Indirect and Direct Design Methods for Design of Reinforced Concrete Pipe

There are currently two acceptable methods by which concrete pipe may be designed per the AASHTO Bridge Design Specifications: the direct design method and the indirect design method. The evaluation of applied load is similar for both methods, however, evaluation of the pipe’s capacity to resist applied load differs between the two methods. The indirect design method uses physical three edge bearing (TEB) testing at the production facility based on a relationship between the forces in the pipe wall in the installed condition compared with forces in the pipe wall from the TEB test. The direct design method follows the conventional design procedure for concrete members where demand versus capacity is determined using load and resistance factors to account for variability in applied loads and resistant capacity of the structure. Because of advances in computer technology, the direct method has become easier to apply than it was in the past. However, the indirect method, which has been used for approximately 70 years, has demonstrated conservatism and is a proven design method. Comparison of similar installations using the two methods has resulted in disagreements with respect to the minimum required reinforcement, however, both methods are adequately conservative, and each may have its place depending on the size and strength of the pipe. This paper presents the fundamental differences between the two design methods and offers some guidance on when to use each of them.