*Disclaimer
The following information provides guidance for the proper use of outrigger pads to set up cranes or other devices with outriggers or stabilizers. For more information refer to OSHA’s standard on cranes and derricks in construction (1926 Subpart CC). It is an employer’s/user's responsibility to devise policies and practices to establish workplace safety, including performing due diligence in setting up cranes and aerial equipment in accordance with the equipment manufacturers as well as state and federal requirements.
What Size Outrigger Pads Do I Need?
Selecting the right size outrigger pad for your aerial device stabilization is vital when purchasing to ensure your equipment is operated safely. Here you can find some general rules of thumb as well as OSHA recommendations to assist in selecting the right outrigger pad for your lifting requirements.
General Calculations to Determine the Correct Outrigger Pad (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 Outrigger Pad 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.
This information was originally developed by Altec through the OSHA and Altec Alliance Program.