How-To Calculate Load Weight: the basics

An important aspect of rigging is measurement — there are a number of important things to determine and consider before you calculate load weight and proceed with the lift.

Read on for basic tips from our Hercules SLR experts and learn how to properly calculate a load's weight. The lifting slings and rigging hardware used to raise your load should not only support the object's weight, but its volume, height, centre of gravity, and any other aspects of the load that could make lifting awkward.

Estimation is important — you must be able to accurately guess a load's weight and centre of gravity. Inaccurate estimations can lead to severe consequences.

Evaluate the load you will lift. Evaluation must include the load's weight, centre of gravity location, balance, stability, and nature — all should be reasonably determined before you proceed with the lift. NEVER guess the weight of a load.

How-To Calculate Load Weight: method to establish load weight

• Inspect the load for any identification or mark that indicates weight. If found, check that it's the weight of the entire load, not just a single component of an assembly.
• Check supporting documentation for load weight.
• Check any drawings/diagrams that accompany the load for its weight listing.
• If the load is still on the transport vehicle, determine the load's weight via a weighbridge.
• Estimate the load's weight with available technical data, like tables or weights.

How-To Calculate Load Weight: total weight on angular lifts

How-To Calculate Load Weight: example 1 — steel sheet/block

Figure 1

• Calculate the weight of a steel plate (shown in Figure 1) 2ft wide × 5ft long × 1 inch (0.0833ft) thick.
• Use the formula: Volume = Length × Width × Height
• Unit weight of steel is 490 lbs/ft³
• Volume = 5ft × 2ft × 0.0833ft × 490 lbs/ft³
• Weight = 408.3 lbs

How-To Calculate Load Weight: calculate force in slings

It's important to understand the different angles that will impact the load to lift. The included angle is the angle created between opposite sling legs (e.g. 0–90°). Using the included angle is known as the Trigonometric rating.

The vertical angle is the angle created by one sling leg from the vertical (e.g. 0–45°). This is called the Uniform Method of rating. All new slings must use this method.

The horizontal sling angle is the angle that forms between the sling leg and surface of the load.

Figure 2

Determine Vertical Share

When the centre of gravity is equal between pick points, the sling and fittings will carry an equal share of the load.

Centre of Gravity and Sling Loading

Figure 3

When the centre of gravity isn't equal between pick points, the sling and fittings won't carry an equal share of the load. The sling attached closest to the centre of gravity will carry the greatest share of the load. In this example, Sling B carries more weight than Sling A.

How-To Calculate Load Weight: additional loading

Before you rig a load to lift, consider these factors that may affect the load in addition to its weight:

• Wind blowing against the load
• Shock loading
• Additional below-the-hook lifting devices
• Loads frozen to the ground
• Loads snagging
• Water, snow, or ice accumulation on load
• Dynamic side-to-side movements
• Extreme temperatures

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