The Shipping Incident Case Study

There was an injury that occurred at this shipping and loading facility. They wanted us to do a mechanical design and root cause analysis of what in this system failed and how to cause this injury. So it was at a shipping and receiving facility where they had sort of a unique inverted caster floor system that the wheels were pointing up and they could move the pallets around on the floor with really relatively minimal effort, but they still had to get the shipping pallets out of the trucks onto that floor. So they had designed a custom winching system that would rig up to the pallet, drag it out of the truck onto the caster floor so that they could get it to its next destination. The rigging system consisted of what's called a double stud anchor, and this is an example of a double stud anchor that's installed into some L track.

Now, sometime during the process of dragging it out using the winch, there was a mechanical failure and this lock body piece from this double stud anchor wound up going through someone's leg that we know exactly which component became a projectile. We just need to figure out how it became a projectile and if that was as a result of improper operation and rigging or as a result of improper system design. Unfortunate aspect of this investigation is that a lot of the original evidence wasn't retained, so we didn't have access to the specific cargo or pallet that they were hooking up to understand how the failure occurred at that location. We actually have video of the incident, which is always helpful and not helpful because it gives you a picture of one half of the incident and obviously the other half happens inside the truck. On the video footage, you can see the injured person collapse after their leg is injured, and from the X-rays that we have, we have a component of the double stud anchor is actually embedded into their left leg, causing a commuted or a severe fracture to their tibia and also a fracture to their fibula.

So because we didn't have the original evidence that we could analyze, we had to go to exemplars to understand how this component failed. In this system, we contracted with a local fabrication and testing facility that built a testing jig to our specifications so that we could pull on this piece in all kinds of different orientations to see what kind of failure mode we got. Can we replicate the failure mode to get this specific piece to become a projectile? So instead of the double stud anchor pulling directly out of the L track as one piece, the L track would fail and kind of buckle under it and it would start to bend and impinge on the particular piece. That did become a projectile.

We built the jig. We really understood inside and out the failure mechanism, and we were the only people to get the insight into the two-stage failure mechanism. Once we had that insight, we went back to the video and we could actually see that there was evidence of a two-stage failure. If you know what you're looking for, you can see a little bit of wobble of the back of the truck. You can see the crane operator look. He looks back at the winch and continues pulling, and then the projectile comes. There was testimony that the floor locks on the truck had not been released, so it was essentially a situation where we have a winch pulling on a movable object. So there's already a failure in the rigging and then a failure in giving the okay signal to the winch operator to begin pulling on the load. And then once you begin pulling on the load, you recognize there's been the first stage failure and you continue pulling on the load to the two stage failure. It becomes a improper rigging and winch operation as opposed to the design of the system, which if you don't undergo this two stage failure and pull against a movable objects, it performs as expected.

‍