This is calculated by dividing the compressive force on the spreader by the section area of the spreader. Compressive Stress – this is the governing stress of the spreader.The spreader experiences the following stresses: The next step is the calculation of stresses. It is expected that the lift point which is closer to the COG will take more load.įig 4: Sling angle and sling loads Stresses The Dynamic Hook Load (DHL) is determined for this and using a simple mathematical formulation, the sling loads are determined. The next step is to calculate the loads on the slings above the spreader, i.e., the line tensions in the slings 1 & 2. If not, the object will experience a tilt during lifting which is not desirable. The sling lengths are to be accordingly adjusted. 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 angles of the slings 1 & 2 (in figure 1) made with the vertical must be less than 45 degrees (16.3.4.2, DNVGL-ST-N001). Dynamic Amplification factor – depending on the environment of the lifting (onshore or offshore), a Dynamic Amplification Factor is to be added to the load.Rigging Weight – additionally, the rigging weight below the spreader is to be added.The sling which is closer to the CoG is expected to take more load 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. Further, the location of the CoG of the lifted object has critical effect on the sling loads. Lifted object weight and CoG – the lifted object’s weight is to be borne by the spreader beam.Next, we analyze the loads which affect the spreader’s design.įollowing are the loads which affect the spreader: If we resolve the forces, we can immediately see that the lifting beam will be primarily under bending stress (see figure below).įig 3: Forces and moments on 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. Lifting beams, on the other hand, are designed to take bending loads. There can be some lateral-torsional buckling too if it is an I-beam spreader. However, the primary load on the spreader is compressive stress. 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. 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. If we resolve the forces on the whole beam, we get the force diagram as above. A spreader beam is designed to take primarily compressive loads, as can be seen in the figure above. 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:įig 1: Arrangement of a spreader bar Spreader beam vs Lifting beamĪt the outset, it is important to clarify the difference between a spreader and a lifting beam. 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. Spreader beams are universally applied gear which is widely used in various types of lifting operations, onshore and offshore.
0 kommentar(er)
