For information, contact greg.harrigan@nasa.gov or vinay.k.goyal@aero.org.

NASA Engineering and Safety Center Technical Bulletin No. 21-05

National Aeronautics and Space Administration

www.nasa.gov

08/02/21

Industry Recommendations from Arecibo Observatory

Zinc Spelter Socket Joint Failure Analysis

A structural analysis and forensic investigation concluded that the Arecibo Observatory M4N socket joint failure in August 

2020 was primarily due to cumulative damage caused by initially low structural design margins and a high percentage 

of sustained load, resulting in zinc creep deformation, progressive internal socket wire damage, and eventual loss of 

joint capacity. Open spelter sockets of this type are used throughout industry in stay cables. Recommendations are 

proposed to prevent failures of similar socket joints, including verification of positive stress margins in socket joint 

wires for all failure modes, periodic visual inspections with pass/fail criteria for zinc extrusion that are tied to structural 

qualification, and revisiting codes/industry standards to capture lessons learned.

Figure 1: NASA Investigation of failed Arecibo Aux M4N cable. Top: socket (left); 

zinc extrusion (center); pulled out cable (right). Bottom: forensic and finite element 

model recreation of M4N failure progression.

Background 

The Arecibo Observatory’s telescope consisted of an instrument 

platform suspended above the dish by stay cables connected to 

three towers. In August 2020, an auxiliary cable slipped from its 

socket joint on one of the towers, eventually leading to the total 

collapse of the observatory in December 2020. 

NASA structural analysis and forensic investigation concluded 

that the M4N Arecibo socket joint failure was primarily due to 

cumulative damage caused by initially low structural design 

margins and a high percentage of sustained load, leading to zinc 

creep deformation, progressive internal socket wire damage, 

and eventual loss of joint capacity. Visual inspections identified 

progressive zinc extrusion, which in hindsight was evidence of 

cumulative damage due to creep [1].

Socket Termination Overview and Mechanics

Zinc spelter socket joints are terminations in stay cables used 

throughout industry that transfer loads between adjacent 

structures. The socket termination comprises stay cable wires 

that are unraveled, broomed, and then embedded/bonded into a 

zinc casting inside a conical volume. Cable tension wedges the 

zinc material against the slanted conical surface, so that a large 

compression zone develops within the zinc such that failure 

occurs outside the socket joint in the cable span. Stay cables in 

the United States are regulated by ASCE 19-10 and 19-96 [2].

Findings

Finite element analysis and forensic investigation of an open 

conical zinc spelter socket with 1x127 cable strand showed 

non-uniform stress distribution across wires at half the cable 

breaking load, with outer wires stressed near ultimate strength 

but with residual elongation capability. 

Traditional design/build verification methodologies for similar 

socket terminations may not adequately consider constituent 

stresses and localized stress concentrations in demonstrating 

positive structural margins; consequently, these socket 

terminations may be vulnerable to time-dependent cumulative 

damage from fatigue and creep.

Analysis also showed that in applications with a high percentage 

of sustained (dead) load and a design factor of safety of 

approximately 2, there is a greater potential for zinc creep. Creep 

will visually manifest as zinc extrusion from the socket and was 

shown to further reduce wire capacity at the socket termination.

Forensic investigation also found internal damage due to environmental conditions, which in combination with wire defects 

may have further degraded capacity of the socket joint without 

clear external indication.

Recommendations 

1. Socket joint constituents should be verified to have positive 

structural margins for strength, fatigue, and creep failure 

modes for the service life of the socket for all design load 

combinations. 

2. Periodic visual inspection of socket joints should include 

pass/fail criteria for zinc extrusion tied to a structural qualification test program that verifies the creep failure mode. 

Qualified processes such as cable replacement and socket 

joint refurbishment should then be defined to restore joint 

capacity in the event of failed inspection.

3. ASCE 19-10 and 19-96 codes should be revisited to ensure 

that the design factors consider time-dependent creep effects 

in dead load dominated structures, environmental conditions, 

and workmanship sensitivity to wire defects or brooming.

References

1. “Arecibo Observatory Auxiliary M4N Socket 

Termination Failure Investigation,” 30 June 2021, 

NASA/TM-20210017934. 

2. American Society of Civil Engineers (ASCE) 19-10/ASCE 19-96 

“Structural Applications of Steel Cables for Buildings.”