Australia, CICA, Cranes & Lifting, Features, Safety

Playing Jenga or working with a crane?

CICA Jenga Crane

In part two of his April article, Klaus Meissner looks at setting up supporting stacks for safe mobile crane outrigging.

Most mobile cranes need to be operated on firm support in 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.

It’s friction that prevents contact surfaces from sliding by horizontal forces.

The amount of friction force is proportional to the vertical force and the coefficient of friction between the two contact surfaces. Friction coefficients depend on the materials and texture of the surfaces and the state of the contact; there are huge differences from dry over greased to wet and special materials like PTFE (better known as Teflon). Steel to PTFE has a friction coefficient of 0.04, but steel to greased steel is about 0.12, steel on dry wood is the same as wood to dry wood – around 0.5. So a contact of steel on dry wood usually has friction forces exceeding the horizontal forces created by the crane.

For metal outrigger pads resting on steel plates the horizontal force generated by the loaded crane may be quite the same as the maximum friction force which can be transferred.

Beware of slippery (wet, greasy or greased) contact surfaces!

If there is grease/oil at the contact surfaces the same wood-wood or steel-wood combination has a friction factor dropping easily to 0.1! All of these friction factors are the values observed before any movement (static friction or adhesion), they apply only up to the point when the horizontal force equals the friction force. Now the horizontal force may overcome the friction force and the two materials start moving relatively to each other. Usually, the friction coefficient then drops further (dynamic friction).

It is better not to rely too much on friction without extra friction material (e.g., rubber mats). We understand the need to secure heavy steel counterweight slabs on platforms of trucks or trailers to avoid them sliding off the platform by using rubber mats and adequate lashing.

With horizontal forces acting on the stack exceeding friction forces, the outrigger pad may slide off the stack and/or the stack may start moving and collapse. Some accessory manufacturers offer plates with recesses/cavities to capture the outrigger pad. Blocks with interlocking shapes or blocks with special high friction surfaces to build up stacks are available in some sizes.

Timber stack with wedges to create leveled layers.

Considering the following will assist in building up solid and reliable support stacks (compare fig. 4, a setup which includes many, but not all considerations given below):

 Check the ground beneath the stack for sufficient bearing capacity, including sufficient distance to trenches, pits, excavations, or underground facilities. 

 Ensure the ground has been prepared and leveled to start with a horizontal base. Use wedges on inclined harder ground (e.g., road surfaces) to create a horizontal base, dig into an incline for a levelled base or use compacted, levelled gravel as a top layer on the soil.

 Consider using a steel mat on softer ground.

 Use graded hardwood timbers or synthetic blocks, which are engineered as crane support, for building the stack.

 Timbers used should be approximately 2 by 1 times (width by height), timbers over time may lose their sharp edges and develop the tendency to roll.

 Avoid mixing and matching different timber/block sizes in the same stack, at least ensure all timbers or blocks in a layer are of same thickness.

 Avoid mixing different timber grades (i.e., types of wood) in the same layer to avoid an uneven settlement of timber under pressure.

 Create friction between layers of timber by using dry wood; some engineered systems have special surfaces that increase friction or use slotting mechanisms between layers.

 Ideally increase friction between the outrigger plate and upper layer of the stack by using a rubber mat.

 Be aware of friction between the lowest layer and the ground, take measures as necessary.

 Plan the direction of the crosswise laid timber or elements to ensure that the upper layer directly underneath the outrigger pad is placed parallel to the axis of each outrigger.

 Ensure that outrigger pads do not separate the upper layer, lay timber, or blocks of upper layer without gaps, and are wider than the outrigger pad. Consider using an additional plate.

 Arrange the other layers preferably with solid blocking to avoid gaps, if unavoidable, gaps should not exceed 2/3 of the width of the timbers/blocks. As a minimum, do not reduce contact surface between layers in the direction from top down of stack.

 The stack should follow the simple 1-to-3-rule, always 3 times wider than high (the pyramid),

 Ensure that the outrigger pads are in the centre of the stacks with sufficient distance to the edges in case of moving.

 Create stacks high enough to retract the outrigger jacks as much as possible (ideally the jacks are extended equally), more extension of the jacks always creates less horizontal stiffness. This will have you operating slower and may create more wear in the sealing of the jacks.

 Never use intermediate blocking under the outrigger beams.

All mobile cranes are designed with means for access in positions with outrigger jacks partially extended only, cranes on stacks may be positioned significantly higher. 

Don’t forget means for access.

Check the setup again prior to commencing work and observe it regularly during working. The crane may “walk” due to changing horizontal forces, and remember, the ground may subside when working over a longer period, over a single outrigger or quadrant.

Last but not least – If it doesn’t look right, rethink.

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). 

Send this to a friend