Understanding Stiffness vs. Strength
Considerations when choosing stabilizer pads, outrigger pads, crane pads, crane mats, and cribbing.
Two important properties to understand when evaluating stabilizer pads, outrigger pads, crane pads and cribbing products are strength and stiffness. Products used in this application must be both strong to prevent breaking (physical failure), and rigid to resist bending (functional failure) and distribute load over an intended area.
Why is the difference between strength and stiffness important to proper product selection? According to David Duerr, PE, 2DM Associates, Inc., the stabilizer pad, outrigger pad or crane pad chosen 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”. Stiffness is required to spread the load, and Strength/Stiffness is required to prevent the crane from going out of level.
When evaluating outrigger pads, crane pads, and plastic cribbing blocks, it is important to understand how strength and stiffness attributes of different materials and designs affect the safety and performance of the products in this application. Selecting the right combination of strength and stiffness to safely and properly support your equipment is a critical decision.
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/Rigidity: The ability of the material to resist bending/deflection (functional failure)
A material’s strength and stiffness properties are not directly related.
Stronger does not mean stiffer. An outrigger pad may have enough strength to not break (physically fail). However, this same pad may not be stiff enough to effectively distribute load to the ground due to excessive deflection or (functional failure). The ability of an outrigger pad to spread load is based on the stiffness of the pads relative to the stiffness of the 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 which will cause more pad deformation/deflection and could lead to both functional and physical failures. This challenge intensifies as the stiffness of the ground increases.
Strength and stiffness relationships
The strength and stiffness of a pad will depend on the material and the thickness of the material. Material strengths and stiffness properties are generally understood though the use of standardized material testing and analysis.
Plastic is stronger than wood and most wood is stiffer than plastic. Because wood is not as strong as plastic, thickness must be increased to not break (physically fail). Because plastic is not as stiff as wood, thickness will need to be increased to more effectively distribute load and resist deflection (functional failure).
The load carrying capacity and stiffness of a pad is not proportionate to its thickness. When you double the thickness, you get 4x the strength. The same doubling of the thickness of material will increase the stiffness by a factor of 8.
Doubling the thickness of material
- Increases the overall strength by a factor of 4.
- Increases stiffness by a factor of 8.
- Increasing thickness significantly increases the load distribution performance of outrigger pads and crane pads. The reverse is also true. Decreasing the thickness of outrigger pads and crane pads significantly decreases load distribution performance and increases chances of physical failure.
To enhance load distribution, thicker and smaller pads are a better choice compared to thinner and larger pads.
Stabilizer pads, outrigger pads, and crane pads having the same thickness and made from plastics or composites are much stronger than outrigger pads made from wood. When exposed to bending, 1” thick plywood can be easily broken compared to 1” thick plastic and composites.
Therefore, properly designed plastic and composite outrigger pads can support significantly higher loads without risk of breaking (physical failure). 1” thick wood and composites are stiffer than plastics. Therefore wood outrigger pads are generally stiffer than plastic pads of the same thickness. Therefore plywood outrigger pads are generally stiffer than plastic pads. Due to higher stiffness and lower strength, wood outrigger pads are more susceptible to breaking (physical failure) under heavy loads.
The difference between strength and stiffness can be thought of this way:
- A stiffer pad with less strength will break (physical failure) if a load or pressure is exerted on the pad that exceeds its limits.
- A stronger pad that is not as stiff will deflect under loads that exceed the combined stiffness of the pad and ground (functional failure).
- Deflection is a warning sign that signals the need for more foundation support due to higher loads, and/or softer soils.
- Wood is an organic material. Therefore the properties and performance of wood outrigger pads change with exposure to the environment, moisture, chemicals and usage. Wood begins to deteriorate and decay the moment it is cut. Exposure to the environment and stress from loads increase its rate of deterioration.
- Polyethylene plastics, or thermoplastics used in making outrigger pads and crane pads are engineered materials and should not be susceptible to environmental exposure, moisture, chemicals and usage. These materials and pads do not deteriorate or decay over time and do not fatigue from loading when used within rated guidelines.
- Composites such as FRP (Fiber Reinforced Polymers) are engineered materials that should not be susceptible to environmental exposure, moisture, chemicals and usage. These materials do not deteriorate or decay and will not fatigue from loading when used within the rated guidelines.
Understanding the different roles strength and stiffness play is essential in the foundational support decision making process. The most reliable way to do this is to work with a supplier or engineer who understands how different materials and product designs interact with equipment loads and the ground.
Seeking out and employing these professional insights should provide you with outrigger pads, crane pads and cribbing solutions with the right combination of strength and stiffness. By doing this, you should achieve Mr. Duerr’s stated objective of “Spreading 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.”