Prevention of aerial tramway failure by safety factors?

Abstract

In December 1996, an accident occurred on a cableway which caused the death of one passenger and injuries to several others. The cause of the tragic accident was the unexpected failure of the axle that carries the deflecting wheels for the wire rope. During the course of the official investigation it was demonstrated that the failure was due to a fatigue crack that had already propagated through a large portion of the cross- section. This fatigue crack had been initiated by fretting. Furthermore, it was found that safety factors prescribed by the regulations for cableways had not sufficiently been considered in the design calculations. It also became clear that if different calculating philosophies are to be combined in a project great care is necessary in order to assure that all the important design parameters and failure modes are considered during the analyses, e.g. finite element calculations are employed together with design standards that contain overall factors, to account for stress concentrations and manufacturing inaccuracies. In general, it may be concluded, that a simple compliance with the factors of safety will not always provide the necessary margin of safety against failure. Safety can only be guaranteed if all the important mechanisms of possible damage have been fully accounted for in the design.