Correct crane stiffness prediction


Ensuring crane integrity and stiffness is vital for safe operation, requiring precise FEA to avoid inaccuracies in stiffness predictions and maintain optimal performance and control.

Ensuring a crane’s integrity and stiffness is crucial for safe and efficient operation. Traditionally, Finite Element Analysis (FEA) models have relied on simplified beam elements to check a crane’s overall strength.

However, these elements can miss crucial details in complex geometries, such as small plates of various shapes, welds, and larger components like gantry systems, leading to inaccurate stiffness predictions.

FEA allows us to determine a crane’s stiffness, natural frequency, and vibration modes, which are essential for smooth operation and safety. Understanding these vibrations helps predict the crane’s response to movements, ensuring optimal performance.

Impact of crane stiffness on control

A crane with insufficient stiffness will experience excessive swaying and flexing during operation. This makes precise positioning of the trolley and spreader difficult, leading to inefficiencies and potential safety risks.  Therefore, proper crane stiffness is crucial for smooth, accurate control.

Terminal operators often specify a minimum natural frequency, essentially the inherent vibration rate of the crane, to ensure efficient and safe control. While design codes guarantee structural integrity, accurate prediction of this frequency is crucial to avoid overly “wobbly” cranes. Measuring the natural frequency of an existing crane is relatively straightforward with specialised equipment that excites the crane and measures its vibration response.

The challenge lies in predicting natural frequency at the design stage. Traditional FEA models using 1D beam elements provide a general idea of crane behaviour but often overestimate stiffness and, consequently, natural frequency. This can lead to the issues described above.

A more detailed approach for accurate predictions

SDC Verifier has tackled these challenges with a hybrid approach. The common beam modelling approach can still be used for overall crane calculations, including stiffness. However, to capture the details and improve prediction accuracy, we incorporate detailed plate models of parts in critical areas.

These areas include: welded connections where complex geometry can impact stiffness as the forces between the members are transferred through the joints; bolted connections; corners and sharp transitions; and gantry systems. In addition to improving stiffness predictions the detailed connections modelled with 2D plate elements also provide valuable insights into fatigue life and local stress concentrations.

Detailed modelling / Credit: SDC Verifier

Analysing stress in welded components can predict where fatigue cranes are more likely to start and develop, allowing for better preventative maintenance to extend the life of the crane. Detailed plate models help identify areas with the highest stress levels.

Addressing these concentrations early in the design phase helps prevent potential failures and ensures the crane’s long-term structural integrity. Using a hybrid approach allows us to model the large part with the well-known beam elements and not the whole crane.

At the same time, it provides a more comprehensive understanding of the crane’s behaviour, leading to accurate stiffness predictions and improved overall strength design and performance.

The Achilles Heel: the gantry system

Beam elements provide a basic understanding of gantry stiffness but miss important aspects like axial stiffness. They overestimate stiffness by not accounting for the bending of individual gantry components. Additionally, these models struggle to capture torsional effects, leading to inaccurate predictions of the gantry’s ability to resist lateral forces.

Detailed modelling of the gantry system / Credit: SDC Verifier

In essence, the gantry system behaves more like a complex lattice than a simple beam. By incorporating detailed plate models for the gantry, we can account for the bending and torsion of individual components, providing a far more accurate representation of its actual stiffness characteristics. This allows us to predict the crane’s overall behaviour with greater accuracy and avoid potential operational issues down the line.

Optimising the modelling approach: balancing accuracy and efficiency

Our targeted approach focuses on detailed modelling for the critical gantry system, achieving significant accuracy improvements while optimising efficiency. While detailed plate modelling of the entire structure would provide the absolute highest level of accuracy, it is important to strike a balance between precision and practicality, often limiting it to the locally important connections modelled with plates.

The good news for those who don’t want to model many areas in detail is that our case study has shown significant improvement can be achieved with a limited approach. As has been shown the gantry system is the primary culprit behind discrepancies in traditional beam element models.

By prioritising a detailed plate model for the gantry within the FEA analysis, we can significantly reduce the mismatch between predicted and measured natural frequencies almost to the point where the two are identical.

This approach allows us to capture the complex behaviour of the gantry, including bending and torsion effects, leading to a more realistic representation of the crane’s overall stiffness.

Case study

SDC Verifier used this approach together with Kalmar Netherlands on the comprehensive verification of an STS crane. Due to modifications made to the crane to increase operational efficiency, a minimum natural frequency requirement was established. Among other calculations such as strength, remaining lifetime, buckling, welds, bolts, etc., a stiffness study was also performed based on the finite element analysis method.

To demonstrate the critical role of detailed modelling of the gantry system in assessing crane stiffness, a series of normal modes analyses were conducted employing various modelling techniques and compared with the actual measurements.

Specifically, the gantry system was modelled using: 1D beam elements, a popular method within the engineering community; a detailed modelling approach with 2D elements (exact copy); and a detailed modelling approach with 2D elements of the similar gantry system (comparable design, but not identical).

Modelling techniques of the gantry system / Credit: SDC Verifier

Based on the received FEA results and real measurements of the natural frequencies, several key findings were revealed:

1. A very small discrepancy of 0.14% between the FEA results and actual measurements.

2. By comparison, modelling the gantry system with simple beam elements can overestimate natural frequencies by more than 32%.

3. Modelling based on similar cranes or configurations proved insufficient, showing over 12% discrepancy.

Last, but not least, a detailed model is necessary for a data set that exactly matches the gantry system. This is required to obtain accurate predictions of crane stiffness and actual results of natural frequencies. This successful collaboration with Kalmar Netherlands showcases the power of SDC Verifier’s advanced FEA modelling techniques.

This article was written by: Wouter van den Bos, Faculty of Mechanical Engineering, Section Transport Technology and Logistics at Delft University of Technology in the Netherlands / founder of SDC Verifier.

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