Correction of Plane Strain Analyses for Corrugated Metal Culverts Subjected to Patch Loads

Today, culverts are often analyzed and designed using plane strain finite element analyses in which a transverse slice of the structure is simulated and assumed equal to any other slice. Although this works well for soil loads, in reality live loads are discrete pressure loads from tires, and the out-of-plane effects can be significant, especially when cover depths are relatively low. While current design specifications define minimum cover that precludes significant localized pressure effects on culverts, many culverts are currently in service that were constructed under very shallow cover depths. In this study, analyses of corrugated metal culverts under truck load were carried out using two-dimensional (2D) (plane strain) and three-dimensional (3D) finite element analyses to assess the difference between structural responses predicted by these two approaches. Thirty-six culvert models with different shapes, spans, and cover depths were created using both 2D and 3D meshes. A significant difference was found between the results of these two analysis methods. Specifically, the thrust from the 3D models under shallow cover can be as much as three times the thrust results from the 2D model, which is unconservative from a safety standpoint. An innovative explanation was provided to explicate the differences. Finally, to address the issue, a reduction ratio method was developed to correct the 2D results to match the corresponding 3D results so that the relatively simple plane strain approach can be used.