Rigging a Multi-Leg Lift

Half Past Eight in the Yard, With Tomorrow's Bridle Half Made Up

The pick sheet says four legs and 6.4 tonnes, and the racking behind you holds two-metre, three-metre and four-metre legs with nothing in between. The tag on each of them states a working load limit that belongs to one condition — a straight vertical pull, one leg, no bend, no side load — and the assembly you are about to build will never once be in that condition. Everything between the number on the tag and the number the leg will actually see tomorrow is arithmetic, and it has to be finished tonight, while the hire desk still answers the phone and the racking is still open.

Two questions get muddled at this hour and they belong to different people. What the machine can hold is a chart question — configuration, radius, deductions, the ground under the floats — settled elsewhere by someone with the manual for that serial number in front of them. What the assembly under the hook can hold is yours. Nor are the two sequential, which is the awkward part: the rigging is a deduction from the chart, so the lift planner cannot close his half until you have chosen the bridle, and you cannot finalise the bridle until you know the item is going on that machine at that radius. Somebody has to guess first, and the guess that gets revised least is the rigging one.

A bridle is a chain of separately rated items and is worth exactly what its weakest member is worth: hook, master link, legs, the terminal fittings swaged or spliced onto them, shackles, the lifting points welded to the item, and the steel behind those points. Sizing the slings, finding them comfortable and calling the job done is the ordinary way to build an assembly that fails at the fourth item down the list.

Four Legs, and How Many of Them You Are Allowed to Count

Hang four fixed-length legs off one master link onto a rigid item and the arrangement is statically indeterminate: more unknowns than equations to settle them with. Which legs take the weight is then decided by things nobody measured — a leg a few millimetres longer than its neighbour, a lug 3 mm off the pattern, a base frame that is not flat, a splice bedded in further than the others. Two take most of it, two ride along, and which two can swap as the item settles. That is not a defect in the sling; it is what happens when you constrain something more ways than it can respond to.

The trade handles that with a convention rather than a calculation, and there are two conventions in circulation. The European standards approach it as a mode factor applied to a single leg's working load limit: BS EN 13414-1 for wire rope slings and BS EN 818-4 for grade 8 chain both publish factors for two-, three- and four-leg assemblies in stated angle bands, and the factors given for three- and four-leg slings are built on three legs carrying, not four. North American practice on a rigid load with fixed-length legs is more cautious again and credits two. Neither number is derivable. Both are statements about how much you are permitted to assume, and the one that governs your assembly is the one printed on its tag and repeated on its certificate.

If you want the legs to genuinely share, build sharing into the arrangement rather than hope for it. Chain shorteners set a length; they do not equalise, because once the load is on a shortener is as fixed as the link it holds. A bridle reeved so one continuous part runs over a free sheave equalises to within its own friction. Two independent two-leg slings taken to the ends of a spreader is determinate and the cleanest answer available. And an item that is itself flexible — a bundle, a long girder, a stack of sheets — redistributes as it deflects and shares far better than a welded skid ever will.

Three legs are worth a thought before you reach for four out of habit. Three points define a plane, so a three-leg bridle finds one hang and settles there, which removes the indeterminacy that four legs introduce. The catch is that it only works while the centre of gravity projects inside the triangle those three points make. Outside it, the item tips until something that was never meant to be a lifting point becomes one. On an item whose weight is genuinely off to one corner, three legs is the arrangement that shows you the problem at 100 millimetres off the ground instead of at head height.

The Centre of Gravity Is Not Where the General Arrangement Put It

On a fabricated skid the drawing gives an outline and the equipment schedule gives masses, and somebody still has to do the moment sum that turns them into a point. Do the sum on paper, then treat it as a first estimate rather than a fact, because the classic errors are all things a drawing cannot show: an oil charge shipped in the gearbox, ballast added at the works, a motor uprated after the general arrangement was issued, water left in a vessel that was pressure-tested and not fully drained.

Expressed the way the arithmetic wants it, the offset is a fraction of the span between lift points. A centre of gravity a fifth of the span off the middle puts seventy per cent of the weight on the near pair and thirty on the far pair, and it does that regardless of how good the slings are. On a four-leg pick the offset exists in two directions at once, so the pair carrying the larger share is not a side of the item but a diagonal corner of it — and that one corner leg is what the whole bridle has to be sized against. Sizing against the average of four legs on an off-centre load is the most common way to end up with a sling that is comfortably rated for a load it is not carrying.

The measurement, in the end, is the trial lift, and it is a measurement rather than a formality. Take the item a hundred millimetres clear, stop, and look at the hoist rope against something vertical. A load that hangs out of level is telling you the hook is not over its centre of gravity, and the direction it tilts tells you which way the hook has to move. Correct it by moving the hook, not by shortening a leg, unless the legs are adjustable by design and the certificate says so — a leg shortened to level a load has changed its angle, its tension and its rating all at once, and none of that is written down anywhere.

1. Do the moment sum from the equipment schedule before you leave the yard, and write down the assumption rather than carrying it in your head.
2. Ask what is inside the item as shipped: oil, water, ballast, packing, and anything fitted after the drawing was issued.
3. Set the bridle for that estimate, take the load 100 mm clear, stop, and read the tilt against a vertical reference rather than by eye across the yard.
4. Move the hook toward the leaning side and repeat until the item hangs level, or until the offset is large enough to want a spreader instead.
5. Land it, and only then reassess whether the sling sizes chosen against the estimate still cover the corner the trial lift found.

