I was thrilled to have a guest spot on Tom Scott's channel to discuss the Hyatt Regency Walkway Collapse and the relationship between a society and its engineers. Huge thanks to Tom for having me on his channel.

In the summer of 1981, the Hyatt Regency Hotel in Kansas City, Missouri hosted a large party in the multi-story atrium area. During the event, suspended walkways on the second and fourth floors collapsed, killing more than 100 people and injuring over 200 more. At the time, the accident was the worst structural failure in U.S. history. In the wake of the disaster, investigators discovered a change in the original design of the walkways that had been proposed by the fabricator and mistakenly approved by the structural engineer. What, at first glance, seemed like a subtle adjustment to the design turned out to be the root cause of the failure.

Two suspended walkways spanned the atrium in the Hyatt Regency, with the second floor walkway directly below the fourth floor walkway. Each was supported by a series of box girders suspended by hanger rods and retained by nuts and washers. The original design called for a single pair of hanger rods which would pass through each fourth floor girder to the second floor girders below. The fabricator responsible for constructing the walkways objected to this plan because it would require screw threads for the full length of the hanger rods which could be easily damaged during construction. So, they proposed to split the hanger rods into two sets: one to connect the fourth floor walkway to the roof, and one to connect the second floor walkway up to the fourth floor girders. If you don’t notice the significance of this change, you’re not alone. It was approved by the engineer without a detailed review or calculations which would have revealed its inherent flaw.

Imagine that you and a friend are hanging on a rope. The original design is the equivalent of you both holding onto the rope independently, whereas the design change is the equivalent of your friend holding onto your ankles. The total weight supported by the rope is the same in both cases, but your likelihood of maintaining a grip is not. This subtle change was identified by investigators as the primary cause of failure. With so many people on the walkways that evening, the load on the connections was too great. The box girders split open, slipping past the washers and nuts, leading to the collapse of both walkways.

There is an implicit handshake between a society and its engineers. We have hardly a choice but to trust that the constructed environment we live in is safe and sound. When an engineer seals a design, he or she takes responsibility for its accuracy and its safety to the general public. But, to err is human, and that includes engineers. So, we acknowledge that fact by developing conventions and processes that can catch and correct mistakes before they get too far. And that includes studying and learning to avoid the errors made in the past. The failure of the Hyatt Regency walkways is an important case study taught to nearly every engineer with the goal that such a tragedy will never occur again.