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).
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 fl oats and multiply by 3. Such as a 2×2 fl oat = 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|
|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|
*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:
|24 inch diameter||452 square inches|
|36 inch diameter||1017 square inches|
|48 inch diameter||1808 square inches|
|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/fl oat square inches = amount of pressure per square inch on the blocking material.
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.