How cranes stay up: stability, load charts and ground

A mobile crane is a balance. On one side is the load, hanging at the end of a long boom. On the other side are the crane's own weight and its counterweight. Between them is a tipping line, usually the line through the outrigger floats or the edge of the tracks on the side of the load. As long as the crane's stabilizing moment exceeds the load's overturning moment, the crane stays upright. This lesson explains that balance, how the load chart describes it, and why the ground under the crane matters as much as the crane itself.

The calculations here are for understanding. The manufacturer's load chart, operator's manual and outrigger load information always govern, and lift calculations for critical lifts are prepared by competent people using the actual chart for the crane on site.

Moments and the tipping line

A moment is a force multiplied by the distance from a pivot. The load creates an overturning moment about the tipping line equal to its weight times its horizontal distance beyond that line. The crane's weight and counterweight create a stabilizing moment equal to their weight times their distance behind the line.

Several things increase the overturning moment without anyone touching the load: booming down or telescoping out increases the radius; the boom deflects under load, which pushes the hook outward; the load swings outward when the crane slews quickly; wind pushes on the load and boom; and a crane set up out of level moves the load further out on the downhill side. Every one of these effects is small, and every one of them has contributed to overturns.

Radius: the most important number

The radius is the horizontal distance from the crane's centre of rotation to the centre of the hook, with the load suspended. It is not the distance from the edge of the crane, and it is not the boom length. Capacity falls quickly as radius grows, so a few metres of error can turn a safe lift into an overturn.

Reading a load chart

A load chart is the manufacturer's table of rated capacities. It is specific to the crane model, and often to the serial number. A typical chart for a telescopic crane lists boom lengths across the top, radii down the side, and capacities in the body of the table, for one particular configuration of outriggers and counterweight and one working area, such as 360 degrees or over the rear only.

Three features of real charts are easy to miss:

• Charts usually mark which capacities are limited by structural strength and which by stability. Near the crane, the boom or another component would fail before the crane tipped; far out, the crane would tip first. Either way the chart value is the limit, but it explains why a crane can collapse without tipping when overloaded at short radius.
• Chart capacities already include a margin below the tipping point. For example, ASME B30.5 bases capacities on outriggers at no more than 85 per cent of the tipping load and on crawlers at no more than 75 per cent. That margin covers dynamic effects and small errors. It is not spare capacity.
• The notes are part of the chart. They state what must be deducted, the conditions such as level within a stated tolerance, the maximum wind speed and which configurations are allowed. A capacity read without the notes is meaningless.

Deductions: what counts as load

Everything hanging below the boom tip counts as load. The chart capacity is the gross capacity; the net capacity available for the item being lifted is what remains after deductions. Typical deductions include the hook block or headache ball, the slings, shackles and spreader beam, and, on many cranes, a stowed or erected jib and auxiliary boom head even when the load is on the main hoist.

Configuration and working area

A crane's capacity changes with its configuration. Outriggers that are only partly extended, reduced counterweight, a different boom mode or working over the side instead of over the rear can each cut capacity sharply. Many cranes now allow partial outrigger extension with a matching chart, and the load moment indicator must be set to the configuration actually used. A wrong setting tells the operator the crane has capacity it does not have.

Working areas matter too. Some cranes have more capacity over the rear than over the side or front, and some charts forbid lifting over the front at all. When a load is swung from the rear to the side, the capacity can drop below the load mid-swing. Lift planning must check the capacity for the worst position along the whole swing, not just at pickup and set-down.

Ground: the foundation of every lift

A crane is only as stable as the ground under it. When the load swings over an outrigger, that outrigger may carry most of the crane's weight plus the load. If the ground under it settles, the crane tilts, the radius grows and the overturning moment increases. That can start a chain that ends in an overturn within seconds.

The pressure an outrigger puts on the ground is its load divided by the area that carries it.

Hidden hazards under the surface

The most dangerous ground is the ground that looks fine. Backfilled trenches, buried tanks, basements, culverts, catch basins, old foundations, utility vaults and recently compacted fill can all collapse under an outrigger. Asphalt and concrete slabs can crack and punch through. Frozen ground can be very strong in the morning and weak by afternoon. Edges of excavations and retaining walls are especially risky because the soil near them can slide toward the opening, a topic covered in HSE-217.

In Canada, the employer, the constructor and the crane owner share responsibility for confirming the ground is adequate. CSA Z150 and manufacturers require the crane to be level within a tight tolerance and on firm, supporting ground. On many sites, the constructor provides a crane pad or a geotechnical assessment for planned crane locations.

Before a crane sets up, someone should confirm with drawings, locates and a walk of the area that nothing is buried under the outrigger positions. During the lift, a person should watch the outriggers and mats for settlement and stop the lift if any sign appears.