# The Assessment of the Technical Condition of Complex Fatigued Load-Carrying Structures
## 4 Step III--Technical Conditions Assessment
### Finite Element Model Boundary Conditions  
The use of numerical techniques allows determining the impact of the detected defects and non-conformities on the condition of the structure. However, to obtain representative results, appropriate boundary conditions consisting of the material model, constraints, and loads must be applied. The material model depends on the characteristics of the construction material and the type of analysis. Constraints depend on the foundation and fastening methods implemented in a structure, whereas loads depend primarily on the type of structure, its purpose, and specific operating conditions. Figure 14: Vibration measurements. Figure 15: Amplitude–frequency spectrum of structure vibrations. In the case of mineral materials processing installation, the structure was divided into two substructures: the first was responsible for material processing and the second for carrying the general loads and integrity. The structure's health issues were associated with the material processing part and it became the primary role of the investigation while the analysis of the second substructure was omitted. This approach provides valid results if the boundary conditions reflect the constraints of the object. This was achieved by applying the constraints to the material processing at the fastening spots within the second substructure. In the case of the conveyor, the whole structure was highly integrated and the approach could not be implemented. The finite model was constrained at the spots where the steel structure fastened to its foundations. The selection of analysis type and loads application is an individual matter for a given structure and depends on whether or not it is subject to industrial regulations. In the case of structures that are subject to standards and legal regulations, instructions on the load conditions and analysis type can often be found in regulatory documents. If the analyzed structure is subject to this class of documentation, it is necessary to read them and apply any stated commands. In their absence, determining the load conditions and the type of analysis to be processed is an individual matter. The standards and regulations approach is correct when designing new objects and structures; however, it may not be suitable for fatigued ones. Structures facing health issues often suffer from factors (e.g., constant amplitude loads, random loads, heat loads, corrosion, etc.) that are not covered by regulation considerations, but still need to be identified and eliminated. This often requires out-of-the-box approaches resulting in an individual set of loads and analysis cases. Considering the detected defects and the structures under investigation, individual sets of loads and analysis cases were established for each. In Tables 1 and 2, load and analysis cases are presented for materials processing installation and loose material conveyor, respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Load Case (LC)**} & \multicolumn{5}{c}{**Loads**} \\ \cline{2-6}  & **Dead, Live and** & **Dipping Tubes and** & **Under Pressure** & **Temperature** & **Storm Wind** \\  & **Extraordinary Loads** & **Thimbles Weights** & **Loads** & **Loads** & **Loads** \\ \hline Dead weight & + & + & – & + & – \\ \hline LC1 horizontal wind & + & + & + & + & + \\ \hline LC1 vertical wind & + & + & + & + & + \\ \hline LC2 & – & – & + & + & – \\ \hline \hline \end{tabular}
\end{table} 
Table 1: External forces included in the FEM (Finite Element Method) analysis—mineral materials processing installation.