What a Shallow Bridle Does to the Thing It Is Lifting

The inward pull in the last column of that table has to go somewhere, and it goes into the item. At 45 degrees each leg squeezes its lift point toward the centre with a force equal to the vertical share it carries; at 30 degrees it squeezes harder than it lifts. That closes a cabinet's panels and creases its corners, bends a skid's lugs inward, and does to a bundle exactly what a ratchet strap does. Fabricators know it, which is why the lifting detail on a decent drawing states a minimum sling angle or a minimum spreader dimension, and that sentence is a requirement rather than advice.

Padeyes are the sharpest version of the problem. A padeye is a plate: stiff in its own plane, and comparatively weak across it. A leg that runs off to a corner rather than in the plane of the plate loads it out of plane, and the fabricator's detail normally states a tolerance in degrees for exactly that. Shackles are the fitting that lets a leg swing to where it wants to be, which is why the bow goes to the sling and the pin goes through the padeye when several legs meet at one point — the legs bear on the bow, they can rotate freely in it, and they cannot work the pin loose. Side loading a shackle away from the plane of its bow derates it, and the maker publishes that reduction against angle in its catalogue. There is no generic figure worth committing to memory; go and look it up for the shackle in your hand.

Two more deratings live at the sling itself. A wire rope bent tightly round a corner loses capacity as the ratio of bend diameter to rope diameter falls, and the maker publishes the efficiency against that ratio. Synthetics do not lose capacity gradually at an edge; they cut, and a roundsling can part on a rolled plate edge that looked harmless in the yard. BS EN 1492-1 and BS EN 1492-2 both address edge protection, and the radius that decides it is the one on the real corner with its weld cap and its burr, not the one on the section detail.

When the geometry cannot be fixed by buying longer slings, the answer is a beam, and the two kinds of beam do different jobs. A spreader is loaded in compression: it holds the lift points apart so the legs above it can stand steep, and it takes the inward pull out of the item entirely. A lifting beam is loaded in bending: it lets you pick a long or awkwardly balanced item from a single point above it, and it is correspondingly heavier for the same capacity. Both are below-the-hook devices with their own rated capacity, marking and inspection record — ASME B30.20 and ASME BTH-1 in the United States, BS EN 13155 in Europe — and both are a deduction from the crane's chart capacity. The forgotten spreader is very often the heaviest single item in the assembly and the one nobody weighed.

The Bridle Is a Lifting Accessory, Not a Bag of Parts

Make an assembly up in the yard and you have created a lifting accessory, not gathered some components, and it has to be identifiable as one. Each leg carries its own mark and certificate; the assembly needs a statement of what it is rated to and in what mode, because a four-leg bridle's rating is a property of the legs, the master link, the terminal fittings and the mode factor together. An illegible tag is not a paperwork irritation — the rating and the mode live on it, and a sling nobody can read is out of service, tonight, without argument.

The examination regimes are worth knowing by name, because none of them is the same thing as your pre-use look. The Lifting Operations and Lifting Equipment Regulations 1998 require thorough examination of lifting accessories at the interval stated in the Regulations or under an examination scheme drawn up by a competent person, and LEEA's Code of Practice for the Safe Use of Lifting Equipment sets out how that is done. OSHA 29 CFR 1926.251 requires rigging equipment to be inspected before use on each shift and lists the conditions that take a sling out of service, with ASME B30.9 behind it holding the removal criteria by sling type. What none of them replaces is the look you give it while you are making it up, in decent light, with time to swap a leg.

That look is short and material by material. Wire rope: broken wires, a kink, corrosion at the ferrule, a thimble that has closed. Grade 8 chain: stretch measured over a stated number of links against the maker's own figure, gouges, a link that is bent or twisted. Webbing and roundslings: cuts and abrasion at the eyes, damaged edge stitching, glazing from heat, chemical staining, and any sign of the core showing through a roundsling cover — because that cover is there to protect and to signal, and a visible core means both jobs are over. Hooks: throat opening against the maker's figure, twist across the throat, and the understanding that a latch retains the sling and carries nothing.

1. Read every tag before anything goes in the bag, and reject the leg rather than the tag if you cannot read it.
2. Check the mode the assembly is rated in matches the mode you are about to use it in — a four-leg bridle at 40 degrees is not the same entry as the same slings used in a basket.
3. Lay each leg out full length rather than judging it coiled, and run a gloved hand along it if the light is poor.
4. Match shackle size and grade to the leg and to the padeye hole, and confirm the pin can be fitted so a moving leg cannot back it out.
5. Fit the edge protection now, cable-tied in place, rather than intending to fit it at the hook in the morning.
6. Weigh or tally the finished assembly, spreader included, and send that figure to whoever is closing out the chart calculation.

What Goes on the Sheet, and What the Next Shift Inherits

Write the assembly down while it is still in front of you. Leg length and type, the mode, the angle the geometry produces, the worst leg's tension and the rated capacity that covers it, shackle sizes, the spreader and its weight, the centre-of-gravity assumption and how you reached it, the wind limit and who reads the anemometer, and the point in the sequence where the trial lift happens and everybody stops to look. Half a page turns tonight's reasoning into something the morning crew can check rather than re-derive at the hook with the driver waiting.

Name what would invalidate it, too. A different lift point than the fabricator's detail shows. An item heavier than the docket, discovered on the trial lift. Less headroom than measured, which arrives as a shallower angle and moves the tension the wrong way on a curve already steepening. Any of those means the sheet is revisited, not adjusted verbally between two people standing under a suspended load.

Then send the rigging weight back up the chain — legs, master link, shackles, spreader, chain blocks, tag lines — because it is a deduction from the chart, and it is the one number the lift planner cannot get anywhere except from the person holding it. Keep the record thin but real: which slings went out on which job, what came back damaged, what was removed from service and why. Lifting arguments settle months later between a hire company, a principal contractor and an insurer, and the party that wins is the one who wrote down what the bridle was, what it was rated to, and what it was hanging in.