Usually when people hear that a mobile crane tipped over, they assume that it was due to the load weight. However, the truth is that loss of stability is inescapable when operators disregard the crane manufacturer’s guidelines concerning crane ratings. Simply put, the cause of a tipping accident is most likely the wrong boom angle or boom length.
A mobile crane is comprised of three basic components: the carrier (chassis), superstructure, and boom. The center of gravity of each component affects the center of gravity of the entire crane assembly. Preventing the crane from leaning too much to one side or tipping forward is the objective of two mechanisms: the counterweights and outriggers.
Counterweights are planted on the cab’s underside, on the back of the crane. The amount of weight needed for a specific operation depends on the load’s weight, the boom’s angle, and radius. Counterweights are removed when the crane is not performing a lift.
Outriggers use hydraulics to lift the whole truck (tires included) off of the ground. The outriggers are made up of the outrigger pad (also known as the “foot” or the “float), and the beam (the outrigger’s leg). Even when you’re working with loads that are under your crane’s lifting threshold, extending the outriggers properly is critical. Occasionally, mats (or wood floats) are lined up to form a base underneath the pad in order to evenly spread the weight over the ground surface, which can help preserve the pavement underneath the crane.
There is a special vocabulary when it comes to improving equipment stability with counterweights and outriggers. Below are a few of the terms you might come across while on the job:
Blocking- Also referred to as “pads”, “dunnage”, “mats”, or “cribbing”, blocking is used to support lattice boom sections and evenly distribute loads to the ground during disassembly and assembly.
Allowable Ground Bearing Pressure- The maximum amount of pressure approved on the supporting surface. The number is measured in Pascals (Pa) or pounds per square foot (psf).
Deflection- Bending of supporting materials as downforce is applied.
Crane Pad- An area of compressed concrete, mats, steel plates, or soil used to support a mobile crane during a lift.
Crush Rating- The amount of pressure that an outrigger is allowed and rated to carry. This depends upon the strength of the materials involved.
Displacement- The difference between the fully-loaded horizontal position of the outrigger pad and the unloaded horizontal positions of the outrigger pad.
Crane Mat- Used to lessen the ground bearing pressure from a crane’s crawler tracks, tires, or outriggers.
Ground Bearing Pressure (GBP)- The pressure that a crane pushes on the supporting surface, measured in Pascals (Pa) or pounds per square foot (psf).
Downforce- The pressure created from the outriggers onto the outrigger pads through the outrigger float.
Ground Load Rating (GLR)- The amount of weight that can be used on the outrigger pad based on the ground’s ability to support it.
Effective Bearing Area- The area underneath a crane mat that is sufficient in distributing the applied load to the underlying surface.
Ground Bearing Capacity (GBC)- The capability of the surface/ground to support a crane and its operation.
Outrigger Pad/Float/Foot- These help even out the load to the supporting surface by attaching to the outer part of a crane’s outrigger. As with several crane parts, they go by many names, typically depending on where you are. You may refer to them as “feet”, “outrigger floats”, “outrigger pans”, or simply “pads”.
Ground Conditions- How well the ground can hold up the crane and its load. These conditions include slope and soil compaction.
Maximum Outrigger Reaction Force- The maximum amount of weight that the equipment can exert through its outriggers.
Supporting Materials- These include cribbing, blocking, and mats, as described in OSHA 1926.1 OSHA 1926.1402(a)(2).
Regardless of how heavy your load is, you should always follow the correct setup procedures. Stick to your crane lift plan when the lift exceeds 75 percent of the crane’s rated capacity or demands the use of more than one derrick or crane to perform. Be sure to ask these questions prior to setting up your crane:
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