CICA Tech Corner

UPDATE TO CICA GUIDANCE NOTE ON GROUND PRESSURE

The CICA technical committee has reviewed the CICA guidance note on crane stability and ground pressure, recommendations of safety factors for allowable ground pressure and allowable axle load calculation have been incorporated in the latest edition of the guidance note.

THIS ARTICLE GIVES A PREVIEW OF THIS

guidance note, it briefly discusses some key points on crane stability and ground pressure. For more details, refer to the full guidance note which provides guidance to assist on: • Clarifying responsibilities for ground capacity assessment. • Ultimate ground pressure vs allowable ground pressure. • Determining the loads and forces. exerted by mobile cranes to the ground. • Reading and interpreting manufacturer information. • Determining the suitability of the crane mats. • Interpretation of industry adopted rule of thumbs.

Effective assessment of ground conditions is essential prior to the safe set up and operation of cranes. To reduce the risk of crane accidents, planning activities shall be carried out by a competent person(s) to assess the capability of the ground to withstand the loads and pressures imposed by the lifting equipment.

RESPONSIBILITIES

All parties involved in the planning, set up and use of cranes on site must be aware of the fundamental criteria, planning issues and risk assessments that are needed to ensure lifting operations proceed in a safe and stable manner. All parties have shared responsibilities to ensure safe operation on site.

GROUND CONDITION

Typically, a mobile crane will be supported via one of, or a combination of the following ways: • Natural ground, for example, existing natural ground or pavement. • Constructed crane pad, for example, hardstand. • Existing or purpose-built structure, for example, concrete slab or steel grillage. • Combination of above, for example, piles driven into natural ground that require the crane or a grillage platform to be positioned on it.

Each of them will require a method of calculation in their capacity to provide a suitable setup arrangement. Some will simply use industry adopted rules of thumb. Some will require key input from the site and the work of a design engineer with knowledge in the civil, structural and geotechnical area.

CRANE LOADING

How a crane transfers its load to the supporting surface will depend on the type of crane. There are usually three types of loads: • Axle loads, commonly from pick and carry cranes. To understand a correlation between axle load and ground pressure, further assessment

Figure 3 - Manufacturer software is provided with static analysis of specific load conditions. by a suitably qualified and competent engineer is required. The engineer will consider factors such as type of ground and resistance, angle of repose and zone of influence. • Outrigger or pedestal loads, these are typically through the use of mats and effective bearing area. • Crawler Track Loads, these are typically based on the track width and effective contact length.

Calculation of the load exerted by the crane outrigger or track should consider both the vertical load (from the crane, rigging gear and object loads, etc.) and the load caused by the moment acting on each outrigger or track under different load cases (see Figure 1 and Figure 2).

During lift operations, crane boom length, slew angle and slew arc may vary, these will change the forces on the outriggers. It is often assumed that the maximum loads and forces will occur during operation at maximum capacity, this is not always the case, for example, outrigger loads could be at their highest without any load on the hook at minimum radius due to the backward moment from the counterweight. In some load cases, total load of the crane may be imposed largely on one outrigger or one crawler track when the boom or counterweight is slewed over that outrigger or over the side of that track. Beyond the load analysis above, crane operations may often require additional considerations. Other loads to consider include: • dynamic loading due to wind pressure • lateral loading due to incorrect set-up or different settlement of supports • dynamic loads caused by slewing or a swinging lifted load • dynamic loads caused by crane traveling with load • emergency loads The person or company in control of a lifting operation should ensure the engineer/designer for the design and assessment of the ground capacity is aware of these loads. Suitable design and safety factors should be applied (refer to the guidance note for recommended safety factors) to determine allowable ground pressure or allowable axle load from ultimate ground pressure or ultimate axle load.

MANUFACTURER SOFTWARE

When properly used, manufacturers’ resources are the preferred method of calculation. Manufacturer software is provided with static analysis of specific load conditions (see Figure 3). Conditions under which the results are calculated must be interpreted correctly and limitations of the calculation must be considered when using the results in lift planning. The guidance note provides links to software developed by crane manufacturers (CICA National Marketers) and provides guidance on how to read the results.

CRANE MAT SIZE

Crane mats (timber, steel, HDPE, etc.) are used to distribute the load of the crane to the ground. The suitability of the crane mat used is determined by: • the area of the mat is suitable to distribute the load to the ground at a stress level less than the ground bearing capacity and • the strength and integrity of the mat and its ability to handle the load exerted by the crane.

If crane load and crane mat size are known, the pressure imposed by the crane on the ground can be calculated by using pressure and area. This pressure is then compared with the maximum permissible ground pressure to check whether the mat size is suitable for the lift.

If crane mat size is unknown, it can be calculated by dividing the crane load by the maximum permissible ground pressure.

Required crane mat bearing area (m2) = Crane Load (tonne) ×9.8m/s2 Maximum permissible ground pressure(kPa)

In the guidance note, sample calculations on crane mat size calculations are given.

CICA has developed a ground pressure App called “CICA Calc” to calculate ground pressure or crane mat size, based on the formulas and ground pressure capacity estimates provided in the Queensland Mobile Crane Code of Practice 2006. This App is available from your phone’s App store. The full “Guidance Note for Ground Pressure Calculation” is available to CICA Members. Contact Alice Edwards alice@cica.com.au