Load control is an important issue to address any time a rigging task is undertaken. There are many questions which must be answered before a rigger can choose the safest and most efficient rigging system for a given load.
Can any part of the load (fluid, sand, etc.) shift during the lift?
Is the load’s CG above or below the sling attachment points?
Should the sling assembly used to make the lift be a single leg, 2 leg, 3 leg or 4 leg bridle?
Will it be basketed or choked at any location or will the slings be attached by sling hooks or shackles at the load’s pad eyes?
Whenever possible, a rigger should:
Stabilize the load so as to prevent any shifting of the load’s contents. A container that is ½ full is always one of the most dangerous types of loads. Transfer the contents and move the container empty or fill it all the way up so it can be lifted as a solid volume with no shifting.
Attach to the load ABOVE the CG when possible. Two leg lifts are obvious for adhering to this rule. On 3 leg and 4 leg sling bridle lifts, the rigger should plan for a worse case scenario. For instance, a sling leg becoming disconnected and the load flipping over which causes a shock loading, resulting in load damage and or crew injuries. When a load’s attachment points are below the CG, inspect the pad eyes, shouldered eye bolts, etc. for previous wear and damage. ALWAYS use a positive device as the “connector” between the sling and the pad eye - for details check out 5 Tips for Rigging to Positive Connections. A shackle or similar type of clevis connector should be used instead of a sling hook which has a sheet metal hook latch.
Single leg lifts are okay, but if the load offers a variety of attachment points, use two slings at separate locations on the load instead of one. If there is a failure of the single lifting attachment point, shackle or sling, it is hoped that the second sling could withstand the shock load and hold until the load is landed. Use a 3 leg instead of a 4 leg bridle since the actual loading on a 4 point attachment generally results in the load being carried by one of the diagonal pairs. The old rule of thumb regarding 4 leg bridles, which have no means for adjustment, “Two legs carry the load, which the other two balance”. When forced to use 4 sling legs to lift a load, use two, 2 leg bridles and place both master links into a rigging shackle. This allows for multiple “hinge-points” and helps to equalize the loading on all four sling legs. A strongback or spreader bar can help equalize leg loading.
When no pad eyes are available, use a choker hitch versus a basket hitch to create a slightly higher degree of compression and better friction due to more square inches of contact. Optimize either hitch by placing a full-wrap around the load contact point for maximum potential holding power, “double-wrap basket or double wrap choker hitch”.
Use positive attachment devices for connecting to the load. Create the situation such that a component would have to fail rather than “slide out” resulting in the loss of the load.
Happy Rigging, Mike Parnell.
This is an article from The Professional Rigger newsletter, 1989.