Hyatt Regency Walkway Collapse

 Connection details of a cross beam on the fourth-floor elevated walkway in the Hyatt Regency Crown Center in Kansas City, Missouri, United States. The failure caused the deaths of 114 people, injuring 200 more. It was the worst structural failure of its time (1981).
(a) Original connection design; each nut takes the load (P) of one walkway (because the load of the NEXT walkway is transmitted through the single, continous steel rod, not through the nut)
(b) Modified design (done due to constructibility issues) that ultimately led to the failure of the walkway due to one nut having to bear the the load of two walkways (2P)

July 17, 1981:

A structural failure leads to the collapse of a walkway at the Hyatt Regency in Kansas City, Missouri killing 114 people and injuring more than 200.
The two walkways were suspended from a set of 1.25 in (32 mm) diameter steel tie rods, with the second floor walkway hanging directly under the fourth floor walkway. The fourth floor walkway platform was supported on 3 cross-beams suspended by steel rods retained by nuts. The cross-beams were box girders made from C-channel strips welded together lengthwise, with a hollow space between them. The original design by Jack D. Gillum and Associates specified three pairs of rods running from the second floor to the ceiling. Investigators determined eventually that this design supported only 60 percent of the minimum load required by Kansas City building codes.
Havens Steel Company, the contractor responsible for manufacturing the rods, objected to the original plan of Jack D. Gillum and Associates, since it required the whole of the rod below the fourth floor to be screw threaded in order to screw on the nuts to hold the fourth floor walkway in place. These threads would probably have been damaged and rendered unusable as the structure for the fourth floor was hoisted into position with the rods in place. Havens therefore proposed an alternate plan in which two separate sets of tie rods would be used: one connecting the fourth floor walkway to the ceiling, and the other connecting the second floor walkway to the fourth floor walkway.
This design change would prove fatal. In the original design, the beams of the fourth floor walkway had to support only the weight of the fourth floor walkway itself, with the weight of the second floor walkway supported completely by the rods. In the revised design, however, the fourth floor beams were required to support both the fourth floor walkway and the second floor walkway hanging from it. With the load on the fourth-floor beams doubled, Havens' proposed design could bear only 30 percent of the mandated minimum load (as opposed to 60 percent for the original design).
The serious flaws of the revised design were compounded by the fact that both designs placed the bolts directly through a welded joint connecting two C-channels, the weakest structural point in the box beams. Photographs of the wreckage show excessive deformations of the cross-section. During the failure the box beams split along the weld and the nut supporting them slipped through the resulting gap between the two C-channels which had been welded together.
Investigators concluded that the basic problem was a lack of proper communication between Jack D. Gillum and Associates and Havens Steel. In particular, the drawings prepared by Jack D. Gillum and Associates were only preliminary sketches but were interpreted by Havens as finalized drawings. Jack D. Gillum and Associates failed to review the initial design thoroughly, and accepted Havens' proposed plan without performing basic calculations that would have revealed its serious intrinsic flaws — in particular, the doubling of the load on the fourth-floor beams.

Along with the Quebec Bridge collapses and Galloping Gertie, this is a textbook civil engineering failure.  It is one of those things that a quick review of a design change can overlook, but 114 people died because of it.  That was more than the two collapses at the Quebec Bridge (75 and 13) and the Tacoma Narrows Bridge (0) combined.  All because of overloaded one-and-a-quarter inch nuts.