Outrigger pads are used to distribute loads across a larger area to reduce ground-bearing pressures and provide a safe and solid platform for your equipment. To achieve a distribution of the load across the surface area of the pad, the pad must be strong enough to resist physical failure and stiff enough to resist deflection and avoid functional failure.
A common misconception is that stacking thinner outrigger pads of the same material will act and perform the same as a single pad of the same thickness and material as the two stacked pads. Unfortunately, stacking two pieces of 1” thick material does not provide the same strength and stiffness performance to distribute loads as a single 2” thick piece of the same material. This is true for all materials used as outrigger pads, such as Steel, Wood, or Synthetics. In addition, attempting to establish a non-bonded connection between two individual parts, such as a male/female or slot and rail design, will not provide enough connection between the parts for them to perform like a pad that is double the thickness. Any performance improvements are subject to the material and the method of connection.
A single pad that is the same thickness as two individual stacked pads has twice the strength and four times the stiffness. A single pad 2x thicker than two single pads is 4x stronger and 8x stiffer.
How is that possible?
Here’s why. The strength and stiffness of an outrigger pad are determined by its material properties and its dimensions (thickness). Strength and stiffness are not directly proportional to thickness. The stiffness of an object increases/decreases by the cube of the increase/decrease in thickness. If you double the thickness (2x), the cubed result is an 8x increase in stiffness (2 x 2 x 2 = 8x). The strength of the material increases 4x when the thickness is doubled (2 x 2 = 4x). Therefore, a single pad that is 2x thicker than two single pads gives you 4x the strength and 8x the stiffness.
When pads are stacked, the cubed effect does not apply because the two pads move independently of each other. If two identical pads are stacked to the same height as a single pad, the resulting strength is only doubled (1 + 1 = 2x), and the stiffness is equal to the increase/decrease in the stacked height times 2, (2 x 2 = 4x).
Thinking about this differently, to achieve the same performance as two 1” thick pads stacked on each other, you would only need one pad 1.25” thick to match the strength or 1.5” thick to match the Stiffness. (Going back to the headline: 1” +1” only equals 1.5”)
Is there any way to match the performance of a single thick pad?
Multiple layers of pads can be bonded or pinned to act as a single pad of the overall thickness. Material can be bonded together with glue or bolts, causing the multiple layers to function as a single layer or pad of the overall thickness. However, options such as alignment slots and rails do not have the same effect as bonding the pads together. Slots and rails or other forms of connection that do not pin or adhere the layers together do not provide enough connection between the parts to allow the stacked parts to perform as a single unit.
The load distribution performance of an outrigger pad is not directly proportional to its stiffness. Load distribution does not increase by a factor of 2 when stiffness increases 2x, nor does load distribution double when stiffness increases 8x. Load distribution is a function of the interaction between the bearing area of the outrigger float, the type and thickness of the material, and the properties of the ground. Calculating or projecting the load-spreading capabilities of outrigger pads and other supporting materials must be left to lift planners, professional engineers and others who are skilled in this area.
Stacking pads of a similar material or rigidity may seem like a way to improve load distribution performance, but the difference between using a single thicker pad vs stacked thinner pads is significant.
Stacking approaches that do not include bonding or pinning do not provide enough connection for the individual parts to function as a single pad 2x thickener.
For the purpose of improving load distribution and reducing ground bearing pressures, using a single, thicker pad is the better choice to increase strength and stiffness compared to stacking two thinner pads.