Sags & Tensions of Span Attachments

Another significant behavior of most wire/cable materials is that even modest, everyday tension loads on it will cause some permanent plastic stretch. This behavior is called Creep. Some wires/cables may have been installed for years, but have never experienced the maximum design loads that would have created permanent stretch that way. We must account for this to ensure our design considers this possibility. If a constant tension was exerted onto a material sample for a long period of time, the amount of creep that would develop would be highest in the first few years and then reduce over time to negligible amounts.

Wire and Self-Supporting Cable manufacturers place their materials under tension for 1,000 hours to determine the amount of creep that will occur based on the loads applied. Separate test results are normally generated at 15%, 20%, 25% and 30% of the Ultimate or Rated Tensile Strength of the material. These results are extrapolated on log paper to estimate the amount of creep expected in 100,000 hours (~ 10 years). After this point the additional creep that could develop is considered by the Industry to be so negligible that it is not worth considering any further. The 10 year projection is expressed in terms of a polynomial, similar to the Initial Curves, for ease of use within software tools. This polynomial represents what is called the "Final with Creep" curve for the material.