Posted on June 26, 2013

forestry-craneThe following information provides guidance for the proper use of cribbing and blocking 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).

Your crew’s safety is Altec’s number one priority. Outlined below are the measures needed to protect you from injury as well as guidance on training, selecting materials and using cribbing and blocking to ensure your unit is set up properly. Knowing your equipment has safety features is not enough. You also need to know how to safely use the equipment.


  • Ground conditions means the ability of the ground to support the equipment.
  • Support materials means blocking, mats, cribbing or similar supporting materials or devices.
  • Blocking is commonly used to provide a foundation for heavy loads such as outriggers.
  • Cribbing is used when a heavy load must be supported at a height greater than blocking can provide.


  • Employers must ensure personnel are trained and competent to safely use and/or construct cribbing and blocking.

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 suffi cient 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.

Personal Protective Equipment (PPE):

  • Personnel should wear steel-toed boots, gloves and eye protection when handling cribbing and blocking.

Use of 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 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:


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:


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 Blocking 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.

OSHA Alliance LogoThrough the OSHA and Altec 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.