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Considerations when choosing outrigger pads, crane mats and cribbing

When evaluation outrigger pads, crane mats and cribbing blocks, it is important to understand how strength and stiffness attributes of different materials and design affect the safety and performance of the products in this application. Selecting the right combination of strength and stiffness to support your equipment safely and properly is a critical design. Products used in this application must be both strong, to prevent physical failure, and rigid, to resist functional failure.

Why is the difference between strength and stiffness important to proper product selection? The outrigger pad or crane mat must spread the load from the crane over a large enough area so that the bearing pressure to the ground surface is acceptable whilst also providing stiffness support to the crane so it will not go out of level.

For purposes of the following discussion strength and stiffness are defined as follows:

  • Strength: the ability of the material to support a load without breaking (physical failure)

  • Stiffness: the ability of the material to resist bending/deflection (functional failure)

Strength and stiffness relationships

Stronger does not mean stiffer. An outrigger pad may have enough strength to not break, however, this same pad may not be stiff enough to effectively distribute load to the ground due to excessive deflection.

The ability of an outrigger pad to spread load is based on the stiffness of the pads relative to the stiffness of the ground/soil. If the pad is not rigid enough, the load will be concentrated on a smaller area on the pad. This results in increased ground bearing pressure (GBP) which will cause more pad deformation and could lead to both functional and physical failures.

Composite vs Hardwood

The strength and stiffness of an outrigger pad will depend on the material and the thickness. Material strengths and stiffness properties are generally understood through the use of standardized material testing and analysis.

With the correct composite material, it is stronger than wood, where most wood is stiffer than plastic. Because wood is not as strong as these composite materials, thickness must be increased to not physically fail. Because the composite materials are not as stiff as wood, thickness must be increased to distribute load and resist deflection more effectively.

Properly designed composite outrigger pads can significantly support higher loads without risk of breaking (physical failure). Whereas hardwoods are generally stiffer than composite pads of the same thickness. Due to higher stiffness and lower strength, hardwood outrigger pads are more susceptible to breaking under heavy loads.

Hardwood is an organic material and brings with it plenty of exposure. The properties and performance of hardwood outrigger pads change as it becomes exposed to the environment, moisture, chemicals, and excessive usage. Wood begins to deteriorate and decay, and with continued use under load distribution pressure, it heightens the safety risk of both operators and equipment.

Polyethylene plastics used in making outrigger pads are engineered materials and should not be susceptible to environmental exposure, moisture, chemicals, and proper usage. These composite materials and pads do not deteriorate or decay over time and do not fatigue from loading when used within rated guidelines and conditions of use, provided by us at System Rigging.

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