PLAYING JENGA OR WORKING WITH A CRANE?

Part one of a two-part article written by Klaus Meissner focusing on how to set up supporting stacks for mobile crane outriggers to ensure they are operated safely.

MOST MOBILE CRANES NEED TO BE operated on firm support on a levelled position. If an uneven ground cannot be compensated by outrigger jacks or when the crane needs to be raised, stacks of timber or similar material are required. From time to time unique solutions are on display; but the fact that it worked does not automatically mean it was a sound set up. It is important to understand the forces acting on and, within the stack, to prevent mistakes.

According to AS1418.5, all-terrain and crawler cranes are designed to be operated on firm and level ground. Levelling prevents the boom working “uphill” (i.e., load radius smaller than indicated by RCL), “downhill” (i.e. real load radius greater than calculated by RCL) and even more critical, the boom under “sideways bending” when working parallel to the incline (i.e. sideways deflection not measured by RCL). The capacity charts of the crane are usually based upon the assumption that a crane is levelled within 0.5 per cent (0.3°) gradient, unless otherwise specified by the manufacturer.

Crawler cranes require some preparation of the ground on the job site to create a levelled area with sufficient ground bearing capacity under the crawler tracks. All-terrain cranes can be levelled by using the outrigger jacks, but it may be necessary to create stacks to equalise the support area, due to the limited stroke of the outrigger jacks. The jacks will then be used to make final adjustments. When working on inclined terrain or raising the entire crane to increase hook height, such stacks can become quite high.

Any support stack, especially when it is high, has the potential to collapse.

To set up proper stacks it is necessary to understand how forces act.

Primarily outriggers and the stacks under each outrigger are loaded with the vertical outrigger forces, of which the maximum value can be found in the cranes’ documentation.

As shown in the Figure 2 the vertical forces acting on each outrigger changes with the configuration, weight and radius of the lifted load and slewing position.

Calculation tools are available to determine the actual forces, which contain the required technical data on geometry and weights of the mobile cranes and their components.

The figures with outrigger loads are taken from CICA’s Guidance Note on “Crane Stability and Ground Pressure”.

More information about outrigger forces and ground pressures under crawlers and links to download calculation software of the mobile crane manufacturers are provided in this CICA guidance note.

Unfortunately, the vertical forces from the empty or loaded crane are not the only forces acting on outrigger jacks and support stacks. The varied loading of the crane with or without load creates horizontal forces at the outrigger jacks by bending momentum in the outrigger beams. These forces act in the longitudinal direction of the outrigger beam and change with the loading condition.

LOADER CRANES (aka vehicle loading cranes) often have to be set up on stabilisers with the wheels still in contact with the ground, transferring forces from the loader crane to the ground. By taking horizontal forces inand out-of-operation, this wheel contact minimises horizontal forces acting onto the stabilisers. Nevertheless, stabilisers of loader cranes are designed taking into account horizontal forces of at least 5 per cent of the vertical force (AS1418:11).

MOBILE CRANES operating on outriggers are set up lifting the entire vehicle off the ground to activate all dead weight as ballast, and to activate the outriggers creating the tipping lines. All horizontal forces in- and out-of-operation of a mobile crane have to be taken by the outriggers and their supports. These horizontal forces may reach values of between 10 and 15 per cent of the vertical force and will act on the supporting stacks!

FURTHER HORIZONTAL FORCES MAY BE CREATED BY

• Unlevelled stacks by (a) unprepared base or (b) stack built from material of different thickness or stiffness used in the same layer. These forces act in the direction of the inclination, their amount depending on the gradient,

• P icking up loads out of vertical or wind acting on crane and load,

• A brupt crane movements, with the direction of the forces depending on the movement,

FIGURE 3 SHOWS FORCES ACTING ON TIMBER STACKS FOR LEVELLED AND INCLINED STACKS.

Changing loading conditions in the outriggers results in varying horizontal forces. Operators may recognise a sudden noise when an outrigger plate under changing vertical load releases the horizontal tension by a sudden small movement. It happens when the horizontal force is equal to or exceeding the friction between e.g., outrigger plate and steel mat. Keep an eye out for the second part in the next edition of Cranes and Lifting.

Klaus Meissner has worked for more than 30 years in different R&D positions on mobile and tower crane design (Demag, Peiner) and provides consultancy on product safety and technical compliance matters. He is chairman of the workgroup for the European product safety standard for mobile cranes, EN13000 and is member of the supervisory board of the ECOL foundation (European Crane Operator License.