My Cart: 0 item(s)
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:
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:
WLL = MBS / DF
Determining Design Factor: If the safe working load and MBS are known, you can solve for the design factor:
DF = MBS / WLL
Factors Influencing Design Factor:
The appropriate design factor depends on several considerations:
- Severity of Consequences: Greater risk to personnel or property necessitates a higher design factor.
- Rigging Material Reliability: New, dependable materials allow for lower design factors compared to older or questionable materials.
- Load Conditions: Static loads typically require lower design factors than dynamic loads (varying or shock loads).
- 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):
- General Rigging Hardware: Minimum design factor of 5.
- 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:
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:
- Acceleration, deceleration, and lift speed
- Attachments; the number, size, and arrangement of sheaves and drums
- Conditions producing corrosion and abrasion
- Length of lift, and so forth
- The best source of information for design factors would come from the manufacturer of the rigging equipment