Why a Published Weight and an Actual Weight Diverge

Why this matters

The number on a submittal sheet is not the weight of the thing on your hook. It is the weight of one defined configuration of that product, computed or measured at one point in its life, under a basis the document usually does not print. Every real object departs from that basis, and the departures are lopsided: field-added options, absorbed water, product left inside, coatings, packaging and the rigging itself all add, and the one common subtraction is a shipped-loose component you are not allowed to take credit for on paper. So the published figure is a floor for planning, never the rigging weight. Where you cannot close the gap with evidence, the weight is unknown, and unknown is a stop rather than a rounding problem.

What a published weight is actually a statement about

Ask what basis produced the number before you use it.

A catalog or submittal weight is the manufacturer's figure for a base model in a stated configuration. It is usually a calculated weight built from nominal dimensions and nominal material densities, so it excludes weld metal, paint and galvanizing, and it rounds.

A shipping weight includes the crate, the skid and the packaging, and excludes anything shipped loose in a separate carton. It is the figure the freight bill was written against, which means it was measured to a commercial tolerance, not a rigging one.

An operating weight includes the fluids, charge and consumables the unit runs with. For equipment that is factory-charged, the refrigerant is already inside the operating figure and adding it again double-counts. For equipment filled in the field, it is not.

A drawing weight on a fabrication drawing is the sum of the modelled parts. Fireproofing, insulation, lagging, jacketing, internals installed after the drawing was issued and the contents of anything hollow are outside it unless the drawing says otherwise.

The four numbers can differ from each other by more than any of them differ from a careful field estimate, which is why the first question about a weight is never "what is it" but "which document, and what basis."

What the number never contained

Work this list against the object in front of you, not against the document.

• Field-installed options. Economizers, hail guards, filter racks, sound curtains, motorized dampers, extra coils. Each was ordered separately and each has its own weight, held by its own supplier.
• Contents. Water in a sump, pan, trap or coil. Product in a tank. Sludge in a sump that has not been cleaned in years. Water is heavy per unit volume and a partly full vessel is the worst case because it also moves during the lift.
• Absorbed water. Mineral fibre insulation and its jacketing hold water. Rooftop equipment that has sat through winters carries more than the same unit did new, and the amount is not knowable from any document.
• Coatings applied later. Fireproofing, epoxy linings and additional galvanizing are added after the drawing.
• Attached hardware that stays with the piece. Bolted-on brackets, spool pieces, valve trains, control panels, curb adapters, and the stub of any line that comes off with the unit rather than at a flange.
• Ice and snow. Seasonal, invisible in a photograph taken in June, and concentrated on the surfaces that also catch wind.
• Packaging still on it. Crating and blocking may still be attached when you pick, and it is not always sensible to strip them first.

The single subtraction that matters is a component listed as as-configured but shipped loose and never installed. Take that credit only after you have looked and seen the empty mounting, and even then treat it as a reduction in the estimate, not a licence to size hardware closer to the line.

What lives below the hook that no product document covers

The rigging is part of the load and it is the part the product document could never have included. On a crane, the gross figure on a load chart already contains the machine's own boom and structure, but it does not contain the hook block, the overhaul ball, slings, shackles, spreaders, lifting beams, stowed jibs or any other weight hanging below the boom point. Those are deductions from chart capacity, taken before you compare anything to the load.

This is a re-basing, not an addition: you are not adding rigging to the load, you are subtracting rigging from the machine's published capacity, because the chart was published on the assumption that the hook was bare. Getting the direction of that correction backwards is one of the two ways a crew ends up over capacity while believing they have margin. The other is the weight itself.

Rigging weight also grows with the geometry you chose. A four-leg bridle long enough to hold a shallow angle is heavier than a short one, and a spreader bar bought to keep the legs steep can weigh more than the bridle it replaced. Sling angle in this library is always measured from horizontal, and a spreader is often the right call precisely because it holds that angle up; just carry its weight into the deduction rather than pretending it is free.

Worked example: a rooftop package unit

A shop is setting a replacement rooftop unit and pulling the old one. The submittal for the new unit gives a shipping weight of 1,850 lb. Here is the whole build-up, with every correction printed as its own line and its source named.

The refrigerant line is a re-basing rather than an addition, and it is printed at zero on purpose: the submittal figure already contains the factory charge, so adding a charge weight would count it twice. If this unit were field-charged, that line would carry a real number and the note would read the other way.

