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I have never heard of that explanation as a contribution to the failure of that walkway... Is that something recent that has come out? It made me curious, because until 9-11 it was considered the most deadly structural disaster in US, and probably still is in the category "due to engineer error". So I pulled out Salvadori's book "Why Buildings Fall Down" (yes, it is written for the layperson but still a good historical ref), and the diagram showing the failure does show the hanger rod nut pulling through the joint where the two channels were welded together.
But Salvadori explains the main reason for failure was the original design called for a single long rod to support two walkways, which was more difficult to construct then if you use two shorter rods. The shop dwgs showed two rods connecting to the 4th floor box beam instead of one, which the engineers mistakingly approved (or in my shop dwg stamp, "REVIEWED" and took no acceptions....) The result was the fourth floor walkway box beam now supported the second-floor walkway hanger rather then the load going directly into the hanger rod. Now it was supporting its own walkway weight and live load, so the load could have been 2x the design load in theory, causing the fourth floor box beam to fail by allowing the nut to pull through the joint between the channels.
Hard to explain without a dwg, but imagine a two story bldg where you take a second story column, instead of being continuous to the first story column, you move it 2 feet over onto a beam. Now that column load is going into the beam and the connection between the beam and column, and the connection was not designed for that, so it fails. (Not explaining for you Harold, for someone not familiar with the incident.)
I can certainly see how a poor or incorrect weld in this instance would be a huge contributing factor to the failure.
Sounds like Salvadori may have glazed over the weld issue for simplification purposes. Either way there were shop dwg coodination mistakes. I also wonder why they did not just use a HSS section rather then build their own? Were HSS just not that popular in the late 70s? Or throw a PL 1/4 on the bottom of the channels to reinforce that area...
Andrew Kester, PE
Structural Engineering Consultant
Lake Mary, FL