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The first gusset plates suspected to have failed was U10. But which one is U10? Graphics observed so far say nothing.
The cascading failure resulted in failure of (was it?) 6 (or 8?) other plates. Still under study is why this plate failed at this time. What was the unique event that finally triggered the failure?
Gusset plates were one half the required thickness. Plates should be thicker (stronger) than connecting beams. Were they literally same or lesser thickness than the beams? In which case, why was this weakness not physically obvious to experienced construction workers?
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