Today’s guest video is from Grady at Practical Engineering.
He’s talking about a tragedy:
and the lessons we learned and the reasons why it happened are fascinating.
Also, he made his own props.
Grady, take it away.
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 over 100 people and injuring over 200 more.
At the time, the accident was the worst structural failure in US history.
In the wake of the disaster,
investigators discovered a change to the original design of the walkways
that was 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 girder below.
The fabricator responsible for constructing the walkways objected to this plan
because it would require screw threads for the entire length of the hanger rods,
which could easily be 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.
In this setup, which represents the original design,
the load of the two walkways is independently transferred to the hanger rods.
Notice how I can lift each girder without affecting the other.
Now, let’s look at the design change.
In this configuration,
notice that the entire weight of the second-floor walkway is being borne
not directly by the hanger rod,
but instead, by the girder above.
If I lift the fourth-floor walkway, the second-floor walkway is lifted as well.
The hanger rods are still carrying the same load at the top,
but the two nuts on the upper girder are supporting the weight of both walkways.
This simple change effectively doubled the load on those bolted connections.
Imagine that you and a friend are both 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 hanging 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 hardly have a choice but to trust that the constructed environment that we live in
is safe and sound.
When an engineer seals a design,
he or she takes responsibility for its accuracy and safety to the general public.
But the err is human and that includes engineers.
So, we try to develop conventions and processes
that can catch and correct from mistakes before they get too far.
And that includes studying and learning from 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.
Practical Engineering is filled with great videos, so go subscribe!
I would recommend starting with Grady’s video on hydrostatics.
a guest video with a prop that has been to the moon.