Flexibility Factor or Pipe Stiffness: Significant Stiffness Considerations

Stiffness requirements for metal and other flexible pipes are often misunderstood and misused. Stiffness, which often is confused with strength, is defined in several ways that do not directly compare. The stiffness of metal pipes is traditionally limited by a calculated flexibility factor. Alternatively, plastic pipes are usually rated in terms of laboratory-measured pipe stiffness. However, the AASHTO design method uses a calculated flexibility factor for plastic pipe design that appears to relate to metals. Laboratory-measured “stiffness” is used here to investigate the geometry, material, and environmental factors that must be considered to evaluate functional stiffness and compare pipe performance.