Currently, in the AASHTO LRFD Bridge Design Specifications, reinforced concrete pipe can be designed according to one of two methods; the indirect design method, or the direct design method. The Indirect Design Method (IDM) uses tables to select pipe class (thickness, reinforcement, and concrete strength) for a given fill height and installation type. The method is based on three-edge-bearing (TEB) tests. The Direct Design Method (DDM) Method is a more theoretical design method where four separate structural design limit states are considered: flexure, shear (diagonal tension), radial tension, and crack control. These two methods may give different answers for design of equivalent pipe, depending upon size and installation requirements. The IDM is based on a comparison of field moments versus test moments, and may be an overly conservative simplification when flexure criteria are not the governing limit state. The DDM has simplifying assumptions for reinforcement conditions, as well as limitations on the steel and concrete properties that may not allow for a pipe to be designed to its true strength. Thus, an inappropriate selection of design method can result in unneeded expense. Recent suggestions for determining appropriate guidelines for which design method to use have ranged from taking the lower steel area requirement of the two designs, to using the DDM for pipe larger than 36” and the IDM for pipe 36” and smaller.
The objectives of this research were to: (1) review the strengths and weaknesses of the IDM and DDM (2) recommend guidelines for when each method is most appropriate for use; and (3) recommend revisions to the AASHTO LRFD Bridge Design Specifications, Section 12.
Products Availability: Agency final report was submitted to AASHTO Technical Committee on Culverts (T-13). (NCHRP Staff: W. Dekelbab)