John R. Yannaccone, P.E., Principal Mechanical Engineer
Incident Description: One morning a worker reported to his scheduled job site. Due to the specifics of this roofing job, the employer elected to rent a ladder hoist to lift supplies and equipment from the ground to the roof. The employer had assembled and setup the ladder hoist prior to the worker’s arrival and the worker was unaware if the hoist had been used prior to his arrival at the job site. He placed the first load of two bundles of shingles on the platform of the hoist, started the engine and began to raise the platform and shingles up to the roof by stepping on the foot pedal. When the shingles were about 25 feet up, the platform and shingles began to rapidly fall, striking the worker’s right knee and then his right foot. He sustained serious crush injuries to his right foot in the incident.
Analysis: The investigation of this incident began with the inspection ladder hoist, identifying both its capacity as well as the cause of the failure. While the ladder, platform and engine all appeared in good condition, there was some damage to the operating handle, likely due to it being hit by the falling platform and/or shingles. There was damage on the bottom of the platform consistent it impacting the travel stops on the bottom of the ladder when it fell. The wire rope used to lift the platform was in poor condition with signs of rust and physical damage to the wire strands. A portion of wire rope, separated from the remaining length of the wire rope, was found still attached to the platform. The inspection showed that this wire rope was not cut, but had broken, based on the various locations of the breakages and the clear evidence of fracture observed at the ends of the strands. In fact, the strands were so brittle that further breakage was observed as a result of handling during the inspection. This evidence was consistent with the application of a load to the wire rope that exceeded it strength. Further inspection of the wire rope revealed additional areas, other than the failure point, where the strands were corroded, degraded or broken.
The specific ladder hoist was rated to have a 200-pound capacity. Based both on photographs of the incident site and the statement of the worker, he placed two bundle of shingles on the platform just before the hoist failed. The shingle manufacturer’s data indicated each of these bundles weighed 56¼ pounds, for a total payload of 112 ½ pounds at the time the wire rope failed. This weight equates to less than 60 percent of the ladder hoist’s capacity rating.
The wire rope on this ladder hoist was measured at 5/32 of an inch in diameter. While the strength of wire rope varies widely, this diameter has an allowable working load of over 500 pounds and a breaking strength in the 2800-pound range. Therefore, if the wire rope was in good condition, it should have been significantly strong enough to raise the two bundles of shingles without failing. The wire rope failed due to severe degradation and damage.
While inspection of the wire rope can and should be performed by anyone involved in the supply, setup and use of the ladder hoist, it should always be performed before the equipment is sent out on a rental. The condition of the wire rope at the time of the inspection clearly indicated it had reached the end of its service life and needed to be replaced due to degradation. Because this degradation is an effect of long-term processes, rather than occurring over a short time, the rental company should have detected the need for replacement well before this failure occurred, had they inspected the equipment on a regular basis. A thorough inspection of equipment should be performed each time it is returned from a rental and/or before it is sent out for a rental to detect damage or degradation due to age, frequency of use or misuse.
Based on the condition of the ladder hoist and the lack of documentation of inspections or maintenance by the rental company, it was clear they had supplied the worker’s employer with a ladder hoist that was not safe for use.
Result: The rental agency settled with the worker shortly after the expert report was served.John R. Yannaccone | Ladder Failure | Mechanical Engineer