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Finite Element Analysis of a Container Crane CP&A

Proprietary Software

CP&A has developed a suite of applications that all work together to permit engineering designs to be completed efficiently. We have used the custom software to design all types of cranes, bulk handling equipment, buildings and other structures. Additionally we have written custom software to write input files and to read the database files from finite element programs.

Beam Cross Section Calculator

CP&A Section

CP&A Section is an interactive environment to create and modify frame element sections. Section stores each section by name in a library. Sections may be defined using equations and variables. Sections may be imported into popular FEA packages such as SAP2000.

Container Crane Analysis

Multiple Model Support

Update load cases/combinations and run analysis for multiple models simultaneously.

Custom Code Checking Software Stress

CP&A Stress and Fatigue

CPA/STRESS & FATIGUE calculates stresses and fatigue damage on sections for different load combinations. CPA/STRESS & FATIGUE combines constant loads, variable loads, lateral loads in two directions, and angled loads (i.e. wind) from the worst angle. A combination table file defines how the analysis loads are combined. Combination tables are used to define the load combinations to be analyzed to calculate the maximum stress and fatigue damage.

Custom Code Checking Software Buckling

CP&A Thin Buckling

CPA/BUCKLING checks thin panel plate buckling. Buckling may be checked either manually (checking of individual plates) or by using file input (checking multiple members and plates). CPA/BUCKLING reads stresses from CPA/STRESS binary output file.

This can help optimize crane designs of all types. Many crane manufacturers can benefit by actually calculating thin panel buckling rather than using the prescriptive methods for stiffener/diaphragm spacing as outlined in CMAA 70 and AIST TR6.

Ship to Shore Container Crane Forestay Connection Analysis

Connections

CP&A stress may pass the governing forces to connection sheets. The connection sheets check multiple load cases and multiple failure modes. These governing forces may also be passed to FEA models of local connections with a fine mesh.

Container Crane Wind Tie-Down Analysis

Non-Linear Tie Downs

The most accurate way to analyze tie down demands for STS container cranes is a non-linear analysis. Tie downs are tension only and wheel loads are compression only, this is inherently non-linear. CP&A have developed a program that quickly analyzes tie down demands on the dock.

Why Beam Elements?

Beam elements offer many advantages over shell and solid elements when it comes to modeling a full crane structure. The largest advantage is code compliant checks related to structural stability and buckling.

To the untrained eye, it can appear that beam element models do not handle connection checks. However, forces from the global model can be passed on to local connections checks for more detailed analysis. This is sometimes referred to as breakout or sub-modelling. Creating a convergent mesh for an entire ship to shore crane is not practical or cost effective. Often times a mesh that is not fine enough is covered up by stress averaging between node points. This can produce a nice looking image but the results are not accurate.

To summarize, global stability, stress, fatigue and buckling are checked and verified with beam elements. Local connections are performed with either code compliant connection calculations or local shell/solid finite element models with convergent meshes.

What is Stress Averaging?

Finite Element Analysis Stress Averaging On

Stress Averaging On

Finite Element Analysis Stress Averagine Off

Stress Averaging Off

Stress averaging is when the finite finite element analysis (FEA) program averages the stresses calculated at each node point. Each solid element has an associated internal force and/or stress at each joint. If there are large discontinuities sometimes stress averaging is used to cover up these large stress variation. Large stress variations between elements indicates that the model is not properly meshed and needs to be refined to properly capture the variation in stress.

The two images above show what stress averaging can do. The image on the left has stress averaging on, and the image to the right does not. Notice that when stress averaging is turned on the element stress reduces from 62 ksi to 41 ksi. The two images are the same load case with the exact same geometry, the only difference is the use of stress averaging. When stress averaging is turned off, it is obvious that the mesh has not converged and needs to be refined. With stress averaging turned off, one element calculates the stress as 62 ksi and the adjacent element calculates 20 ksi at the same intersecting node point. With stress averaging on, these two numbers are averages and the program displays a nice smooth plot with an average stress at 41 ksi.

 

The mesh should be refined until these two solid objects calculate similar stresses at the joining node point. There is no guarantee a refined mesh will converge to the average stress, this is why mesh convergence is so important.

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