built to hold 40 people, but “at least twice that” were on the bridge when it collapsed. So many people were on the bridge because they were returning to their cars (as discussed above), and because of ineffective crowd control. There were too many people on the bridge despite officers stationed on either side. Why was the crowd control ineffective? It’s not known at this point, but we’ll put a question mark here. The next step of the investigation will be to replace that question mark with reasons for the ineffective crowd control. Once we’ve done that, we can come up with solutions that will keep an event like this one from occurring in the future.