The Evolution of the Application of Highway Live Loads to Buried Concrete Pipe

Abstract

For many years, the application of highway live loads to the surface, and their distribution down to buried concrete pipe, was consistent and reasonably easy to understand. However, near the beginning of the new millennium, the application of highway live loads through soil began an evolution in in the United States that resulted in a myriad of changes. The effect of live load on a buried pipe is a result of the application of the live load at the surface, the assumed distribution of that live load through the soil, and furthermore, the dissipation of the load through the structure itself. Modifications have been made to all of these parameters in the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (AASHTO LRFD) Bridge Design Specifications within the last two decades. The intent of this paper is to review the history of highway live load design for concrete pipe and to discuss their development within the AASHTO LRFD Bridge Design Specifications. Despite the fact that highway live loads themselves have barely changed over the decades, the live load distribution factor has undergone more than one change in the AASHTO codes over the last several years. Meanwhile, the dissipation of the load through the pipe itself was never really addressed within the AASHTO until recently, but a method developed by the concrete pipe industry has been used for years. This has led to inconsistencies with regard to how engineers would address these issues, depending upon which references were consulted for their designs. This paper reviews the history of highway live load design on buried concrete pipe, including tabular and graphical examples of results from the various methods, and provides suggested applications.