Surface Loading & Deformation # 2004 Milan
Gravity flow thermoplastic pipes use profile wall geometries to provide maximum wall stiffness with minimum wall area. This practice minimizes the material used in manufacture, and thus the product cost. In the case of some thermoplastic pipe, polyethylene in particular, this practice has led to a need for design procedures that address pipe with low hoop stiffness (EA/R) and thin wall elements. Pipe with low hoop stiffness carry greatly reduced load relative to that predicted by traditional design theories. However, the compression capacity of pipe with thin elements may be limited by local buckling of the elements, rather than by the currently considered design limits of hoop compression and global buckling. Several US projects have addressed these issues, leading to new AASHTO design procedures for thermoplastic culverts. This paper presents a detailed, yet simplified procedure for evaluating the compression forces applied to thermoplastic pipe walls and for evaluating the capacity of a profile wall pipe to carry those loads. The new procedures include : 1) consideration of pipe hoop stiffness to determine earth loads, 2) the use of the constrained soil modulus, Ms, to replace the traditional modulus of soil reaction, E'; and 3) consideration of local buckling to proportion profile wall cross-sections.