Stiffness vs. Strength
Two important properties to understand when choosing outrigger pads or crane pads
The outrigger pad you use must “Spread the load from the crane over a large enough area that the bearing pressure to the ground surface is acceptable [and] provide support to the crane that is stiff enough that the crane will not go out of level as the loads from the crane change during lifting operations” – David Duerr, P.E.
When evaluating different outrigger pads and crane pads, it is important to determine the appropriate material, thickness, and construction that will provide the right combination of strength, stiffness and safety to properly support your equipment.
For purposes of the following discussion strength, stiffness and failure 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 distribute a load and resist deformation or deflection (functional failure)
A material’s strength and stiffness properties are not directly related. For example: engineered thermoplastic outrigger pads have a much higher strength than wood outrigger pads. Meaning, they are capable of supporting significantly higher loads without breaking or causing permanent deformation to the pads. Wood outrigger pads are generally stiffer than engineered thermoplastic. Due to this stiffness, wood outrigger pads are much more likely to break under heavy loads.
The difference between strength and stiffness can be thought of this way:
- A stiffer pad that is weaker will break (physical failure) if a load or pressure is exerted on the pad that exceeds its limits. This can cause catastrophic results.
- A stronger pad that is not as stiff will deflect under high pressures or loads that exceed the combined stiffness of the pad and ground (functional failure). Deflection is the warning sign that signals the need for more foundation support due to higher loads, and/or softer soils.
Wood is not an engineered material, therefore the specific properties of wood outrigger pads constantly change with exposure to the environment, moisture, chemicals, use, and deteriorate over time. Wood outrigger pads will deteriorate from their first use with exposure to moisture, chemicals and stress from supporting loads.
Thermoplastics used in making outrigger pads and crane pads are engineered materials and are not susceptible to environmental exposure, moisture, chemicals and use. These materials and pads do not deteriorate over time and do not fatigue from loading.
Understanding the roles that strength and stiffness play is essential to the decision process when choosing foundational support to minimize risk. When considering pads and crane pads for your equipment, it is highly recommended that you work with a competent supplier or engineer that is able to understand and communicate the pros and cons of different material and design options for your environment and application.
When considering the purchase of thermoplastic outrigger pads it is very important to work with professionals who understand your applications and how different materials and product designs interact with equipment loads and ground bearing pressures. Specific material properties and product designs are proprietary and should be evaluated when choosing outrigger pads that will provide the right combination of strength, stiffness and safety to properly support your equipment.