Analysis of the catastrophic failure of a dockside crane jib

Abstract

The catastrophic failure of a crane jib is discussed in the paper. The final failure was determined by an existing crack, which had propagated up to a considerable part of the cross section of one of the main tubular elements, which constituted the jib frame. The crack originated from a seam weld, which was used to join a stiffener on top of the tubular element where some cracks were found few years before the failure and that, actually, resulted to be a misguided attempt to strengthen the jib structure at that point.A stress analysis of the crane jib is firstly carried out, in order to analyse the stress levels nearby the failure region and to discuss the fatigue design with reference to current standards. In the second part of the paper, a crack-growth model is developed in order to achieve an estimate of the propagation period and a simple collapse model, based on beam theory, is used together with the material properties determined by experimental tests, to determine whether the final failure was determined by fracture or by plastic collapse.The analyses conclude that the fatigue design requirements were not satisfied and that the final failure was determined by plastic collapse of the critical cross section, after a relatively long stable propagation period. Some considerations about the necessity of scheduled and qualified inspections are also drawn.