02
Pricing the tolerance instead of ignoring it
A centre of gravity is an envelope. The governing case is at the edge of it, and on a tandem lift the edge is worth more than any of the factors.
Give the centre of gravity a tolerance of 600 mm - 5 percent of the span, which is modest for an unweighed module - and recompute at the worst position:
- Share on B at the nominal position: 397 kN
- Share on B at the worst position: 441 kN
- Increase: 11.1 percent
Compare that with the factors on the same lift, none of which is larger than 10 percent individually. On a tandem lift the centre of gravity tolerance is usually the biggest single uncertainty in the load path, and it is the one most often entered as a point.
03
What the tolerance costs on the ground
The heavier crane's ground check passes at the nominal centre of gravity and fails at the edge of its tolerance. Nothing else about the lift changed.
Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render
Crane B at the nominal centre of gravity
558 kN on the hook at 20 m radius, on 3.5 m mats over a platform confirmed at 550 kPa.
| Hook loadshare, rigging, contingency, dynamic and tandem allowance | 558kN |
|---|---|
| Worst outrigger loadFR at 317.6 degrees of slew | 1306kN |
| Pressure on the ground | 515kPa |
| Ground bearing utilisation | 93.6% |
Passing, with a margin nobody would call generous but nobody would reject either.
Open this example in the calculatorCrane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render
The same crane at the worst centre of gravity
The centre of gravity 600 mm towards crane B, which is inside its stated tolerance. Same crane, same mats, same ground, same radius.
| Hook loadagainst 558 kN at the nominal position | 617kN |
|---|---|
| Worst outrigger loadagainst 1306 kN | 1416kN |
| Pressure on the groundagainst 515 kPa | 558kPa |
| Ground bearing utilisationfailed | 101.5% |
Failing, on a position the module is entitled to have its centre of gravity in. The difference between the two examples is one number that nobody measured.
Open this example in the calculator04
The share moves during the lift
Continuously, and the operators control it. That makes hook height a load case rather than an operational detail.
Everything above is a static picture. On the day, four things move the share.
Relative hoist. Raising one hook relative to the other tilts the load, which moves its centre of gravity horizontally relative to the hooks and transfers load. This is the largest and most immediate effect and it is directly under the operators' control.
Travel and slew. If either crane's hook moves horizontally relative to the other, the geometry changes and the share follows.
Sling stretch. The more heavily loaded crane's rigging stretches more, which tilts the load slightly and transfers some load back. A small self-correcting effect, and not one to rely on.
The load flexing. A long module is not rigid. Its own deflection changes where its weight is delivered.
Three controls that follow, all of which belong in the plan:
State a maximum permitted tilt, and give the supervisor a way to see it. A level or a taut string is enough.
Nominate a lead crane. One crane's operator controls hoist and the other follows. Two operators each correcting independently is how a share runs away.
Fit load indication on both hooks where the margin is tight. If the design depends on the share being within a range, measure the share.
05
What a tandem lift plan has to carry
Everything a critical lift plan needs, plus eight things that exist only because there are two machines.
Additional to a normal critical lift plan
- 01The share calculation, with the centre of gravity toleranceBoth shares, at the nominal position and at the worst position inside the tolerance. The worst is the design case.
- 02The tandem allowance, named and justifiedA factor covering share uncertainty, stated separately from the dynamic factor and the weight contingency.
- 03Both cranes' duties at their own governing radiusTwo machines, two charts, two radii. Neither is the other's.
- 04Both cranes' ground checksThey are different machines on different ground, and the heavier share is not always on the better ground.
- 05A nominated lead craneOne operator controls hoist; the other follows. Written into the sequence.
- 06A maximum permitted tilt, with a means of seeing itA number and an instrument, not a judgement.
- 07The communication arrangementOne signaller for both cranes, or a defined protocol between two. The failure mode is two crews acting on different information.
- 08What happens if the share is not what was predictedA hold point, a measurement, and who decides whether to continue.
06
Seven ways a tandem lift goes wrong
Five are about the share. Two are about the two crews being one operation.
1. The centre of gravity used as a point. The share depends on it linearly, and on the worked lift the tolerance was worth more than any factor.
