Introduction

Steel structures and industrial mechanical equipment generally have some connection with reinforced concrete elements. This interface is usually made by means of anchor bolts, which are anchored in the concrete in order to transmit the forces coming from the supported components. This article will look at one of Kot's success stories, in which a failure analysis was carried out on the base connection of a piece of mining equipment. The anchor bolts on this piece of equipment broke, causing it to tip over, as shown in Figure 1. The aim of the analysis was to identify the causes of the failure and propose solutions so that the equipment could return to operation safely.

Figure 1: Faulty connection at the base of the equipment.

Methodology used

The failure investigation consisted of an analytical stage to check the transmission of forces between the components of the steel-concrete connection, and a computational stage to check the foundation footing supporting the equipment. In this way, as well as identifying the cause of the failure, it was also possible to assess the conformity of the structure's design.

The design of anchored connections must take into account the joint behavior of the materials involved. Both the failure modes related to the concrete and the limit states associated with the steel of the anchor bolt must be verified in order to define the materials and dimensions of the connection elements. Figure 2 shows the failure modes that can occur in this type of connection.

Figure 2: Failure modes associated with anchored connections.

Based on the equipment's load plan, the stresses on the connection components were determined. The machine under analysis was supported by a layer of rubber, used to absorb the vibration developed during its operation. As a result, in addition to the traction and shear in the anchor bolts, the lever arm due to the rubber led to the development of bending moments in the anchor bolt bars, which proved to be the critical stress in the analysis.

The dynamic nature of the equipment's loads was also taken into account. Based on the defined stresses, the stresses in the cross-section of the anchor bolt bars were calculated to check for material fatigue. As the equipment can operate in different directions, the stresses caused by the bending moment in the bar can be compressive or tensile, as shown in Figure 3, causing a wide variation in stresses.

Figure 3: Representation of the equipment's movement during operation.

Considering the high magnitude of the stresses, the fatigue analysis indicated that even a small number of equipment operating cycles could lead to a reduction in the service life of the anchor bolts, justifying the failure that occurred. This conclusion was corroborated by a metallurgical analysis carried out on the broken metal, which found that the fracture surface exhibited cracks with characteristics typical of nucleation and propagation by fatigue under bending stress, followed by a brittle rupture of the rest of the section. Flaws in the manufacturing process of the anchor bolts were also identified and contributed to their failure.

The finite element model of the foundation footing was used to carry out structural and geotechnical checks. At this stage, it was observed that the base of the footing had a compressed area lower than the minimum value recommended by the standard, indicating a risk of the structure losing stability.

Proposed solutions

To restore the connection between the equipment and the concrete, the proposed solution was to use padlock-type anchor bolts, which are inside steel tubes embedded in the concrete. With this system, the base can be recovered without the need to demolish the concrete, requiring only the drilling of holes to remove the broken anchor bolts and the installation of new pipes. In addition, the anchor bolt maintenance process is easier and can be carried out without the need to remove the equipment.

A prestress value was also recommended for tightening the anchor bolts to avoid load variations on the bars during the operation of the equipment. The proposed system can be seen below in Figure 4.

Figure 4: Representation of the proposed system for fixing the equipment.

To prevent shear forces from being transmitted to the foundation by the anchor bolts, it was proposed that a shear plate be installed at the base of the equipment. It was also recommended that the rubber layer on the base be replaced with a high-strength grout capable of leveling the base and withstanding the vibrations caused by the operation of the equipment.

To solve the design non-compliance related to the stability of the foundation, an increase in the height of the footing base was proposed, as shown in Figure 5. In this way, the region with loss of contact between the structure and the ground is reduced to the permitted normative range, in order to ensure the safety of the foundation.

Figure 5: Proposed solution for reinforcing the footing.

Final considerations

The analyses carried out made it possible to identify the cause of the failure of the anchored connection: the rubber layer at the base of the equipment leverages the shear force on the anchor bolts, causing the metal to break. The computational approach also identified a non-conformity in the foundation design, related to the stability of the structure.

The work carried out led to the proposal of solutions to the problems identified. The proposed changes aim to ensure the safety of the equipment's operation and are easy to implement and maintain throughout its useful life.

The assessment of connections between steel and concrete elements must take into account the joint behavior of these materials and the specific limit states associated with this type of connection.

A Kot conta com um time de profissionais qualificados em integridade estrutural, prontos para oferecer as melhores soluções de engenharia para os nossos clientes. Entre em contato com a nossa equipe para mais informações!

Siga também nossas páginas no LinkedIn, Facebook e Instagram para continuar acompanhando nossos conteúdos.