The basis for such proof against failure (e.g. by yielding, elastic instability or fatigue) is the comparison between calculated stresses induced by loads and the corresponding calculated strengths of the constituent structural and mechanical components of the crane.
Proof against failure may also be required in respect of overturning stability. Here, the comparison is made between the calculated overturning moments induced by loads and the calculated resistance to overturning provided by the crane. In addition, there may be limitations on forces that are necessary to ensure the stability and/or to avoid unwanted displacement of portions of the crane or of the crane itself, for example, the jib support ropes becoming unloaded or the crane sliding.
The effects of differences between actual and ideal geometry of mechanical and structural systems (e.g. the effect of tolerances, settlements, etc.) shall be taken into account. However, they shall be included specifically in proof of competence calculations only where, in conjunction with applied loads, they may cause stresses that exceed specified limits.
When applying this part of ISO 8686 to the different types of cranes, operating in the same service and environmental conditions, equivalent resistance to failure should be sought.
5.2 Methods of proof of competence calculations
There are two general approaches to structural design or proof of competence.
a) The allowable stress method: where the design stresses induced by combined loads are compared with allowable stresses established for the type of member or condition being examined. The assignment of allowable stress is made on the basis of service experience with consideration for protection against failure due, for example, to yielding, elastic instability or fatigue.
b) The limit state method: where partial safety factors are used to amplify loads before they are combined and compared with the limit states Imposed. [or example, by yielding or elastic instability. The partial safety factor for each load is established on the basis of probability and the degree of accuracy with which the load can be determined. Limit state values comprise the characteristic strength of the member reduced to reflect statistical variations In its strength and geometric parameters. This method is a prerequisite if this part of ISO 8686 is applied together with ISO 20332 and/or the 2nd order method.
Annex A gives a more detailed description of the application of the two methods.