Proper cribbing and blocking helps operators set up cranes or other equipment safely on a variety of ground surfaces. Blocking provides a foundation for heavy loads such as outriggers. Cribbing is used to support the outriggers at greater height than blocking can provide.
These measures will help protect you from injury and guide you in choosing the right method of supporting your outriggers or stabilizers. Your safety is Altec’s number one priority, and we want you to know how to safely use your equipment.
For more information refer to OSHA’s standard on cranes and derricks in construction (1926 Subpart CC).
Terms To Know
- Ground conditions means the ability of the ground to support the equipment.
- Support materials means blocking, mats, cribbing or similar supporting materials or devices.
Selecting Material
- Material must be physically capable of handling heavy loads induced by the unit outrigger.
- Material must not be cracked, deformed, rotten, bent, worn or damaged.
- Select material that is larger than the foot of the outrigger.
- Material should be of sufficient thickness, width, and length to prevent shifting, toppling or excessive settlement of the load.
- Material must be free of oil, grease or debris and kept dry to prevent slippage.
Use of Cribbing and Blocking
- Personnel should wear steel-toed boots, gloves and eye protection when handling cribbing and blocking.
- Material must be placed on a firm level base or foundation to properly disperse the weight of the load.
- Ensure no debris is under the cribbing or blocking which may prevent it from resting firmly on the ground or surface.
- It should be placed directly beneath the outrigger and stacked to avoid slipping.
- Frequently inspect cribbing and blocking during unit operation for settling, slippage, cracking, bending, crushing or shear failure.
- Cribbing should never be taller than twice the width of the blocking base.
General Calculations to Determine the Correct Blocking Size
- Rule of 5: Take the maximum lifting capacity in tons of the unit such as 50 tons, divide that by 5 = 10 sq. ft. of cribbing or blocking.
- Rule of 3: Take the square footage of the cranes floats and multiply by 3. Such as a 2×2 float = 4 sq. ft. Take 4 sq. ft. x 3 = 12 sq. ft. of cribbing or blocking.
Examples of Soil Types and Load Bearing Capacities
| SOIL TYPE | LOAD BEARING CAPACITY |
| Virgin Ground | 22 psi |
| Asphalt | 29 psi |
| Compressed Crushed Stone | 36 psi |
| Clay/Silt Soil, Firm | 43 psi |
| Mixed Granular Soil | 51 psi |
| Firm Compacted Gravel | 58 psi |
| Firm Compacted Gravel (more compacted) | 72 psi |
| Firm Compacted Gravel (more compacted) | 109 psi |
| Brittle Weathered Rock | 145 psi |
| Concrete | 1000 psi |
*check the load chart or consult the unit manufacturer for the maximum outrigger down pressure for a single outrigger.
Outrigger Pad/Float Size in Square Inches
| ROUND | |
| 24 inch diameter | 452 square inches |
| 36 inch diameter | 1017 square inches |
| 48 inch diameter | 1808 square inches |
| SQUARE | |
| 24 X 24 | 576 square inches |
| 36 X 36 | 1296 square inches |
| 48 X 48 | 2304 square inches |
Specific Calculations to Determine the Correct Blocking Size
Maximum outrigger down pressure* ÷ outrigger pad/float square inches = amount of pressure per square inch on the blocking material.
Example:
62,800* ÷ 1296 (sq. outrigger pad 36×36) = 48.5 lbs psi
Now consult the soil type chart above. With 48.5 lbs psi, the unit can be safely set up on “Mixed granular soil” or any soil with a higher load bearing pressure than 48.5 lbs psi.
Through the OSHA Alliance Program, Altec developed this content for information purposes only. It does not necessarily reflect the official views of OSHA or the U.S. Department of Labor.