Crane Stability & Site Setup
Outriggers, cribbing, ground bearing pressure, tipping point, safety factors, ground conditions, and excavation hazards.
- Explain the purpose of outriggers and when they must be fully extended
- Describe how outrigger pads and cribbing distribute ground pressure
- Define ground bearing pressure and its importance for crane setup
- Identify the tipping point and factors that affect crane stability
- Assess site conditions including ground conditions, underground utilities, and overhead obstructions
Lección 1
Outriggers, Cribbing & Ground Bearing Pressure
The Purpose of Outriggers
Outriggers are extendable beams with hydraulic cylinders and pads that deploy from the crane's carrier to increase stability and distribute the crane's load to the ground. They serve two critical functions: widening the crane's support base and transferring the combined weight of the crane plus load directly to the ground, bypassing the tires and suspension.
When Must Outriggers Be Fully Extended?
Outriggers must be fully extended when using full rated capacity from the load chart. The load chart capacity values assume outriggers are fully extended unless a separate chart section specifies partial extension or "on rubber" capacities. Using partially extended outriggers when the load chart assumes full extension can result in tipping.
Full Extension Required for Full Capacity
Load chart rated capacities assume outriggers are fully extended. Never use full capacity values with partially extended outriggers unless the chart specifically provides reduced capacity values for that configuration.
Outrigger Pads and Cribbing
Outrigger pads (cribbing) are placed under the outrigger floats to distribute the outrigger loads over a larger ground area. Without pads, the concentrated force from an outrigger float can exceed the ground's bearing capacity, causing the outrigger to sink and the crane to become unlevel - or worse, tip over.
Cribbing materials include:
- Hardwood timber mats
- Steel plates
- Engineered composite pads
- Stacked timber cribbing for heavy loads
The size of the pad must be calculated based on the expected outrigger reaction force and the ground's bearing capacity.
Ground Bearing Pressure
Ground bearing pressure is the load per unit area transmitted to the ground beneath the outrigger pad. It is calculated by dividing the outrigger reaction force by the area of the pad in contact with the ground.
Different ground conditions have vastly different bearing capacities. Compacted gravel can support much more pressure per square metre than soft clay or recently filled ground. The operator must assess the ground and select appropriate cribbing.
Outriggers increase stability and distribute load to the ground. They must be fully extended when using full rated capacity. Outrigger pads (cribbing) distribute outrigger loads over a larger ground area to prevent sinking. Ground bearing pressure is the load per unit area transmitted to the ground.