Checks and Inspections

Checking of compliance with the safety conditions in time of the structure or structural element means assessing whether, due to events that may affect the state of stress or deformation of the structure itself, a dangerous situation can be reached.

The problem is linked to the necessity of evaluating the reliability of the construction or of the single structural element, considering the random variability of the phenomena connected to it: the loads acting on the structure and the resistance of this, or of all the parameters whose uncertainty can influence on the final behavior. In this sense, it is also necessary to take into account the structural model adopted during the calculation and the actual transposition in real construction.

The variability of the resistance of the material is linked to the reference voltage. The references are those of the technical standards and requirements in force at the time of construction of the structure.

Load variability is largely related to their nature. In general, they can be divided into two categories: permanent loads and variable loads.

While the former can be considered as the structural own weights, the seconds are of more varied nature:

1. actions and loads due to the construction and assembly methods of the structure;
2. loads related to the operating mode of the structure (accidental use, thermal effects, dynamic loads due to appliances and machines);
3. loads due to natural phenomena (snow, wind, seismic action) or caused by improper use (shocks, accidents).

Permanent loads depend on some factors that cannot always be defined exactly in the design phase, as they too are subject to limited statistical variability: specific reference weights may vary or there may be geometric tolerances in terms of thickness, lengths, additional elements, such as to require a probabilistic assessment.

The loads due to construction, assembly, and testing can generally be considered deterministic. In fact, they are established on a case-by-case basis according to the construction of the structure. Otherwise, instead, those states of compulsion induced by a succession of phases or assembly operations that remain in the structure or the structural element even after assembly is completed (forcing, impressed deformations, etc.) must be treated. These, actually, are to be considered real permanent loads of difficult evaluation and statistical quantification.

The variable loads resulting from the use of the structure cannot be defined by observations of the type conducted for permanent loads: the observations must be repeated over time for each structure. In fact, during the life of the structure, its destination may change and therefore the relief of the data necessary to build the histogram will have to consider a period at least of the same order of magnitude of the expected life of the structure.

The loads caused by natural phenomena or by uncontrolled use must be described with a different model. In fact, they cannot be predicted a priori as they are independent of the user’s will. That is, they differ from the previous ones due to the impossibility of defining a “plate load” which in a certain sense could constitute a limitation, even if qualitative, to their maximum value. These loads can, therefore, be defined only through a statistical analysis over time and it is no longer possible to identify their distribution independent of the time parameter. Their values must be related to the return period of the value itself. Once the value of this period has been set, the value of the action must be defined, which on average is exceeded only once in the predetermined time interval. In this sense, the probability that a structure is stressed in an anomalous way, even above the minimum calculation actions set by the Standards, increases with the aging of the structure itself.

The assessment of the reliability of a structure is possible only if, alongside the definition of the loads and the material, the characteristics of the limit state against which we want to protect ourselves are specified.

The limit states can be defined as states which, if reached, corresponding to the decommissioning of the structure or part of it towards some precise danger. They can be divided into two categories:

• ultimate limit states which correspond to reaching the maximum bearing capacity of the structure or part of it;
• service limit states that can be defined based on the suitability of the structure to be used and to last over time.

Final limit states of the structure may include:

1. the partial or global loss of balance by the structure, considered as a rigid body;
2. the transformation of the structure or part of it into a mechanism as a consequence of the formation of plastic hinges;
3. failure of a part of the structure due to excessive deformation before a mechanism has been reached and therefore due to lack of ductility;
4. the loss of global or local stability of a structural element.

Among the final limit states, can also be mentioned one, relating to fatigue failure. However, it must be considered differently from the previous ones because the value of the loads associated with it are those due to the normal use of the structure. On the other hand, the fatigue phenomena are relevant only for certain structural types, so it is necessary to define the load spectra and the fatigue behavior of the material and construction details.

Among the operating limit states, the following can be considered:

1. the deformability of the structure or its parts which can prove harmful to its use;
2. localized phenomena such as plasticization, buckling, sliding of the bolted joints, cracks in the welded joints, which pose dangers concerning corrosion and which limit the use of the structure;
3. vibrations due to wind or machinery that can make the structure unusable or unusable or that can cause an increase in the state of stress due to the resonance phenomenon.

The verification of the safety of a structure is therefore identified with the analysis of the set of loads acting during the operating phase and in exceptional situations, as well as in ascertaining the maintenance of the resistance conditions of the various structural elements.

• Lift the load slowly and check that this occurs without difficulty and that there are no abnormal noises, evident deformations or sagging of the structure;
• Check the functionality of the “up and down” limit switches;
• Check the functionality of the brake, checking that the mass is braked inadequate time and that there is no slipping of the load, after completing the maneuver;
• Carry out the same checks also for the carriage translation and bridge sliding movements, checking the functionality of the relative stops, without bringing the load to the maximum height (raise the minimum necessary height from the ground);
• Check the correct sliding of the trolley on the beam and make sure that there is no noise, noticeable deformation or sagging of the structure;
• Check the operation of the emergency stop button which must inhibit all movements; the machine must stop in the shortest possible space without presenting anomalies or dangerous slippages and oscillations would compromise its stability.

During these phases there may be a reduction in some speeds (lifting and/or translation); this fact is normal and must not be considered as a “bad test index” or failed test.

Static test execution:

The execution of this test must take place by lifting the nominal load at a small distance from the ground, adding the necessary surplus and proceeding without shocks, up to an overload value equal to 25% of the nominal capacity. No requirement is given by the Machinery Directive 2006/42 / EC regarding the elastic deformation of the crane in the test conditions, in accordance with the prevailing orientation, which considers this parameter not representative of the quality of the machine.

The static test must be performed in the following ways:

• Lift the load used for dynamic tests, stop it in a suspended position at a height of 50 cm and leave the mass suspended for a time of not less than 10 minutes;
• Check that the suspended mass (nominal load and overload) does not give way (the lifting brake must not slip) and there are no obvious deformations or sagging of the structure;
• Check the operation of the load limiter, if installed, which must exclude and deactivate all the functions of the crane with the exception of the downward movement.

Structure check after testing:

After testing, it is necessary to carry out an accurate visual inspection from the machine in order to verify that the execution of the tests did not give rise to obvious mechanical deformations or breakages.

Issue of the test report:

The issue of the test report immediately follows the execution of the load tests if successfully passed by the lifting equipment.