Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 301 The second example of a large structure that failed because of deficient communication concerns a 40 tonne portal crane operating in a harbour. Portal cranes are a form of crane where a large rectangular framework forms a flat-topped arch with a gantry across the top. This whole portal frame can move on tracks via a railway bogie attached under each end. The lifting gear itself is on a small trolley that can move crossways along the horizontal gantry. A driver’s cabin was also positioned under the gantry. In common with many other shipyard gantry cranes, the design consisted of twin overhead box girders rigidly fixed to rectangular box section legs at one end and hinged at the other end to an A-frame structure. The crane failed during service whilst lifting a shipping container, and the mode of failure of the crane was by fracture through a welded joint between the bottom plate and the vertical members of the gantry box girder; this allowed collapse of the horizontal beams to occur over the fixed legs causing the other set of hinged legs to pivot outwards. The net result was that the driver’s cabin was crushed in the collapse. Figure 5 : Fracture surface of the failed box section which led to collapse of the portal crane. The letter ‘O’ indicates the outer edge of the section, where the cracking started. The fracture surface is shown in Fig. 5 where the symbol ‘O’ marks the outer side of the leg. The bottom plate of the gantry box girder was 17 mm thick, with the end plate at position ‘O’ being 8 mm thick and the side plates 6.5 mm thick. Examination of the failed joint indicated that final fracture had occurred from a pre-existing fatigue crack that had a length of the order of 1 metre at the time of failure. Heavy corrosion on the fracture surface indicated that the crack had been growing for a considerable period of time before the final fracture occurred. No unusual defects could be observed at the presumed initiation site for the fatigue cracking. The crane had been in operation for about 7.5 years and had last been inspected by an authorised inspection agency some 2 months before the failure. It was designed as a Class 2 crane for medium material handling duty, supposedly intended to work 2,000 hours per annum with a design lifetime of 20 years, i.e. 40,000 hours. At the time of failure the crane had been operating for some 22,515 hours, well inside the design life but with a greater number of hours, pro-rata, for the 7.5 year service life. There were two main points of interest regarding this failure, firstly, that the design should have been done to Class 3 for heavy material handling duty, based on the operational service conditions and, secondly, that the inspection methodology had not taken account of the likely mechanism of failure of this portal crane. With respect to the first point, the manufacturer was aware that this crane was intended for harbour operation, moving shipping containers and should have queried the operator’s specification of a Class 2 crane for this duty. It is worth noting that more recent designations of crane classes indicate that Class 2 equates to 100,000 to 500,000 load cycles while Class 3 equates to 500,000 to 2,000,000 load cycles. This crane was definitely in a Class 3 environment. Lack of clear and full communication between operator and designer regarding the duty cycle and crane class was therefore a prime cause in this failure. With respect to inspection, the existence of a crack around a metre in length at the time of the failure should have implied that it would have been detected at the last inspection which took place 2 months before the failure. Speaking with the person who carried out the inspection of the crane, he stated that this critical joint (in terms of structural integrity) had been visually inspected from the ground level. It was also clear that the paint coating on this crane structure was flexible and of high quality; thus it was detached from the steel structure at this positon on other parts of the structure but was O