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Understanding Rigging Design Factors: Safety Through Calculation

Design Factor: The Cornerstone of Safe Rigging

The design factor (DF) is a crucial concept in rigging, ensuring safe working loads for your equipment. It's the ratio between a rigging component's minimum breaking strength (MBS) and its permissible working load (WLL). 

Mathematically expressed as:

  1. DF = MBS / WLL

Breaking Strength Explained:

  • The MBS refers to the maximum load a rigging component can withstand before failure. 
  • It can be tensile strength (lifting/pulling) or compressive strength (bearing) depending on the specific application.

Formula Applications:

This formula is a two-way street:

Finding Safe Working Load: Knowing the design factor and MBS, you can calculate the safe working load using the rearranged formula:


Determining Design Factor: If the safe working load and MBS are known, you can solve for the design factor:


Factors Influencing Design Factor:

The appropriate design factor depends on several considerations:

  1. Severity of Consequences: Greater risk to personnel or property necessitates a higher design factor.
  2. Rigging Material Reliability: New, dependable materials allow for lower design factors compared to older or questionable materials.
  3. Load Conditions: Static loads typically require lower design factors than dynamic loads (varying or shock loads).
  4. Load Weight Certainty: Precise knowledge of the load weight allows for a potentially lower design factor.

OSHA Regulations:

The Occupational Safety and Health Administration (OSHA) mandates minimum design factors in 29 CFR 1926.1431(g)(3):

  1. General Rigging Hardware: Minimum design factor of 5.
  2. Rotation-Resistant Slings: Minimum design factor of 10.
Design Factor as a Percentage:

Sometimes, design factors are expressed as percentages of MBS. To convert a percentage to a whole number design factor, simply divide by 100:

Percentage Design Factor = Whole Number Design Factor (%) / 100

Example Calculation:

Imagine a project using a new 12-strand ½ inch synthetic rope with a manufacturer-rated MBS of 12,500 pounds. Applying the mandated design factor of 5 and the formula for permissible working load:

  1. WLL = MBS / DF = 12,500 pounds / 5 = 2,500 pounds

This safe working load assumes a new rope. A higher design factor would be necessary for a used or compromised sling.

Determining the Right Design Factor:

  • The ideal design factor considers all load types acting on the rigging component, including:
  • Acceleration, deceleration, and lifting speeds
  • Attachments (number, size, arrangement of sheaves and drums)
  • Corrosive or abrasive environments
  • Length of lift

Manufacturer Guidance:

For the most accurate design factor recommendations, consult the rigging equipment manufacturer's specifications.

The proper design factor for any rigging component demands consideration of all loads: