To perform analyses of machine, building, and conveyor structures using the Finite Element Method (FEM), the professional responsible must evaluate the regulatory criteria that address the problem and the behavior of the computational model. This procedure is important so that the evaluation performed by the professional represents the actual behavior of the structure. Previously, one of Kot's directors, Rafael Brasil, posted on the company's blog introducing FEM. A quick read may remind you of some important details about the method! Click here and check it out.

This text presents the report of one of the engineers responsible for the Structural Assessment of Conveyors, described in the article: Succes story: Structural Assessment of Conveyors – Part 1. Here we will present the impacts that could have been generated in the result of a structural analysis if the boundary conditions had been assumed incorrectly.

Introduction

As shown in the initial analysis of the conveyor, this equipment is used to transport bulk materials. This transportation is carried out by a conveyor belt, similar to a conveyor belt. Figure 1 shows an example of a conveyor belt.

Figure 1: Conveyor belt. [1]

For the belt to be able to transport any solid material, it must be sufficiently tensioned. This tensioning causes high stresses in the structures, causing deviations in the direction of the belt.

Evaluated boundary condition

When starting the study, the Kot team quickly noticed the first point of concern in the support devices of the marine terminal transporter. 

Pins are the most common connecting elements found in machines and structures. In this case, all it took was a little inattention to mistakenly consider that the connection of this conveyor has pinned behavior, i.e. it restricts all degrees of freedom except rotation around its Z axis. 

However, when the team responsible was studying the case, they realized that this device is made up of two rollers that allow it to move in an X direction.

To better illustrate what a wrong structural assessment can cause, the comparative results for each of the above-mentioned boundary conditions are shown below.

Results

The wrong analysis of the support elements would have failed the structure, as it would have been considered that a pin-type device causes an increase in stresses in the support regions of the conveyor, as shown in Figure 2.

Figure 2 - Comparison between analyses. [2]

Discussion of results

It turned out that the company that designed this structure was right to use a sliding support. This is due to the fact that adopting the freedom of translation of the rollers not only reduces the stress on the metal structure, but also prevents shear forces in the longitudinal direction of the conveyor from being transmitted to the supports.

It is also noted that these sliding devices are supported on isolated reinforced concrete pillars that reach heights of almost 30 meters. Shear forces at the top of these pillars, even if of low magnitude, would cause high bending moments at the bases of these pillars.

In this sense, it can be concluded that the design decisions significantly reduced the cost of incorporating this equipment, when comparing the two situations analyzed.

However, from the point of view of using the structure, the design is flawed, since the absence of horizontal restraint in practically all the conveyor trusses causes high displacements, reaching 4.0 centimeters during belt stretching and 6.7 centimeters in cases where the structure is set back due to temperatures of around 20°C.

Figure 3 shows the longitudinal displacement of the conveyor during belt stretching.

Figure 3: Shrinkage of the structure during belt stretching. [2]

Figure 4 shows the same situation on a reduced scale, showing some of the structure's shrinkage rates in centimetres.

Figure 4: Focus on structure shrinkage for belt shrinkage condition. [2]

Conclusion

It was possible to observe that misinterpreting the boundary conditions of a structure can influence the numerical results.

In this sense, it is always important that the professional responsible for analyzing a structure uses critical thinking to interpret the data from a computational model. Kot Engenharia a team of qualified professionals with extensive know-how in the field of structural analysis, who are prepared to ensure the Structural Integrity our clients' buildings and machinery. Consult our team for more information!

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References

[1] Abecom, (S.I). Available at: https://www.abecom.com.br/o-que-e-correia-transportadora/ - Accessed on 31/07/21, at 11:00.

[2] Kot Engenharia Collection.