By Dica CEO Kris Koberg
It can be difficult to know if the ground beneath your crane will support the weight and pressure under your outriggers, yet knowing this is critical to safe crane set up. In response to this need, United States standards and regulatory bodies now place greater emphasis on understanding ground conditions and using crane mats or outrigger pads that are sufficient to provide the necessary foundation support.
Here are 10 crane safety tips to guide you to a successful crane set up:
- Know your responsibilities. In the United States, OSHA states that cranes must be assembled on ground that is firm, drained and graded sufficiently, in conjunction with supporting materials, such as blocking, cribbing, pads and mats, to provide adequate support and levelness. ASME states that outrigger blocking or cribbing must have sufficient strength to prevent crushing, bending or shear failure. And it needs to be of such thickness, width and length as to completely support the float, transmit the load to the supporting surface and prevent shifting, toppling or excessive settlement under the load.
- Know the bearing strength of ground and soil. As this is an extremely complex combination, it’s a good idea to seek the advice of a geotechnical engineer. A low-cost way to determine ground conditions is to use a dynamic cone penetrometer, which is portable and easy to use. This information can be compared to ground bearing pressure charts for different soil types.
- Identify any sub-surface hazards and avoid if possible.
- Evaluate and improve the ground if needed. Ways to improve the ground include compaction, removing loose surface layers or bringing in rock or other dense inorganic material. If the ground is wet, allow time to get the moisture out.
- Know the maximum pressure the crane will exert on each outrigger during the operation, or the maximum outrigger reaction force. Crane manufacturers provide this information for each crane model, and generally it can be found in your operation manual. Many also offer free software solutions that allow you to input the lift data, which then outputs the outrigger reaction forces that will be generated.
- Select the right size crane or outrigger pad. Undersized pads can put you in an unstable condition, but oversized pads are inefficient in terms of purchase, labour and transportation costs. Several methods are commonly used in the industry. Dica has evaluated each of these methods and has found various methods produce widely varying conclusions and most methods don’t take into account the ground condition. Dica strongly suggests that you should use a sizing method that takes into account ground conditions (ground bearing capacity) and actual outrigger loads.
- Choose crane pads or outrigger pads that are designed and constructed to meet or exceed the bearing, flex and shear strength required. Their purpose is to distribute the load from the outrigger float over a large enough area that the bearing pressure to the ground surface is acceptable. They must be stiff enough that the crane will not go out of level as the load swings.
- Never place blocking, cribbing, pads or mats under the outrigger beam. Only use them under the outrigger floats or pontoons.
- Monitor every lift. If the outrigger pad or crane mat is deflecting or bending, stop the lift. If the pad or mat is being driven into the ground, stop the lift. The load pressure is in excess of the ground bearing pressure. A larger surface area or improved ground conditions are needed to support the load.
- Use common sense. If it doesn’t look right, stop. If it doesn’t act right, stop. If it doesn’t feel right, stop. Products are never a replacement for common sense. Use your common sense.
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DICA employs a three-step fitting process, including identification of the key data points for the equipment and the maximum outrigger reaction force. Then we work with customers to determine soil bearing capacity. Finally, we determine the proper material, construction and thickness needed for the outrigger pad or crane mat to distribute the load over the intended area.