Load Cases and Governing Trolley Positions
Static verification is the first fundamental step in the structural design of crane main girders according to FEM 1.001.
Unlike ordinary beams subjected to uniform loading, crane girders experience moving concentrated forces from wheel loads. This makes identification of the governing load case and load position essential.
Nature of Loading on Crane Girders
The main girder is primarily subjected to:
- Self-weight of the girder
- Weight of the trolley
- Hoisted load
- Dynamic amplification effects
These loads are transferred through crane wheels as concentrated reactions, which move along the span during operation. Because the load position changes, internal forces are not constant.
Maximum Bending Moment Under Moving Wheel Loads
The main girder is commonly idealized as a simply supported beam subjected to concentrated moving wheel loads.
For a single concentrated load P placed at a distance a from the left support of a span L, the bending moment at that section is:
M = P a (L − a) L
As the load moves along the span, the value of a changes and the bending moment varies accordingly.
The governing bending moment corresponds to the load position that maximizes this expression. For crane girders with multiple wheel loads, the most unfavorable combination of wheel positions must be considered.
More on this phenomenon is discussed in the “Moving Loads and Maximum Bending Moment in Crane Girders” article.
Bending Stress in the Girder
Once the maximum bending moment is determined, the normal bending stress in the flange is evaluated as:
σ = M W
Where:
- M – bending moment
- W – section modulus of the girder cross-section
This stress must remain within allowable limits defined by the design framework.
Shear and Combined Stress
In addition to bending, the web of the girder is subjected to shear:
τ = V Aw
Where:
- V – shear force
- Aw – effective web area
In regions near wheel load introduction, local stresses may combine bending and shear effects. Static verification therefore involves checking that the combined stress state remains acceptable under the governing load case.
Conclusion
Static strength verification of crane main girders requires identification of governing load cases and critical trolley positions under moving concentrated loads.
Because wheel loads travel along the span, internal forces vary with position, and the most unfavorable configuration must be evaluated.
This static assessment forms the foundation of crane girder design, providing the baseline for subsequent fatigue and stability verification.