Estimating load weight and finding the centre of gravity

The first question in every lift is how much the load weighs. It sounds obvious, but loads are misjudged every day on construction sites: a pallet of fittings that turns out to be twice as heavy as it looks, a section of pipe that still has water in it, a concrete block that was cast with steel inside, a piece of equipment with its packing crate and accessories still bolted on. Every capacity decision that follows, the choice of sling, hitch, shackle, hoist and crane configuration, depends on that first number. If it is wrong, every other check is built on sand.

The second question is where the weight is. A load hangs with its centre of gravity directly under the hook. If the hook is not above the centre of gravity when the load leaves the ground, the load swings or tilts until it is, and a load that tilts can slide out of its slings, swing into a worker or put far more force on one sling than the others. This lesson covers both questions: how to find or estimate the weight, and how to find the centre of gravity so the load lifts level.

Where to find the weight

The best number is one that someone has measured or certified. In order of preference, look for:

• The nameplate or data plate on equipment. Mechanical and electrical equipment usually lists its shipping or operating weight. Check which one: operating weight includes fluids, shipping weight may not.
• Shipping documents, bills of lading and delivery tickets. Suppliers usually state the weight of what they ship, including pallets and crates.
• Manufacturer's catalogues and submittals. Pipe, steel shapes, valves and fixtures are listed with weights per piece or per length.
• Markings on the load itself. Precast concrete, steel assemblies and many packaged loads have the weight painted or stencilled on them.
• A load cell or crane load indicator. A dynamometer between the hook and the rigging measures the actual weight when the load is just lifted clear, and most cranes have a load moment indicator that shows it.

When none of these exist, estimate the weight by calculation, and then add a margin. Never guess from appearance. A steel plate looks light until you try to lift a corner; a tote of water looks small until you remember that every litre weighs a kilogram.

Estimating by volume and density

Weight equals volume times density. Work out the volume of the material from its dimensions, multiply by the material's density, and you have its weight. Break complex shapes into simple ones (blocks, cylinders, tubes) and add them up.

Here W is the weight in kilograms, V is the volume in cubic metres and the Greek letter rho is the density in kilograms per cubic metre. Strictly, kilograms measure mass, and rigging charts in Canada often show capacity in kilograms or tonnes of mass; the weight force in newtons is the mass times 9.81. On site, people say weight and mean the mass in kilograms, and this course does the same.

These are typical values. Concrete with heavy reinforcing, wet lumber, or soil after rain can be noticeably heavier, which is one reason to add a margin to any estimate.

Worked examples of estimating

The pipe example is a real trap. Drained systems often hold water in low points, and a section cut out of an old riser can surprise everyone. Plumbers and pipefitters learn to assume a pipe is full until they have proven it is empty.

Things people forget to include

• Contents: water, oil, refrigerant, concrete in a bucket, soil in a bin, or debris in a garbage hopper.
• Packaging and accessories: crates, skids, shipping brackets, loose parts boxed inside, and accessory kits strapped to a unit.
• Ice, snow and mud: a load that sat outside overnight in a Canadian or Midwest winter can carry a surprising amount of ice.
• Rigging weight: on a crane, the hook block, slings, shackles and spreader bar are part of the load the crane must carry. A spreader bar alone can weigh hundreds of kilograms.
• Suction and attachment: a load stuck to the ground by frost, mud or a gasket, or still bolted down, can take far more force to break free than its weight. Free it before lifting, and never use the crane to break it loose.

Experienced riggers add a margin to any estimate. A common practice is to round up and add about 10 percent or more when the weight is calculated rather than certified, but the site's lift plan and the lift director set the rule. When the estimate is close to the capacity of any piece of equipment, get a measured or certified weight before lifting.

The centre of gravity

The centre of gravity, abbreviated CG, is the point where the whole weight of a load can be thought of as acting. A uniform block or a straight length of pipe has its centre of gravity in the middle. A pump skid with a heavy pump at one end and a lighter motor at the other has its centre of gravity closer to the pump. A tank half full of water has its centre of gravity low and toward the water, and it moves if the water sloshes.

When a load is lifted from a single hook, it rotates until its centre of gravity is directly below the hook. If the rigging is attached so the hook is over the centre of gravity, the load comes up level. If the hook is off to one side, the load tilts as it leaves the ground, the sling on the heavy side carries more of the weight, and the load may slide or swing. A load that is top-heavy, with its centre of gravity above the lifting points, can flip over completely.

Finding the centre of gravity by calculation

For a load made of several parts, find the centre of gravity by taking moments about one end. Multiply each part's weight by the distance of its own centre of gravity from that end, add these products, and divide by the total weight.

Finding the centre of gravity on site

Not every load comes with a drawing. Manufacturers often mark the centre of gravity on equipment with a symbol: a circle divided into quarters with two opposite quarters shaded. If it is not marked, a rigger can find it by a trial lift: lift the load a few centimetres, watch which way it tilts, set it down, adjust the rigging toward the low side, and try again until it lifts level. Never try to correct a tilting load by hand once it is in the air, and never adjust rigging while the load is suspended.

For long loads like pipe and lumber, the centre of gravity is near the middle, but bundles can shift as they are lifted. Use two slings spread apart, or a spreader bar, so the load cannot tip or slide out.

Stability: lifting points above the centre of gravity

A load is stable when it is lifted from points above its centre of gravity. Think of carrying a bucket by its handle: it hangs steady. Try to lift it by straps around its bottom and it wants to flip. Tall equipment such as a cabinet, a transformer or a water heater should be lifted from lugs at the top, or with slings that pass above the centre of gravity. If the lifting points are at or below the centre of gravity, the load can tip over in the slings, so the rig must be changed or the load must be secured against rotating.

A calculator for load weight and centre of gravity

The program below adds up the weights of a load's parts, adds an estimating margin, and works out where the centre of gravity lies. It uses the pump skid from the example.

# weight and centre of gravity of a pump skid (kg and metres)
parts = [  # (name, weight kg, distance of its own CG from the left end, m)
    ("skid frame", 150, 1.0),
    ("pump", 400, 0.4),
    ("motor", 250, 1.4),
]
total = sum(w for _, w, _ in parts)
moment = sum(w * x for _, w, x in parts)
cg = moment / total
margin = 0.10                       # 10 % added because the weights are estimated
print(f"total weight      {total:7.0f} kg")
print(f"with 10% margin   {total * (1 + margin):7.0f} kg")
print(f"CG from left end  {cg:7.3f} m  (skid middle is 1.000 m)")
print(f"offset from middle {1.0 - cg:6.3f} m toward the pump end")

total weight          800 kg
with 10% margin       880 kg
CG from left end    0.825 m  (skid middle is 1.000 m)
offset from middle  0.175 m toward the pump end

The margin is applied to the weight, not the centre of gravity. If the centre of gravity is uncertain, a trial lift a few centimetres off the ground is the check.