What is Point Loading?
When talking about rigging, finding the distribution of a load is critical in knowing whether it is safe to proceed or if the rigging plan needs additional work. This brings up the question: What is point loading and how does it affect a lift?
Put simply, point loading is placing the bulk of a load or, potentially, an excessive load onto a single point. You may wonder how or where that becomes detrimental. There are several common instances of point loading that have resulted in failure of equipment, the load, or even the ground below the load.
Within the scope of rigging, the most common instances of point loading that come to mind would be point loading of a hook or, on a larger scale, cranes. This is the reason why outriggers exist. In lifting, one example would be hydraulic cylinders and rams. I will briefly detail the examples above, as well as the implications of not addressing the point load and proper methods to overcome the issue.
To start, we will touch on one of the most common problem areas where point loading can be seen: hooks. Whether referring to the hook on a hoist or the hook of a crane, improper rigging techniques often cause issues with point loading.
In this instance, the point load reference occurs when the sling is not properly situated in the saddle of the hook. Instead, the sling finds itself settling or being pulled toward the point of the hook. In this industry, hooks are designed to indicate that a point load has been applied on the current lift, though it is possible to gradually stretch a hook through frequent misuse with lighter loads.
The main obvious concern with this style of loading is the risk of hook failure. While there are certain tolerances built into the manufacture of hooks, repeated improper use or gross negligence will either cause the load to slip from the hook or straight failure of the hook once that tolerance has been exceeded.
To avoid this kind of point loading, the simplest course of action is to ensure the correct size and style of slings are being used, that nothing impinges the load’s ability to freely move and visually inspect all rigging for proper loading and ensuring no damaged equipment is in use prior to beginning the move.
Point loading with cranes arise when the ground bearing pressure is likely to exceed the strength of the ground below the crane. Modern cranes are becoming smaller to increase portability and simplify transport. The adverse effect of this is that the smaller footprint of the crane increases the weight of the load over a smaller surface area, which is why outriggers exist.
The addition of outriggers spreads the weight of the load over a larger surface. It also serves to stabilize the crane, allowing it to lift more without tipping or doing damage to the ground. Outrigger mats further spread the load – in this case, spreading the load seen by the outriggers – to prevent the outriggers or crane from sinking into the ground.
Similar to the issues seen with the outriggers, hydraulic cylinders carry a very small footprint for heavy loads. They require something to assist in spreading the load. This helps to avoid damage to the surface below. Larger capacity hydraulic rams already spread the load over a larger surface area due to the extra weight capability. Typically, hydraulic cylinders refer to taller, thinner units that range from 5 to 25 tons, but can reach up to 100 ton capacity. With those thinner cylinders, there is often a need to include a welded base and collar below the cylinder to distribute the weight, stabilize the cylinder, and allow the load to lift without causing damage to the floor surface.
While this blog discussed two different types of point loading, the commonality is that negligence or failure to account for the point load will inevitably cause damage to the equipment, the load, or both. Knowing the basics of point loading and where to look for it will allow you to ensure each lift runs without a hitch.
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