Spreader beams are universally applied gear which is widely used in various types of lifting operations, onshore and offshore. In this article, we will explore the design of a basic spreader beam and see what design checks are needed to establish the suitability of a spreader beam for particular lifting operation.
The spreader beam discussed here is one with two lifting points, from which the lifting slings go vertically straight down to the lashing points.
At the outset, it is important to clarify the difference between a spreader and a lifting beam. A spreader beam is designed to take primarily compressive loads, as can be seen in the figure above.
If we resolve the forces on the whole beam, we get the force diagram as above. We can see that the vertical downward forces of Fv1 and Fv2 are balanced by the components F1y and F2y, while F1x and F2x are the compressive forces on the spreader. Some bending may be experienced as the forces F1x and F2x are acting at the hole of the pad-eye, which is offset from the centerline of the spreader by some distance. However, the primary load on the spreader is compressive stress. There can be some lateral-torsional buckling too if it is an I-beam spreader.
Lifting beams, on the other hand, are designed to take bending loads.
We can see that it has a lifting eye at its top in the middle, while the eyes below are used to connect the slings to the lifted object. If we resolve the forces, we can immediately see that the lifting beam will be primarily under bending stress
Following are the loads which affect the spreader:
*Lifted object weight and CoG – the lifted object’s weight is to be borne by the spreader beam. Further, the location of the CoG of the lifted object has critical effect on the sling loads. If the CoG is not located at mid-point of the cargo (lengthwise, see Fig 1), then the loads on the slings will not be the same. The sling which is closer to the CoG is expected to take more load
*Rigging Weight – additionally, the rigging weight below the spreader is to be added
*Dynamic Amplification factor – depending on the environment of the lifting (onshore or offshore), a Dynamic Amplification Factor is to be added to the load.