Planning a lift: weights, roles and lift plans

Most crane incidents are decided before the hook moves. The load turned out heavier than the paperwork said, the crane was set up where the radius was longer than planned, nobody checked what was under the outriggers, or the person signalling could not see the load. Lift planning is the discipline of answering those questions in advance, writing the answers down and making sure everyone involved understands them.

This lesson covers how to decide how much planning a lift needs, how to find a load's weight and centre of gravity, who does what on a lift, what goes into a lift plan, and how to run the pre-lift meeting and trial lift. The aim is not to turn supervisors into lift engineers but to make sure they can recognize a weak plan and stop a lift that has not been thought through.

Ordinary and critical lifts

Employers usually sort lifts into two or three categories. Routine lifts, such as unloading standard bundles from a truck well within capacity, can be covered by a general procedure and the operator's own checks. Critical lifts need a written, lift-specific plan reviewed and signed by competent people before the lift.

The 75 per cent trigger is common in employer programs and owner specifications, and Alberta's OHS Code requires a written lift calculation, including load details, crane configuration, the percentage of capacity and a sketch of placement and clearances, whenever a load reaches or exceeds 75 per cent of the crane's capacity. Some employers use 85 or 90 per cent for the highest planning tier.

Finding the weight of the load

Every lift calculation starts with the weight of the load, and the weight is the number most often wrong. Shipping weights may leave out packaging, skids or contents. Drawings may show dry weight without fluids. Fabricators may give an estimate before the item is finished. Good practice is to get the weight from at least one reliable source and check it against another.

• Manufacturer data or a certified weight on the item's nameplate.
• Shipping documents and bills of lading, checked for what they include.
• Calculation from dimensions and material density.
• Weighing with a load cell or dynamometer between the hook and the rigging, which is the most reliable method when available.
• Weighing on a truck scale, comparing loaded and empty weights.

Hidden weight is a frequent cause of overloads. Tanks and vessels may hold liquid or sludge. Pipe may be full of water or ice. Loads lifted from mud or water are held by suction until they break free, which can briefly add a large force. Old equipment may have been modified with heavier parts. Items frozen to the ground or bolted down must be freed completely before lifting; a crane should never be used to pull something loose.

Finding the centre of gravity

The centre of gravity is the point where the whole weight of the load can be considered to act. When a load is lifted, it rotates until its centre of gravity hangs directly below the hook. If the hook is not above the centre of gravity, the load tilts as it leaves the ground, may slide in the slings and can load one sling far more than the others.

For symmetrical loads of uniform material, the centre of gravity is at the geometric centre. For skids, machinery and fabrications, it must be found from drawings, from the manufacturer or by calculation. To calculate it, take moments of each part's weight about a reference point.

A high centre of gravity above the lifting points makes a load unstable: it can topple within its slings. Tall, narrow loads should be lifted from points above their centre of gravity or with a spreader beam and a basket arrangement that cradles them.

Roles on a lift

Every lift needs clearly assigned roles, and every person needs to know who has which role before the lift begins. On small lifts, one person may cover more than one role, but each role must still be covered.

In Ontario, the constructor coordinates crane work on a project and the employer of the crane crew is responsible for its operation; both must make sure the people assigned are competent and, where required, certified.

What goes into a lift plan

A lift plan answers, in writing, the questions that decide whether a lift will be safe. For routine lifts a short checklist may do. For critical lifts the plan is detailed, includes calculations and drawings, and is signed by the people responsible.

The pre-lift meeting and trial lift

A written plan only works if the people doing the lift know it. Before any critical lift, and before routine lifts on a new site or with a new crew, the lift director should hold a short meeting at the lift location. The crew walks the load path, confirms the radius and set-down point, checks the exclusion zone, agrees on signals and confirms who can stop the lift. Anyone who has a concern raises it then.

A trial lift tests the plan with the real load. The load is lifted just clear of the ground, typically a few centimetres, and held. The crew checks that the load hangs level, that the slings are seated and not slipping, that the brakes hold, that the outriggers and mats show no settlement and that the load moment indicator reads what the plan predicted. If anything is unexpected, the load is lowered and the plan corrected before the lift continues.

Multiple-crane lifts

When two cranes lift one load, the share each carries depends on the geometry of the rigging and the load's centre of gravity, and it changes as the load is raised, tilted or moved. If one crane hoists faster or swings differently, it can pick up far more than its planned share. These lifts need an engineered plan, a single lift director, constant communication and usually a reduced percentage of each crane's chart. Lifting with two cranes is never a way to lift a load that is too heavy for one crane without proper engineering.