Quoted breaking strength of rope is usually the optimum strength achieved under laboratory conditions, in accordance with prescribed test procedures, on new rope, pulling at a slow, steady rate in a straight line. Real life applications normally involve many different types of forces and factors which will cause the rope to fail at much reduced loads. Accordingly, the safe working load of any rope is much lower than its nominal strength.

The following article reprinted with permission from Cordage News, a publication of the Cordage Institute provides a good overview of factors to consider when determining the safe working load of the rope.

TOTAL POTENTIAL LOAD (TPL) is the weight to be lifted, towed, restrained, suspended, or secured, not just in a static condition, but in a dynamic condition. Examples: the jerking of a weight being lifted, the heaving of a vessel, the slalom of a water skier, the falling of a body, the swaying of scaffolding, the tethering of a balloon, the wind pressure on antenna guys, a truck being towed out of the mud, and a boat being towed through waves – these and many other situations like them produce dynamic forces with a total potential load that may well exceed, sometimes substantially, the static weight of the load, and this must be taken into account.

WORKING LOAD LIMIT (WLL) of a size and type of cordage or rope must be determined by the user (engineer, operator, or manufacturer).

A DESIGN FACTOR (DF) must be selected based on the TPL, the degree of risk to life, limb and property and the conditions of use. This, in turn, is used to establish the WLL using the formula Minimum Breaking Strength divided by the Design Factor (WLL = MBS + DF).

For critical applications a Design Factor greater than 12 may be necessary. Users must determine the DF as they are the only one who can assess service conditions and establish operating procedures. The TPL applied to a cord or rope should never exceed the WLL. If uncertain, a qualified engineer should be consulted.

Cordage Institute Standards now show a range of Design Factors (5-12)* for selection, and values at the high end of the range, or larger, should be used when:

  • Smaller ropes are used, because they can be more severely damaged by cutting, abrasion and sunlight.
  • Loads cannot be accurately predicted.
  • Operators are not trained.
  • Operations and use procedures are not well defined and/or controlled.
  • Inspection is difficult, infrequent or performed by inexperienced persons.
  • Extreme dynamic loading (shock loads) are possible.
  • High temperatures are present.
  • Strong chemicals are present.
  • Ropes are kept in service for long periods.
  • Rope will be under tension for long periods (creep).
  • Ropes will be subjected to bend ratios under 3:1.
  • Ropes will be used over sheaves at less than recommended D/d ratios.
  • Knots are used (strength is reduced up to 50%).
  • Death, injury or loss of valuable property may result from failure.

*Cancord Note: Some Standards such as CAN/ULC S555 and NFPA 1983, require a DF of 15:1