Improving the Effective Bearing Area
Fact: Materials used as supporting materials such as pads and mats do not uniformly distribute loads to the ground.
Fact: The load distribution performance of materials used as supporting materials such as pads and mats are heavily influenced by the supporting surface.
Fact: The strength and stiffness of the supporting materials play a major role when assessing the bearing area. These properties determine whether a material can resist breaking under load (physical failure) or excessively bend, lose shape which reduces the bearing area (functional failure).
When faced with real-world conditions, using transition layers and stacking material on top of other materials can be an effective way to increase the bearing area of supporting materials under outriggers. However, like most anything, these practices are best when they are planned events and not gametime decisions.
In a Rigging Matters article, Grounded, published in the June 2024 issue of American Cranes & Transport, Kris Koberg, CEO of DICA explains how these concepts can help direct the foundational support the equipment needs for the application.
“On jobsites, it’s not uncommon to see layers of supporting materials used to support different equipment,” says Koberg. “Whether you are involved in specifying the supporting materials, planning the lift, or operating the crane, it’s important to understand the variables that impact the effective bearing area of supporting materials,” he adds.
Adapting to real world ground conditions – FiberMax as a part of a constructed crane support.
Timber Mats
Correctly sized timber mats of known species and conditions are an excellent solution to support crawler cranes. However, timber mats are not well suited for use as an outrigger pad because timber mats are assembled from individual timbers and the only timbers that are engaged in load distribution are the timbers the outrigger float is in direct contact with. As a result, not all timbers in a mat are engaged, which drastically reduces the effective bearing area of a crane mat.
A transition layer of a material with the appropriate stiffness properties that spans all timbers is required to create engagement of all the timbers.
Steel Plates or Road Plates
Likewise, steel plates are another commonly used supporting material. Because of its strength, the effective bearing area of steel plates are often assumed to be 100% of the area of the plate. This assumption may be wrong, as it can be much less. Users should be cautioned not to assume that the entire steel plate is bearing on the ground. Like the timber mat discussed above, stacking wood timbers of an appropriate species, thickness, and length on steel plates will increase the bearing area of the steel plates.
Transition Layer Options
A steel plate, synthetic outrigger pad, or a second layer of timber positioned perpendicular to the first layer can be used as a transition layer. It must be stiff enough to spread the load to the outer timbers, and not physically fail under the load and pressure of the outrigger float. Calculating the appropriate material thickness should be left to a qualified person.
Layering materials enhances strength and stiffness, but it’s important to understand that stacked materials will not perform as well as a single, thicker layer due to the lack of sheer connection between the layers. Each layer behaves independently, which reduces the bearing area. For example, two stacked pieces of material of the same height as a single piece will be half as strong and will deflect four times more than the single layer of the combined thickness.
Increasing Bearing area on Hard Surfaces
The effective bearing area of the outrigger pad or crane pad is also impacted by the ground conditions. Both hard and soft ground conditions can adversely affect the bearing area of supporting materials. Crane users intuitively understand the problems associated with soft ground, but hard ground presents different challenges. The harder the ground, the less effective the supporting materials are in spreading the load.
There are two ways to increase the bearing area when working on hard ground conditions. The first is to increase the stiffness of the supporting materials to overcome the stiffness oof the ground. This can be done by layering materials such as wood timbers, steel plate, steel pad, and synthetics.
The second approach is to soften the ground by using sand or a layer of neoprene between the hard surface and supporting materials. This is typically the more cost-effective approach.
Armed with the facts, equipment users can improve the bearing area of supporting materials, in any ground conditions. The key is understanding effective ways that account for the factors involved.
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