The famed Arecibo Telescope in Puerto Rico collapsed on Dec. 1, 2020, when the 900-ton equipment platform suspended above the 1,000-foot diameter telescope dish crashed through the dish.
The National Academies of Sciences, Engineering and Medicine formed a committee to review the collapse and three earlier investigative reports and to issue a final verdict on the contributing factors and probable causes of the failure.
Habib Tabatabai, professor of civil engineering and an expert on structural cables and cable-supported bridges, served as a member of the committee, which published its final report on Oct. 24 and presented it to the U.S. Congress on Nov. 13.
This committee attempted to answer a question not addressed in the other inquiries: Why did multiple cable failures occur at Arecibo when no records exist of this specific type of failure (cables pulling out from their sockets) occurring anywhere else in the long history of these structural cables?
A suspended platform
The telescope’s platform was suspended with steel cables from three towers located around the dish. All cables terminated in “spelter” sockets at their ends. The spelter sockets were filled with molten zinc during manufacturing to surround the individual wires and secure the cables at their ends.
The visual inspections of cables in 2018 and 2019 had indicated that some of the cables had pulled out of their end sockets to varying degrees following the 2017 Hurricane Maria. The earlier investigative reports and the National Academies’ also reported that “creep” of zinc in the cable sockets was a major contributor to the cable pullout and subsequent collapse.
“Creep” is the long-term deformation of materials under sustained stress.
But what exactly caused the excessive creep of zinc?
“Zinc-filled spelter sockets have been used for decades in a variety of industries,” Tabatabai said. “This type of failure of zinc-filled spelter sockets has not been reported anywhere else despite its widespread use.”
Previous investigations also noted that the patterns of cable pull-outs were not uniform. They varied at different locations.
A pivotal moment in the telescope’s history
In 1997, new (auxiliary) cables were added to the structure to accommodate the installation of new components, including a high-energy radar transmitter.
The first cable to fail in 2020, however, was one of the newer cables, followed by failures of older, original cables.
The committee carefully examined the observed failure patterns and concluded that the most plausible explanation is that “low-current, long-term electroplasticity” contributed to the excessive creep of zinc within the sockets.
An important feature of the Arecibo Telescope was that it emitted powerful electromagnetic waves, which generate electrical currents in metals, such as steel cables.
Studying low-current electroplasticity
Electroplasticity can result in a softening effect when very high electrical currents flow through a metal, like zinc, over a short period of time. Tabatabai said it is conceivable that exposing the zinc to low-level electrical currents over a long period of time (decades) could also result in a softening effect.
However, this has not been experimentally evaluated in the past.
Because of his experience working on the Arecibo report, Tabatabai plans to test the concept of low-current electroplasticity of pure zinc in the UWM Structural Engineering Laboratory.
“Typically, electroplasticity has been associated with very high currents over a short period of time,” he added. “We are going to examine the effects of low currents over a much longer period of time.”