Section Idealization of Corrugated Thermoplastic Pipe in AASHTO Design

The AASHTO Load and Resistance Factor Bridge Design Specifications (AASHTO LRFD) thermoplastic pipe design method requires corrugated pipe local buckling resistance to be determined based on corrugation effective area. The effective area may be determined through calculations or physical tests on sections of pipe. When determined through calculations, effective area is based on individual corrugation element (e.g., crest, valley, web) slenderness following methods published by the American Iron and Steel Institute (AISI) for cold-formed steel design. Cold-formed steel members are rolled from constant-thickness steel sheet and have cross-sections divided into elements by distinct corners. In contrast, corrugated thermoplastic pipe cross-sections have variable geometries with non-uniform thickness, elements without distinct corners defining their ends, and elements that may be rounded or have other beneficial features such as intermediate ribs or stiffeners. Applying the calculation method requires idealization of corrugation elements into flat plates of representative clear width and thickness. As corrugation geometries have evolved with the increased use of thermoplastic pipe, there has not been a thorough review of appropriate methods of idealization to ensure current geometries meet the intent of the design method. This paper reviews the existing AASHTO LRFD effective area calculation method, information from background documents upon which the AASHTO LRFD method was based (NCHRP reports 438 and 631), and relevant information from the AISI Specification for the Design of Cold-Formed Steel Structural Members; identifies important concepts for cross-section idealization; and provides recommendations for idealization of corrugation members with curves and intermediate stiffeners.