# Assessment of Simplified Rating Factor Methodology for Shallow Cover Metal Culverts
## 2 Definition and Origin of Minimum Cover Requirements
Furthermore, [PERSON]'s later study [7] restated that field experience has demonstrated that flexural failure under normal live loads is unlikely if the cover depth is greater than 20% of the span or 30.3 cm (one foot), whichever is greater, and the backfill is adequately compacted. Therefore, minimum cover specifications have been established with several factors in mind, including construction loading, potential thermal effects, dynamic effects that could damage the pipe, etc., but the primary impetus behind the minimum cover in current metal culvert construction specifications was to provide sufficient soil depth so that the primary resistance mechanism would be thrust in the structure wall rather than flexure. In \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Condition** & **Minimum Cover \({}^{*}\)** \\ \hline Corrugated Metal Pipe & – & S/8 \(>\) 30.5 cm (12.0 in.) \\ \hline \multirow{4}{*}{Spiral Rib Metal Pipe} & Steel Conduit & S/4 \(>\) 30.5 cm (12.0 in.) \\ \cline{2-3}  & Aluminum Conduit where & \\  & S \(<\) 122 cm (48.0 in.) & S/2 \(>\) 30.5 cm (12.0 in.) \\ \cline{2-3}  & Aluminum Conduit where & \\  & S \(>\) 122 cm (48.0 in.) & S/2.75 \(>\) 61.0 cm (24.0 in.) \\ \hline Structural Plate Pipe Structures & – & S/8 \(>\) 30.5 cm (12.0 in.) \\ \hline Long-Span Structural Plate & – & Refer to Table 12.8.3.1.1-1 \\ \hline Structural Plate Box Structures & – & 1.4 ft. (42.7 cm) as specified in \\  & Article 12.9.1 \\ \hline Deep Corrugated Structural & & See Article 12.8.9.4 \\ Plate Structures & – & \\ \hline \hline \end{tabular}  
* Minimum cover taken from top of rigid pavement or bottom of flexible pavement.  
\end{table} 
Table 1: Minimum cover definitions for metal culverts adapted from Table 12.6.6.3-1 of the AASHTO _LRFD Bridge Design Specifications_[14]. addition to minimizing the demand on surrounding soil, the minimum cover stipulation also facilitates simple analysis and design approaches that do not depend on defining the position and magnitude of the maximum moment and interaction between moment and thrust. However, from the load-rating standpoint, an existing metal culvert that does not meet the current design and construction minimum cover can still have resistance that is sufficient to meet the live load demands. The other critical factor is whether pavement, particularly flexible pavement, should be considered in the definition of cover used in rating factor analyses. NCSPA [15] acknowledges that \"asphalt does at least as good a job of distributing wheel loads as soil\"; however, it then points out that it is not included in the minimum cover definition because, during asphalt placement and compaction, heavy equipment could be over the pipe with insufficient cover. Furthermore, as noted in the background section, those involved in developing and assessing the ODOT and MDOT spreadsheet methodologies concluded that the minimum cover defined for design was overly conservative when applied to rating factor analyses. It is, therefore, clear that pavement should be involved in rating factor analyses, especially for shallow buried culverts.