Guideline for Interpreting AASHTO Specifications to Design or Evaluate Buried Structures with Comprehensive Solution Methods

Design engineers and consultants are required to satisfy the criteria set forth in the AASHTO LRFD Bridge Design Specifications when designing buried culverts within the national transportation infrastructure. Adherence to the specifications is especially difficult when an engineer uses a comprehensive solution method such as a finite element program to predict the deformations and internal forces of the culvert-soil system. Part of the difficulty is in knowing how to apply factored loads as well as to determine their values. A more difficult problem is to assess whether the culvert is structurally safe according to the AASHTO LRFD specifications. Said another way, the design criteria and the corresponding factored capacities as written in the specifications are less than well defined. A succinct interpretation is presented of the AASHTO specification for load and resistance factor design (LRFD) for buried structures with regard to factored loads and design criteria. Tabularized values are provided for load factors and load modifiers to quantify dead loads, earth loads, and live loads, which are required for a LRFD analysis to produce the factored demands. Also presented are tabularized descriptions of the design criteria and the corresponding factored capacities, which are defined and quantified for corrugated metal, reinforced concrete, and plastic pipe materials. For some culverts, the design criteria have been generalized to provide sufficient information that is compatible with comprehensive analysis methods. Although intended especially for design engineers who use comprehensive analysis tools, the information presented is also useful and educational for engineers who use the simplified design solutions in the AASHTO LRFD specifications.