NCHRP Report 473: Recommended Specifications for Large-Span Culverts contains the findings of a study to develop recommended design and construction specifications for metal and concrete large-span culverts. The report describes the research effort leading to the recommended specifications and includes information on field-testing and computer modeling. The methodology used to develop simplified design equations is also included. The material in this report will be of immediate interest to specification writers and to engineers concerned with the design and construction of large-span culverts. Design examples illustrating the application of the design equations are available in the linked NCHRP Web Document 44. The unpublished appendices for NCHRP Report 473 (A, F, G, and H) are available here.

Flexible and rigid large-span culverts, typically ranging from 3 to 9 m (10 to 30 ft), are often a practical structure for crossings, especially on local road systems. The use of these structures is growing, and the available design and construction specifications related to them are in need of improvement. Large-span culverts are complex structures whose design and performance are dependent on the interaction of the structure and the surrounding soil. Properties of the backfill envelope as well as in situ material have a major effect on the performance of these structures, and additional knowledge about these effects is needed.

Under NCHRP Project 12-45, Simpson Gumpertz & Heger Inc. in cooperation with the University of Massachusetts and the University of Western Ontario, monitored the performance of full-scale metal and concrete culverts during backfilling and under vehicle loads. The results of the experimental program were modeled and extended with finite element analysis to create the data necessary to develop the simplified design expressions. The analysis and compilation of prior experience with long-span culverts provided the basis for the recommended design and construction specifications for large-span culverts. These specifications are consistent in philosophy and format with the AASHTO LRFD Bridge Design Specifications and are accompanied by a commentary.