Understanding Rigging Design Factors

Definition of Design Factor

The design factor is the ratio of the minimum breaking strength to the permissible working load. It is always greater than 1. Other terms frequently used for design factor are safety factor, factor, or safety.

Breaking strength is the rigging material or hardware's tensile strength in lifting and pulling operations. It could also be the bearing strength or compressive strength of rigging hardware in certain other situations.

So How Is This Formula Helpful? 

If the required design factor is known and the minimum breaking strength of a rigging component is known, then the safe working load can be determined by rearranging the formula to look like:

What Determines The Design Factor

In general, the appropriate value for the design factor depends upon a number of considerations. Among these are:

• The extent of danger to human life and property
• The dependability of the rigging material ◦ The age and condition of the rigging material
• The load conditions: static, varying, shock loads?
• The degree of knowledge of the load's weight

Practically, OSHA spells out minimum values for design factors at 29 CFR 1926.1431(g)(3):

“All hardware used for rigging must be able to support five times the maximum intended load applied to, or transmitted to, that component. Additionally, slings using rotation resistant rope must have a safety factor of ten.”

Safe Working Load Stated As A Percentage 

To make things confusing, sometimes design factors are indicated by percentages of minimum breaking strength. To convert percentages to whole number design factor values, simply divide the percentage value into 100% to arrive at a whole design safety factor. For example, a piece of rigging equipment that is rated at 20% of ultimate strength has a design factor of:

An Example 

Suppose that your project has a 12 strand 1⁄2 inch synthetic rope for which the manufacturer has stated a 12,500 pound minimum strength. Using the mandated design factor of 5 and the formula for permissible
working load, the safe working load can be seen to be,

This value would be for a new rope. A much higher value for the design factor may be warranted for a sling that has been used or is in less than perfect condition.

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