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Abstract

The suspended platform of the Arecibo Observatory collapsed on December 1, 2020, after 57 years of illustrious service to the scientific community. This article reflects on the collapse from a structural engineering perspective.  It explores issues that most likely contributed, including the cable terminations and their performance under loading, corrosion, fatigue, safety factors, the dynamic demands on the remaining cables when one of them suddenly snaps, and the dynamics of the suspended platform during its pendular swing.  The article is based on publicly available reports from the news media, photographs, and published video recordings of the collapse sequence.  These were expanded with relevant studies collected from the literature and with publicly available technical reports from structural engineering consultants that evaluated the structure during its final months.  It is organized as a sequence of events, starting with the first cable failure on August 10, 2020, the second failure on November 6, 2020, and the final collapse.  Despite the limitations, it is concluded that corrosion does not seem to have played a significant role, and that the failure of the first cable was most likely due to flaws during the fabrication of the cable/socket connection.  These will be reviewed and updated once the forensic analyses of the failed elements are concluded and made public.

Abstract

Structural and forensic analysis concluded that the Arecibo Observatory M4N socket joint failure in August 2020 was due to a socket joint design with insufficient design criteria that did not explicitly consider socket constituent stress margins or time-dependent damage mechanisms. The socket attachment design was found to have an initially low structural margin with a finite service life, notably in the outer socket wires, which degraded primarily due to long-term zinc creep effects that were activated by longterm sustained loading and exacerbated by cyclic loading. Additionally, HAC and wire defects were found in a few outer wires that may also have contributed to initial outer wire failures. The design did not explicitly consider the time-dependent effects of creep and cyclic loading on design capability, account for a worstcase build condition traceable to in-service inspection of features (e.g., zinc creep/extrusion), specify an end-of-life capability requirement associated with service life degradation, or explicitly set service life inspection intervals with pass/fail inspection criteria. In-service inspections showed evidence of progressive zinc extrusion on several Arecibo sockets, which in hindsight were evidence of cumulative damage and effectively a missed opportunity to prevent cable failure. Open spelter sockets of this type are used throughout industry in stay cables. The following recommendations are proposed to prevent failures of similar socket joints: (1) Verification of positive stress margins in socket joint wires for all failure modes, (2) Periodic visual inspections with acceptance criteria for zinc extrusion that are tied to structural qualification, (3) Revisiting civil codes and industry standards based on lessons learned from this analysis. 

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