2. No tandem allowance. The share is the uncertain quantity on a two-crane lift, and it needs its own factor.
3. Each crane checked at the other's radius. Two machines, two charts, two radii.
4. The ground checked for one crane. They stand in different places, and the heavier share is not always on the better ground.
5. The share treated as fixed during the operation. Relative hoist transfers load continuously, and it is under the operators' direct control.
6. No lead crane nominated. Two operators correcting independently is how a share runs away, and it happens fast.
7. Two crews on different information. The single most common contributing factor in multiple-crane incidents, and the cheapest to prevent with one signaller and one agreed protocol.
Common questions
- How is the load shared between two cranes?
- By moments, exactly as a simply supported beam's reactions are. The share on one hook is the load multiplied by the distance from the other hook to the centre of gravity, divided by the distance between the hooks. Two cranes is a genuinely determinate problem, unlike a four-leg sling, so the statics is exact. What is not exact is the centre of gravity, and the share depends on it linearly.
- What tolerance should I use on the centre of gravity for a tandem lift?
- One you can justify from a sensitivity calculation on the component masses, and then design at the edge of it. On the worked 90 t module a 600 mm tolerance - five percent of the span between hooks - moved the heavier crane's share by 11 percent, which is larger than any of the individual factors applied to the lift. On a tandem lift the centre of gravity tolerance is usually the biggest single uncertainty in the load path.
- Why is a tandem allowance applied on top of the usual factors?
- Because on a two-crane lift the share itself is the uncertain quantity rather than the weight. A weight contingency covers not knowing the total, and a dynamic factor covers motion, and neither covers the load transferring between hooks because the centre of gravity is not where the drawing put it or because one operator hoisted faster than the other. The allowance is a separate, named factor covering a separate uncertainty.
- Does the share between two cranes change during the lift?
- Continuously, and the operators control it directly. Raising one hook relative to the other tilts the load, which moves its centre of gravity horizontally relative to the hooks and transfers load between them. Travel and slew do the same. Sling stretch produces a small self-correcting effect not worth relying on. The controls that follow are a stated maximum tilt with a means of seeing it, a nominated lead crane, and load indication on both hooks where the margin is tight.
- What extra does a tandem lift plan need?
- The share calculation at both the nominal and the worst centre of gravity, with the worst as the design case; a named tandem allowance stated separately from the other factors; both cranes' duties at their own governing radii; both cranes' ground checks, because they stand in different places; a nominated lead crane; a maximum permitted tilt with an instrument to see it; a single communication arrangement; and a hold point for what happens if the measured share is not what was predicted.
Sources
Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.
29 CFR 1926 Subpart CCCranes and Derricks in Construction
US Occupational Safety and Health Administration · free to read
The whole US construction crane subpart, free in full: ground conditions, assembly and disassembly, power line clearance, operator qualification, signals, inspection and multiple-crane lifts. The index page, because the duty a reader needs is usually two sections away from the one they searched for.
ASME B30.5Mobile and Locomotive Cranes
ASME · paid document
Construction, installation, operation, inspection and maintenance of mobile cranes in the US, including load rating and the requirement to operate within the manufacturer's chart. It governs the machine; the ground it stands on is 29 CFR 1926.1402 and the calculation is yours.
HSE lifting equipment guidanceLifting equipment at work: planning and organising lifting operations
UK Health and Safety Executive · free to read
The regulator's own plain-language account of what planning a lifting operation means in UK law: who is competent to plan it, what a plan has to address, and how it scales from a routine repetitive lift to a one-off complex one. Free, short, and the closest thing to an official answer to 'what has to be in a lift plan'.
LOLER 1998Lifting Operations and Lifting Equipment Regulations
UK Health and Safety Executive · free to read
The UK duty framework for lifting operations: planning by a competent person, supervision, and thorough examination of lifting equipment and accessories. Like OSHA's rules it governs the process, not the arithmetic.
Run the check properly
Reading about a calculation is not the same as being able to hand one over. These tools produce the traceable record.
Something here wrong, or thinner than it should be? Tell us which paragraph and it gets rewritten. Articles carry the date they were last revised for exactly this reason.