Structural Failures
Why the Arecibo Telescope in Puerto Rico Collapsed
New report examines start and progression of failure of support cables
The Arecibo Telescope is seen from the air following Hurricane Maria, which a new report found started the 39-month sequence that led to the telescope's collapse
The December 2020 collapse of the Arecibo Telescope in Puerto Rico came at the end of a 39-month sequence that began with Hurricane Maria in 2017, according to a new report from the National Academies of Sciences, Engineering and Medicine. The committee behind the report found long-term zinc creep-induced failure in the telescope’s cable spelter sockets as the root cause of the collapse. The mechanisms behind those failures, otherwise unprecedented in more than a century that similar spelter sockets have been used in other structures, can only be hypothesized—but the unique conditions within the radio telescope may have played a role.
Originally built from 1960 to 1963, the telescope featured a platform suspended almost 500 ft above a 1,000-ft-dia reflector, or dish, from three towers. Each tower was connected to the platform by four 3-in.-dia, 575-ft-long main cables. Five backstay cables from each tower to a backstay anchor counterbalanced the platform’s loading on the towers. And in 1997, when additions to the platform brought its weight up to 913 tons, the cable system was also upgraded with 12 auxiliary cables.
The committee identified Maria, which was a Category 4 storm at the time it hit the Arecibo Telescope, as the start of the failure sequence in part because inspections in 2003 and 2011 found no measurable increase in the zinc leading edge separation, while inspections after Maria in late 2018 and early 2019 showed cable slippage had increased from 0.5 in. to more than 1.5 in. on auxiliary sockets at the ground end of one backstay and 1.125 in. at the tower end of one of the main cables.
“The Arecibo Telescope gave fair warning post-Maria that it was in structural distress through increasing cable socket pullout,” the committee wrote in its report.
The first failure occurred on Aug. 10, 2020, when the tower end of an auxiliary cable pulled out of its zinc-filled spelter socket despite being at less than half its design load and it not even being the most heavily loaded, according to the report. The cable had only been in service for 23 years at the time, compared to about 57 years for the main cables.
Then, one of the main cables on the same tower failed at the zinc-filled spelter socket on Nov. 6. Repairs were set to start a few days later, but a second cable pulled out, increasing the loads on the remaining three main cables on the tower. The National Science Foundation declared that safe repairs would not be possible, and the telescope would have to be decommissioned.
Finally, on Dec. 1, another of the three remaining cables failed, increasing the load on the last two above their nominal strength. The 913-ton platform collapsed.
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Unique Site, Unprecedented Failure
The committee members note in the report that they were unable to find another recorded instance of a spelter socket failure. So what was different in this case?
While the committee describes it as only a hypothesis, one theory points to the “plausible but unprovable answer” that the telescope’s “uniquely powerful electromagnetic radiation environment,” in which the cables crossed the beam of a powerful radio transmitter, could have contributed to the socket zinc creep.
Researchers have studied electroplasticity in zinc, and found that electric current flowing through zinc increases its creep rate. However, those lab experiments have only been done at much greater densities and for much shorter periods than the telescope’s socket zinc service. The committee found no data on low-current, long-term electroplasticity in zinc.
So the telescope’s conditions could potentially explain why this failure occurred when a similar one has never been seen before, why the rate and pattern of cable pullouts were not uniform, why a comparatively young auxiliary cable socket failed first as well as the overall timing of the cable wire breaks, according to the committee.
"The only hypothesis the committee could develop that provides a plausible but unprovable answer to all these questions and the observed socket failure pattern is that the socket zinc creep was unexpectedly accelerated in the Arecibo Telescope’s uniquely powerful electromagnetic radiation environment,” the committee wrote.
