A bridge engineer watching a crack (previously described as “tight”) under the Grand Boulevard bridge noticed it had extended to 20′ on May 6, 2016. He immediately ordered the bridge closed, requiring the rerouting of the more than 9,000 vehicles that use the bridge every day. Replacing the bridge is estimated to cost $5 million.
Luckily, due to the quick action of the engineer, there were no injuries or fatalities as could have occurred due to either the bridge catastrophically collapsing while in use, or for motorists on the Interstate below being struck by large chunks of concrete falling from the overpass.
The overpass failure can be addressed in a Cause Map, or visual root cause analysis. The process begins by capturing the what, when and where of the incident (a bridge failure May 6 in Kansas City) and the impacts to the goals. Because there was the potential for injuries, the safety goal is impacted. The re-routing of over 9,000 vehicles a day is an impact to the customer service goal. The closing of the bridge’s overpass/ sidewalks is an impact to the production goal, and the cost of replacing the bridge is an impact to the property/ labor goal.
By beginning with an impacted goal and asking ‘Why’ questions, cause-and-effect relationships that lay out the causes of an incident can be developed. In this case, the impacted goals are caused by the significant damage to the bridge, due to a rapidly spreading crack.
The failure of any material or object, including all or part of a bridge, results from the stress on that object from all sources overcoming the strength of the object. In this case the stress on the bridge was greater than the strength of the bridge. Stress on the bridge results from each pass of a vehicle over the life of the bridge. In this case, 9,300 vehicles a day transit the bridge, which has been in service since 1963.
Stress also results from large trucks traveling over the bridge. The engineers suspect this is what happened, possibly due to an apartment construction project near the bridge. Says Brian Kidwell, an assistant engineer for the Missouri Department of Transportation, “My hunch is a very heavy load went over it. It could have been a totally legal load.” A “hunch” by an experienced professional is included in the Cause Map as a potential cause. This is indicated with a “?” and requires more evidence.
Legal loads on bridges are based on the allowable stress for a bridge’s strength. However, the strength of the bridge can change over the years. It is likely that happened in this case. Previous damage has been noted on the bridge, which also required bracing last month to fix a sagging section. However, the bridge was deemed “adequate” in an inspection eight months ago. Any needed repairs may not have occurred – there’s never enough money for needed infrastructure improvements. It’s also possible that water entered the empty cylinders that make up the part of the span of the bridge (this is called a “sonovoid” design) and they could have filled with water and later frozen, causing damage that can’t be easily seen externally.
For now, more information will be required to determine what led to the bridge failure. At that point, bridges of similar design may face additional inspections, or be replaced on the long waiting list for repairs. For Kansas City, some are taking a broader – and bolder – view and are recommending the older section of the Interstate “loop” be removed altogether.
To view the Cause Map of the bridge failure, click on “Download PDF” above. Or, click here to learn.