Snubbing to Steel Lattice Structures: Lessons Learned
In the fall of 2010 I participated as an incident investigation board member to determine why a light-duty steel lattice structure collapsed, resulting in an injury. Shortly after this accident took place, our investigation team met with and interviewed the crew members who were at the work site that day. One crew member in particular still remains firmly embedded in my memory. During his interview he was very emotional while he described the sequence of events that led to the collapse of the steel lattice structure and the injury to his co-worker. He ended his testimony by stating, “Please tell me what happened. I don’t know what happened. I’d like to know what happened.” We left the interview assuring the crew member that we would find out what had happened.
During the course of the investigation, the board members reviewed all information that appeared to be relevant to the 2010 tower collapse incident. Of particular interest was a 1993 written report that detailed the direct and root causes for the collapses of two 230-kV steel lattice towers. After reading the 1993 written report and comparing the findings with the information we had gathered for our 2010 report, we were struck by the fact that both incidents mirrored one another.
The 1993 Incident
The first incident took place sometime during the early morning hours of August 4, 1993. Two double-circuit steel lattice towers on a 230-kV transmission line located near central California collapsed. The 230-kV transmission line was in the process of being removed in preparation for construction of a new 500-kV transmission line. Removal of six 795-kcmil ACSR conductors from the double-circuit steel lattice towers preceded the structure removal process.
The day before the incident took place, all six conductors were temporarily lowered and snubbed off by a line crew at predetermined elevations on both steel lattice towers. The purpose of lowering the conductors to predetermined elevations was to maintain a minimum of at least 10 feet of ground clearance for the conductors. Doing so alleviated the need to apply protective fencing around the conductor, ground leads and other equipment for protection of the public.
The snubbing of the conductors to the tower legs was also done in lieu of catching off the conductor to a buried dead man, to the foundation projections and/or to heavy equipment. Furthermore, the six conductors were caught off in the heat of the day. From the time the conductors were lowered and caught off to the time of the collapse during the early morning hours of the next day, the temperature dropped approximately 40 degrees Fahrenheit. This temperature differential significantly increased the tension on the snubbed-off conductors.
Shortly after the incident, a review promptly took place so that the company could determine why both steel lattice towers collapsed. The findings determined that the direct cause of both towers collapsing was due to the increased lateral loads applied to steel members by contraction of the conductors during the night. These increased loads resulted in the buckling of various steel members on both towers.
The 2010 Incident
The second tower collapse incident occurred in the fall of 2010 not far from where the first incident took place. The scope of work required the removal of segments of an existing double-circuit steel 115-kV/230-kV lattice transmission line to make way for the construction of a double-circuit steel pole 230-kV transmission line.
The snubbing of the conductors to the tower legs was also done in lieu of catching off the conductor to a buried dead man, to the foundation projections and/or to heavy equipment primarily because of two issues. Ground disturbance was not allowed because of environmental restrictions at the work site during the time the work was being performed. Furthermore, an earthen canal berm located just south of the tower also contributed to the line crew electing to snub the six 954-kcmil AAC conductors approximately 19 feet above the ground.
In this scenario, the 230-kV conductors on the east side of the tower were snubbed off at 19 feet to the southwest tower leg with rigging and chain hoists. Two crew members then climbed the northwest leg of the tower to approximately 15 feet above the ground. They were first tasked with rigging and cutting the three west-side 115-kV conductors so that the conductors could be lowered to the ground. The two crew members rigged a truck winch through blocks to a grip on the bottom 115-kV conductor. The conductor was pulled to tension by the truck winch in order to create a “bubble” in the conductor. The crew members then cut the conductor in the bubble, and the winch operator slacked off the winch and lowered the conductor to the ground.
The same work methods were then repeated for both the middle and top 115-kV conductors. Immediately after cutting the last 115-kV conductor, one crew member climbed to the ground. At this point in the process, the tension on the winch line was slowly being reduced as the conductor was moving. Shortly thereafter, the tower collapsed with the last crew member still on the tower. The crew member sustained serious injuries.
Investigation Findings and NESC Rule 422
The investigation findings for both incidents determined that the direct cause of the tower collapses was unbalanced dead-end tension loads placed on the steel members of a lattice suspension tower. The root cause for both incidents was that management systems were ineffective in mitigating the hazard of rigging to a non-engineered anchorage point. After the 2010 tower collapse, the following was added to Western Area Power Administration’s power system safety manual after one of the organization’s structural engineers analyzed and determined the reason for the tower collapse: “The practice of snubbing (catching) off conductor (or any external forces) to a steel member of a lattice suspension (or other) type tower, not specifically designed for the load’s magnitude and direction of force, shall not be allowed without first completing a technical analysis by a structural engineer.”
NESC Rule 422, “Overhead line operating procedures,” also addresses issues related to the 1993 and 2010 incidents in rules 422B1, B2 and C3, as follows:
B. Checking structures before climbing
1. Before climbing poles, ladders, scaffolds, or other elevated structures, employees shall determine, to the extent practical, that the structures are capable of sustaining the additional or unbalanced stresses to which they will be subjected.
2. Where there are indications that poles and structures may be unsafe for climbing, they shall not be climbed until made safe by guying, bracing, or other means.
C3. Before installing or removing wires or cables, the strains to which poles and structures will be subjected shall be considered and necessary action taken to prevent failure of supporting structures.
Lineworkers who are experienced with wrecking out (removing) steel lattice structures understand that this work can be extremely hazardous. Because rigging incidents can often be traced to a lack of knowledge on the part of a rigger, it is critical that workers involved with rigging activities have a basic knowledge of the principles related to safe rigging practices and are trained in both safety and operating procedures. At a minimum, a safe rigging operation should include rigging to an engineered anchorage point, knowing the weight of the load and rigging hardware, and knowing the working load limit of any hoisting ropes, slings and hardware. Following these guidelines will help to minimize hazards and prevent accidents.
About the Author: Will Schnyer, CUSP, is a division maintenance manager for the Rocky Mountain Region of Western Area Power Administration, a power marketing agency within the U.S. Department of Energy. He is a Certified Utility Safety Professional and has more than 28 years of experience working in the electric distribution and transmission field.