Today, we look at the Kansas City skywalk failure. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Do you remember the failure of the skywalk in a new Kansas City Hotel in the Summer of 1981? A weekend crowd was dancing to big-band music in the hotel atrium. Some were on the floor. Others strolled on three crowded skywalks above the atrium. Two of the skywalks suddenly gave way. 114 people were killed, and 200 more were injured.

That sort of accident is so rare -- a seemingly unprovoked building collapse. Twentieth-century thinking being what it is, the first order of business was fixing blame -- finding out who was entitled to sue whom for what. The National Bureau of Standards finally issued a 300-page report a year later. It leads us on a fairly tortuous path through a sequence of three minor complacencies on the part of otherwise honorable people.

The walkways were hung from the ceiling on steel rods. On one side of the atrium, one walkway was suspended below another. In the original design these two were both to have been mounted on the same rods. Even at this stage of the design, two errors had already been made. Neither of them would have been fatal. The first was that the rod design didn't meet the building code. While that made the design illegal, a stress analysis showed that the rods were still safe. And the whole concept was, after all, so simple! The second problem seems to be only a small detail. The designers didn't clearly explain how the rods were supposed to grip the upper walkway where they passed through it.

The contractor then unwittingly made the final error of judgment. He solved the problem of gripping the walkway. He simply ended one rod, and started another next to it, in the skywalk's cross-beam. The result was subtle but devastating. This change doubled the stresses at one point in the beam. I say the calculation is subtle. But it's still within the grasp of our second-year engineering students. Their applied-mechanics professor now gives it to them as a homework exercise.

So, under extreme loading, the upper walkway failed, and both walkways fell away -- depositing steel, concrete, and people upon the dancers below. It was a terrible moment. But it never would have occurred if the chain of blunders hadn't fit together so perfectly. I'm chilled by the weight of responsibility that vests in these little design decisions. At the same time, I'm encouraged to see that so much safety is inherent in our system of design -- that so many dovetailing errors had to be made before this dreadful accident could ever happen.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Petroski, H., To Engineer Is Human: The Role of Failure in Successful Design. New York: St. Martin's Press, 1985.