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 on the cargo as shown in the figure below:
Fig 1: Arrangement of a spreader bar
Spreader beam vs Lifting beam
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. A simple lifting beam is shown below:
Fig 2: A simple lifting beam
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 (see figure below).
Fig 3: Forces and moments on lifting beam
Loads on the Spreader Beam
Next, we analyze the loads which affect the spreader’s design.
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. This can be found in DNVGL-ST-N001 (2016), 188.8.131.52
Sling Angles and Loads
The angles of the slings 1 & 2 (in figure 1) made with the vertical must be less than 45 degrees (184.108.40.206, DNVGL-ST-N001). It is to be noted that the center of gravity (COG) of the lifted object has to be in line (on the same vertical) as the crane hook. The sling lengths are to be accordingly adjusted. If not, the object will experience a tilt during lifting which is not desirable.
The next step is to calculate the loads on the slings above the spreader, i.e., the line tensions in the slings 1 & 2. The Dynamic Hook Load (DHL) is determined for this and using a simple mathematical formulation, the sling loads are determined. It is expected that the lift point which is closer to the COG will take more load.
Fig 4: Sling angle and sling loads
The next step is the calculation of stresses. The spreader experiences the following stresses:
- Compressive Stress – this is the governing stress of the spreader. This is calculated by dividing the compressive force on the spreader by the section area of the spreader. The allowable compressive stress is calculated in accordance with AISC ASD, Sec E2 by calculating the slenderness ratio (KL/r) of the spreader, and the factor Cc which depends on the material properties of the spreader.
Fig 5: Compressive stress check
- Bending Stress – the bending of the spreader is caused by the force on the pad-eye acting at a vertical offset between the eye of lifting lug and the longitudinal axis of the spreader (see hp in the figure below). Bending is also caused due to the self-weight of the spreader. Together, these moments divided by the section modulus of the spreader give the bending stress. The allowable bending stress is obtained from the AISC ASD
- Shear Stress – the vertical loads on the lifting points cause shear stress. Dividing these loads by the section area of the spreader gives the shear stress
- Combined Stress check – a combined axial compression and bending check is also required for the spreader as per AISS ASD H1
- Lateral torsional buckling – if the spreader section is I-beam, then a lateral-torsional buckling possibility needs to be investigated, depending on whether the flange/web is compact/non-compact. This is done as per AISC ASD Sec F2.
With all the above checks performed, a complete design assessment of a spreader for a specific lifting operation can be done. Spreaders can come in different section shapes, the most common being I-beam, hollow circular, and hollow rectangular.
TheNavalArch has its own app for spreader beam design which performs all the checks diligently and can be used to either design a new spreader or check the suitability of an existing one for any lifting operation. Please do spend some time exploring it through the link below.
Disclaimer: This post is not meant to be authoritative writing on the topic presented. thenavalarch bears no responsibility for the accuracy of this article, or for any incidents/losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any activity which draws on information mentioned in this post. All the figures, drawings, and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.
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