Lift stability and transport stability: determining elasticity

The operations in the pictures below don’t have much in common at first sight. But the opposite is true. In both types of operations, safe execution depends in part on stability. Specifics like mass and center of gravity are crucial when determining safety in lift operations and SPMT operations. The examples shown in these pictures are illustrative and depict safe situations.

Rules of thumb for stability

Stability is a complicated subject. In cases where stability is not the greatest issue, you may decide on stability by one of these two simple rules of thumb.

• GM value (metacentric height) for barges
• Minimum tipping angle for trailers

In operations in which stability is a critical issue, these simple rules do not suffice. To determine stability in these cases, it is necessary to quantify all elasticities that play a role.

Quantification of elasticity in lift and SPMT operations

The quantification of ‘elasticity’ is the common factor in lift operations and in SPMT operations. How do you determine all elasticities from top to bottom? For the barge, total elasticity is the sum of crane elasticity (bending) and the elasticity of the barge against rolling. In this so-called ‘roll stiffness’ of the barge the GM value plays a major role. For the trailer, it is the combination of bending elasticity of the ramps, the elasticity of the tires and the elasticity of the hydraulic setting. 3 point and 4 point hydraulic tilts have different tipping lines, and a different elasticity occurs for different hydraulic layouts.

From elasticity to stability to safe operations

You have quantified all elasticities and all external influences (wind, moments, and other forces) are known. Now you can calculate all movements and rotations of the operation. These movements can easily be judged as acceptable or not. This establishes a clear basis for operations in which stability is critical.