Numerical Structural Analysis of a Single Girder Crane According to Standard NBR 8400

The use of cargo handling equipment in the industry in general is extremely important for logistics, since their contribution in receipt of material, until the production stages. Since they are subjected to severe mechanical stress, it is necessary for them to resist to these loading. This work aims to verify the structural integrity of a single girder crane designed by “ARPI Engenharia”, according to standard NBR 8400. In order to evaluate the crane structure and compare the mechanical stresses with NBR 8400 yield stress, a 3D model was created in the software Ansys Workbench 17.2 and analyzed through the Finite Element Method. This is one of the most widely used methods by Engineers in order to design or solve engineering problems. This paper shows the solution of the static analysis, presenting the stress outputs from Ansys Workbench 17.2. Keywords—Ansys Workbench, Cranes, Finite Element Method, NBR 8400, Structural Analysis.


I. INTRODUCTION
Cranes, subject of analysis in the current study, play an important role in the replacement of labor force by the mechanical method, allowing the transport of high loads in situations where manual labor becomes limited [1]. In view of the current engineering scenario of high competitiveness and incessant search for reduction of costs and waste, it is essential that cargo handling equipment be able to resist to several types of loads during their useful life. In order to guarantee operating reliability and safety, as well as optimum performance and cost-effectiveness, is crucial the use of regulatory standards. In this way, the present study presents a numerical structural analysis of a single girder crane subjected to different cases of load combinations required by the standard NBR 8400 [2],using the Finite Element Method through Ansys Workbench 17.2 software. Many engineering problems can be solved by using differential equation. Nowadays, one of the most used method resolution is the Finite Element Methodthat is among the various numerical methods [3]. It was developed to solve complex engineering problems and is increasingly being used by many engineers with an aim to simulate components and structures during project, structural reinforcements, among other activities . In this context, the purpose of this work is to compare stress outputs from Ansys Workbench 17.2 with yield stress requirements established by NBR 8400 [2]. The contribution of this paper is to provide an example of how computational simulation can be useful and effective for a crane structural analysis, in order to simplify and optimize engineering design.

II.
TECHNICAL CHARACTERISTICS OF THE SINGLE GIRDER CRANE An illustration of the single girder crane in study and its main technical characteristics are shown in the Fig. 1 andin the Table 2, respectively.  Table 2.   [2] provides the guidelines for the correct verification of cargo handling equipment in general. These structures are classified in structural groups, according to their operational functions. In accordance with the technical characteristics of the single girder crane provided by "ARPI Engenharia", responsible for designing the structure, we conclude that the crane is classified as group 3 (as determined by the table 4 of NBR 8400 [2]). Based on the selected structural group, we come to a security coefficient (Mx) of 1. Due to mechanical loads and shocks caused by vertical movements such as lifting, a dynamic coefficient () is also adopted. According to table 5 of NBR 8400 [2], the dynamic coefficient for a lifting speed of 0.083 m/s is equal to 1.15. At last, another dynamic coefficient (h=2) is adopted for horizontal loads due to the deceleration of the crane. According to NBR 8400 [2] the evaluation of structures such as cranes is made by determining the stresses during their operation. Three kinds of loads are considered for the present analysis: Main loads , vertical loads, and horizontal loads. The main loads include the self-weight (SG) of the structure (metallic structure and electric hoist weight) and the operating load (SL) of 10 ton. The vertical load is represented by the dynamic coefficient () that multiplies the operating load by 1.15. Lastly, the horizontal loads (SH) represent the inertia effects caused by deceleration during translation movement and impact due to shock effects.The horizontal load caused by braking is applied in the Finite Element model through an acceleration and a force directly in the electric hoist support in the beam. The acceleration applied in the model is equal to 0.196 m/s² (0.098 m/s² multiplied by Mx and h). The force applied in the model is equal to 1960 N (0.098 m/s² multiplied by Mx, , and SL). The acceleration caused by shock effects is equal to 0.6 m/s² (adopted by [4]). Table 3 summarizes all the loads applied in the Finite Element model and their respective directions according to the model coordinate system.     The approval criterion for the shell elements used in the Finite Element Model is to present Von Mises stress below the allowable stress in all load combinations. Thus, the allowable stresses for the material of the single girder crane for cases I and III are shown in Table 6.

IV. STRUCTURAL ANALYSIS IN ANSYS WORKBENCH 17.2
In the Finite Element Method, the geometry of the component or structure under analysis is subdivided into small elements, in a finite quantity, interconnected by nodes, forming a mesh. This process is called discretization [5]. The analysis is divided into three distinct steps: pre-processing, solution and postprocessing [5]. The pre-processing step consists of geometry modeling, definition of mesh, material properties, and boundary conditions. At this stage, the geometry was modelled on ANSYS Discovery SpaceClaim and exported to Ansys Workbench 17.2. For the solution step, the linear static analysis was selected in order to obtain stress and strain outputs. Finally, the structural response of the single girder crane was evaluated in the post-processing step.The model was built with SHELL181 elements in Ansys Workbench 17.2. The different thicknesses of the plates are shown in the Fig.3 in a color scale.

Fig. 3: Different thicknesses of the single girder crane
The model has a total of 49317 nodes and 49635 elements, including Tri3 and Quad4 first order types. Fig.4 shows in detail the defined mesh.

Fig. 4: Details of model mesh
The single girder crane was considered to be simply supported and the most external plates where restricted. One side was considered fixed and the opposite side had Y and Z displacement restricted, while X (longitudinal direction) was set free, according to global coordinate system shown in Fig. 4.
The most critical result of the static analysis occurred in combination 5, where the electric hoist is located in the center of the single girder crane. As previously mentioned, combination 5 includes the following loads: self-weight (SG), operating load (SL), and braking (FH). The Fig. 5 and Fig. 6 show in details, in a color scale, the Von Mises stress. Note that the maximum Von Mises stress in the center of the beam (Critical region) is 125 MPa.It can also be noted that there is a stress concentration at the extremity plates, where the constrains were set. It can be viewed in Fig. 7.

Fig. 7: Stress concentration at the extremity plate
A value of 160.8MPa is found in this region. It can be explained because of the constrains applied in the area. As the constrains applied simulate a perfect fixed support, consequently it results in a bending moment bigger than the expected in reality, as a rigid joint transfers all the moment thru the joint. Another factor to be considered is that the single girder is connected to the end truck with bolts. It means that the rotational stiffness decreases and actually the connection looks like a semi-rigid joint. Table  6 summarizes the maximum Von Mises stress found for each load combination. demonstrated that the single girder crane structure is able to resist to all load combinations from NBR 8400 [2]. Its structure has enough stiffness to operate with safety and reliability, therefore meets the required criteria of NBR 8400 [2]. The maximum Von Mises stress found was 160.8 MPa and occurred in combination 5. This value is below the allowable stress of 167 MPa.The general objective of the article was reached. The results were able to prove how efficient, practicality, and applicable the Finite Element Method is for a single girder crane structural analysis. Future possible applications and extensions could be a fatigue and buckling analysis of the structure, since the current paper only approaches the yield stress criterion. NBR 8400 [2] also provides the methodology to evaluate fatigue and buckling criteria for cranes and cargo handling equipment, therefore can be useful for possible applications.