The Kansas City Walkway Collapse—The Answer

Yesterday, I wrote about The Kansas City Hyatt Walkway Collapse. I showed a close up of the change made to the original design to make the construction simpler. To recap, in the picture below, to avoid having extremely long threaded rods as in the design on the right, the rods were split in two so that only the ends needed to be threaded, as in the design on the left.

These Are Not the Same

Whatever the difficulties of assembling the walkways with long threaded rods, the design on the right is fine (provided the rods are strong enough). Although at first glance, the two designs seem almost identical structurally, there is actually a big change.

In the design on the right, the nut underneath the 4th level (upper) walkways just supports the weight of that one walkway. The nut down out of the picture for the 2nd level walkway just supports the weight of that one walkway. The rod has to support the weight of both, but it was sized to do that.

In the design on the left, the nut on top of the beam has to support the weight of the lower 2nd level walkway. But now the nut underneath the beam supports not just the weight of that 4th level walkway, but also the 2nd level walkway, the weight of which is transferred by the lower rod. The stress is doubled. It was this seemingly insignificant design change that doomed the walkways to collapse. The picture below shows the stresses in a bit more detail. If one walkway requires a force of P to hold it up, then obviously two require 2P. In the original design there is just P on the nut under the upper level walkway, but under the modified design, where the rod is split, there is 2P focused on that nut, more than it could stand.

The Collapse

The picture above shows where the rods pulled through the support beams under the stress of the design change and a crowd of people dancing.

Actually, it turns out that the walkways were badly under-designed anyway, and could barely support their own weight. Even with this poorly thought-out design change, the forces involved should only have been twice the forces originally anticipated, and designs of structures are typically done with much larger safety factors.

Video

Here is a two-minute video that talks about the disaster and why it happened.

Happy Independence Day

Happy July 4! It is a special July 4 for me since it will be my first as a US citizen. I finally became a citizen on January 6 of this year.