Sign installers are experienced in hooking up various loads to their cranes for hoisting. Yet the routine nature of these operations, combined with the fact that less-experienced workers sometimes assume these duties, can lead to errors that compromise safety. The substantial importance of safe rigging in construction operations is reflected in corresponding standards developed by the US government.
The US Department of Energy (DOE) incorporates a Hoisting and Rigging Technical Advisory Committee that develops these standards and publishes the DOE Hoisting and Rigging Standard (DOE-STD-1090) and accompanying manual. Following its specifications can spell the difference between project safety and exposure to hazards.
SLINGS AND SHACKLES
The most common rigging devices are slings that attach loads to the crane hook and shackles that provide attachment points for hoisting signs and other items. While three basic types of slings are widely used to hoist loads, the most common type is the synthetic web sling. These slings may be composed of nylon or polyester which, when woven into thick strands, becomes extremely strong. Some key advantages of synthetic slings include their light weight, soft-ness and elasticity compared to steel slings (e.g., chain slings or wire-rope slings). These properties reduce the likelihood of marring or otherwise damaging surfaces of signs and hardware.
Nylon slings are considered superior because they incorporate twice the elasticity of polyester slings. This extra stretching capability provides a buffer against sudden shocks that sometimes occur during load movements and handling. But regardless of the type of synthetic web sling used, the fact that they cannot rust represents an additional advantage.
Synthetic web slings, though, are more susceptible to damage from cuts, abrasions, heat, moisture and chemicals than steel slings. Sling manufacturers incorporate red fibers into the cores of synthetic slings which, when exposed due to substantial wear or misuse, indicate that the sling must be removed from use. For all usable slings, it’s essential to use dry-storage racks – ideally located indoors. All slings require regular inspections to ensure that they’re not damaged. Additionally, synthetic slings should never be knotted during use, as tests show this can reduce the lifting capacity up to 50%.
All slings are marked with tags indicating their safe-load limits when used in various configurations. These safe-lifting capacities vary substantially depending on how a sling is used during hoisting. A key factor is the angle formed between the crane’s hook and the point of attachment to the load. As a general rule, slings have optimum load capacities when used in configurations ranging from vertical to a 45° angle from vertical. This is particularly important when loads are extremely heavy.Advertisement
Using two identical slings of adequate length balances the load and enables optimal sling angles. Conversely, hook-ups utilizing shorter slings (e.g. where the crane hook is relatively close to the top of the hoisted sign) should be avoided whenever possible because, in these configurations, the slings’ load capacities are reduced and greater stress is applied to all of the rigging devices.
Because many signs are relatively lightweight for their sizes, the hardware provided by manufacturers to serve as hoist points might be ordinary, forged-steel eye-bolts. As in the case of sling angles, the angle at which a sling is attached to an eye-bolt during hoisting should not be less than 45° to avoid failure due to shear forces.
Before any sign is lifted, installers should always inspect the hoist hardware provided to verify its secure fastening to the sign cabinet. Eye-bolts used for hoisting must incorporate shoulders allowing the bolts to be fastened flush against the top of a sign. Additionally, when eye-bolts serve as hoist hardware, proper orientation of the bolts in relation to the direction of lifting force is critical.
Anchor shackles are forged-steel devices commonly used for attaching slings to eye-bolts or other types of hoist hardware. These devices typically incorporate threaded pins which must be hand-tightened securely into the shackle body prior to use. Never substitute ordinary bolts for shackle pins; this adversely affects the shackle’s rated capacity. Also, never use a shackle in a configuration where tension might make the sling bear on the pin in a way that would unscrew it.
To determine the load capacity of a shackle, its size is stamped into the forged metal by the manufacturer. Shackle sizes ranging from 3/8- to ¾-in. – the sizes most commonly used by sign installers – have load capacities ranging from 1 to 4.75 tons.Advertisement
Whether you’re using a synthetic sling or another apparatus, avoiding “time savers” and shortcuts is key – and could save a life!
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