The water line is the finding. Nobody knows it, and the honest options are to drain the pan and trap before the pick and confirm by eye that they are empty (with the unit locked out under 29 CFR 1910.147 for the mechanical and stored-energy sources and under 1910.333(b)(2) for the electrical disconnect, because you are opening a panel and a drain), or to carry it as an explicit allowance and say so on the plan. What you may not do is leave it off and call the total 2,280.

Now round the total, and round it in the conservative direction. 2,280 lb plus an unquantified water term is planned at 2,500 lb, which is an increase, not a nearest-hundred round. Rounding a load weight down to a tidier figure is the one place where tidiness is a hazard.

The old unit coming off is the harder half. There is no submittal for it, the nameplate is unreadable, and it has sat for nineteen winters with saturated insulation and a rusted pan. The correct finding is that its weight is unknown. It gets weighed.

When the answer is "unknown", and what that costs

Unknown is a legitimate finding and it has one honest consequence: you establish the weight before you lift, or you do not lift. 29 CFR 1926.1417(a) prohibits operating the equipment in excess of its rated capacity, and there is no way to satisfy that against a weight nobody knows. Guessing past it converts a planning problem into a capacity problem at the worst possible moment, which is at maximum radius with the load already off its supports.

The cost of establishing it is usually an hour or two and a rented instrument. The cost of not establishing it is a load that stops rising at half boom-out, a machine that reports over capacity in the middle of a swing, or a sling that parts. Compared against the crew hours already committed to a set, an hour of weighing is a small fraction of the day and the only line item on the plan that removes an unbounded unknown.

Weighing it, and the accuracy of the number you get

The usual instrument is a load cell or dynamometer rated for the range, inserted in the rigging between the hook and the top of the bridle, or a crane load-indicating device. Take the reading with the load broken free of its supports by a small amount and holding still, with everyone out from under it and out of the swing path, and read it from a position where nothing overhead is above you. 29 CFR 1910.184(c) requires employees be kept clear of loads about to be lifted and of suspended loads, and a weighing pick is exactly that.

Then read the instrument's specification rather than its display. An accuracy figure needs both its basis and its character before it means anything:

• Basis. A cell quoted at plus or minus 0.5 percent of full scale on a 20,000 lb cell is plus or minus 100 lb at any reading. On a 2,000 lb indication that is 5 percent of reading, not 0.5 percent. A cell quoted at plus or minus 1 percent of reading behaves the opposite way and shrinks as the load does.
• Character. A single cell's calibration error is a fixed systematic offset. It does not cancel here, because you are not taking a difference between two readings on the same instrument, you are using one absolute reading. It also does not combine in quadrature with anything, because there is only one term.

So on that 20,000 lb cell reading 2,060 lb, the honest statement is that the weight lies between roughly 1,960 and 2,160 lb, and the number you plan against is 2,160, not 2,060. A crane's load-indicating device is not a scale at all: it is an operating aid with the machine's own tolerance and its own configuration assumptions, and it is a cross-check on your figure rather than a source of it.

How to verify you got this right

Read your build-up back against the object. Every line should name a document or an observation, and the lines that name neither are the ones to close first. Then check three things specifically.

Check that nothing is counted twice. Factory charge, factory-installed options and any item already inside the shipping figure are the usual double counts, and they are pleasant to find because they run in the safe direction; the unpleasant one is a rigging item counted as load weight and then deducted from chart capacity as well.

Check the direction of every round. Load weights round up, capacities round down, clearances round out. If any round in your plan made the arithmetic easier and the margin larger at the same time, redo it.

Check that the total you carried into the capacity check is the total below the hook, including the spreader and shackles, and that the chart deduction for block and ball was taken separately from the machine's side. Those are two different corrections in two different places, and merging them is the mistake that leaves a lift short by exactly the weight of a hook block.

References

• 29 CFR 1926.1417, operation of cranes and derricks in construction, including the prohibition on operating in excess of rated capacity
• 29 CFR 1910.184(c), general industry slings, keeping employees clear of loads about to be lifted and of suspended loads
• 29 CFR 1910.147 for mechanical and stored-energy isolation, and 29 CFR 1910.333(b)(2) for electrical work, when draining or opening equipment before a pick
• Manufacturer submittal, load chart and sling tag documentation for the specific unit, attachment and rigging in use
• See related: What Wind Does to a Lift and When It Stops It; What Removes Rigging Hardware From Service Permanently