As there are so many design variations for custom crane applications, it is not possible to identify all the points of interest here. But since structures respond to loads in predictable ways, some of the most likely places to inspect can be identified.
For beam members, look for cracks in the tension zone and at the end connections – for simple beams, the bottom surface is in tension. The middle 2/3 of the length of the tension zone should be checked first. Material near the top surface of the beam is in compression, and the bottom is pulled into tension. If there are cracks in the compression zone, they will not grow. Cracks in the tension zone will grow, but the speed of growth depends on the magnitude of the loading, and the number of loading cycles. A hole must be drilled at the end of the crack to stop propagation, if detected.
Meanwhile, the high shear zones are near the beam ends. Failures in the high shear zones are more likely to be sudden and catastrophic. Shear failures can occur in bolted connections and welds. The connected base material can also fail by shear but usually the shear is combined with tension and compression. Shear can be visualised as the tendency of the material to slide on itself.
DEFLECTION AND BUCKLING
Deflection – the bending or sagging of a beam under the influence of applied loads – does not always necessarily indicate weakness of a structure. A structure can have low strength and little deflection, or high strength and large deflection. The actual deformation that causes visible deflection is too small to be seen with the unaided eye because it is distributed over the length of the member. Local elongation or compression of the material may only be a few thousandths of a centimetre long, but the cumulative effect is visible as deflection. Sometimes paint will crack from these small deformations. Areas of cracked paint can indicate high stress and deserve a close look.
Some visible deformations/deflections are caused by buckling, which is caused by excessive compression. In plate material, buckling can also occur from excessive shear. The surface of a buckled plate will appear wavy or rippled. Free edges of plates and bars are subject to buckling. When viewed along the edge, the buckled plate appears wavy. For straight members such as legs of a gantry crane, buckling appears as a bowing of the normally straight member.
EFFECT OF WELDING
In general, welding creates an internal, local restraint that results in biaxial and triaxial stress and strain conditions. Structural steel is very ductile – i.e. it will undergo a very large deformation before failing. But when steel is loaded triaxially (or biaxially), it behaves like a brittle material. When welds cool, they shrink in all directions. Internal reaction forces are created when welds shrink. This is because the base metal outside of the heataffected zone resists the weld shrinkage force. One example for this is when three welds meet at a common corner, each line of weld shrinks in its long direction. The point at the corner is subjected to tension forces in three perpendicular directions. This corner would be the most likely spot for a crack to form.
Cracking will eventually occur in hard-worked cranes with poor engineering details and/or poor weld quality. In order of importance, keep an eye on the following conditions: previously repaired areas; girder and end tie connections; locations where cyclic flexing of thin-walled elements may occur; sharp corners; welded attachments to the tension flange; jagged or flame-cut edges not ground smooth; welds that meet in corners; arc strikes and tack welds; abrupt changes in structure geometry (stress concentrations); locations where the state of stress regularly fluctuates between tension and compression (stress reversals).
BOLTED CONNECTIONS
When a loose bolt is found, it must be replaced with a new bolt or removed and inspected. It may be tempting for maintenance and inspection personnel to simply retighten loose fasteners on the spot, but ATSM A325 bolts, for example, can only be retightened once or twice. If you don’t know how many times the bolt has been retightened after the original installation, it should be replaced. ATSM A490 bolts should not be retightened.
