Comparison of Measured and Computed Stiffness of High-Density Polyethylene Pipe

Abstract

This paper examines the ability of traditional analytical methods and linear finite element models to predict the stiffness of high-density polyethylene (HDPE) profile-wall pipe in a parallel-plate test. The stiffness of typical pipe 36 in. and larger in diameter can be predicted quite accurately with a traditional analytical method. However, hand stiffness calculations for smaller-diameter pipes can have significant error. The most commonly used formula for calculating pipe stiffness is based on the assumption that deflections are a result of ring-bending stresses alone, an accurate assumption when a solid-wall pipe is being considered. However, for profile-wall pipe, shearing stresses, thrust stresses, and stability can have significant influences on pipe deflections and in turn on the pipe stiffness. Small-diameter pipes typically have larger profile height-to-diameter ratios. These “taller” profiles are more susceptible to the influence of thrust and shearing stresses. A more accurate formula for calculating pipe stiffness, one that accounts for shear and thrust stress, was developed. Stiffness values determined from these traditional analytical methods were compared with results from finite element models as well as from the physical test data. In addition, in parallel-plate testing, significant deflections occur in the pipe rib just under where the pipe comes in contact with the flat plate. These localized deflections reduce the computed pipe stiffness if the pipe deflection is based on the displacement of the plates. If the deflections of the inside diameter of the pipe were used to compute pipe stiffness, higher pipe stiffness values would be obtained.