Calculation Variations Between the Indirect and Direct Design Methods Available to Purchase

Recently produced South Carolina Department of Transportation (SCDOT) reinforced concrete pipe (RCP) design standards capped the use of the indirect design method to RCP with diameter less than 36 in. in diameter and limited the use of the direct design method to RCP greater than or equal to 36 in. in diameter. In the original Marston and Spangler research done in the early twentieth century on concrete tiles up to 36 in. in diameter, flexure was the governing failure mode. In the latter half of the twentieth century, the American Concrete Pipe Association (ACPA) initiated a long-range study to determine the failure modes of large diameter RCP. Out of the long-range research came the standard installation direct design (SIDD) method, which adds crack control and radial and diagonal tension to flexure as potential RCP failure modes. Today, because of the simplicity of the indirect design method and the popularity of the three-edge bearing test, many designers and end users are choosing the indirect design method for large diameter pipe designs, not realizing that this design method does not directly address all of the RCP modes of failure. This paper looks at the direct and indirect design methods and outlines potential issues that may arise by designing RCP using the indirect or direct design methods independent of each other.