What is a crane mat and when is one required?

A crane mat is a structural load-spreading device placed between the crane's outrigger float or crawler track and the ground surface. Its purpose is to increase the effective bearing area so that the force per square foot exerted on the soil is reduced to a level within the soil's safe bearing capacity. Without appropriate matting, a crane's outrigger float may exert thousands of pounds per square foot on the soil beneath it, which can cause the soil to fail, the outrigger to sink, and the crane to tilt or overturn.

Materials and Construction

Traditional crane mats are constructed from large-dimension hardwood timber, typically oak or hardwood species with high compression strength across the grain. Timber mats are commonly 12 inches thick, 48 inches wide, and 8 to 20 feet long, and are assembled from individual timbers bolted together. They are valued for their combination of strength, flexibility, and ability to distribute load across soft surfaces without creating a rigid concentrated bearing point. Composite crane mats made from high-density polyethylene or fiberglass-reinforced polymer are increasingly common; they are lighter and resistant to rot and contamination but carry higher cost per mat.

Sizing Crane Mats

The size of crane mat required for a given lift depends on two variables: the maximum outrigger load and the soil's allowable bearing capacity. The maximum outrigger load must be calculated for the specific crane, configuration, and load being lifted. Most modern crane manufacturers publish maximum outrigger reaction forces for their cranes in the load chart or in a separate specification document. The soil's allowable bearing capacity may come from the project's geotechnical report, from published tables for known soil types, or from field testing by a geotechnical engineer. The required mat area is simply the maximum outrigger load divided by the allowable bearing capacity. If the calculation yields an area larger than a standard mat, multiple mats or a larger custom mat configuration must be used.

Placement and Inspection

Crane mats must be placed on a firm, stable surface with no voids or soft spots beneath them. A mat placed across a small depression will have reduced effective bearing area and may rock under load. Before any lift begins, the outrigger floats should be centered on the mat, the jack cylinder extended until the crane's weight is transferred to the outrigger, and the mat visually inspected for settlement, cracking, or displacement. After the crane is loaded and during the lift, the mats and outrigger floats should be monitored for any sign of settlement. Any observed mat movement during a lift requires immediate cessation of the lift, load lowering, and re-evaluation of the ground conditions before proceeding.