Comparison of Standard Load and Load and Resistance Factor Bridge Design Specifications for Buried Concrete Structures

For the past 30 years, it has been common practice to use AASHTO standard design specifications for underground precast concrete structures. Today, the bridge engineering profession is making the transition from the standard AASHTO load factor design bridge specifications to the load and resistance factor design (LRFD) specifications. The FHWA has mandated that all culverts, retaining walls, and other standard structures designed after October 2010 be designed according to the AASHTO LRFD bridge design specifications for a project to receive federal funding. This paper compares related provisions from both specifications used in designing underground concrete structures, such as underground utility structures, drainage inlets, three-sided structures, and box culverts. Although many provisions of these two codes are the same, important differences could have a significant impact on the amount of reinforcement, member geometry, and cost to produce buried reinforced concrete structures. Many AASHTO LRFD code provisions that differ from the standard specifications include terminology, load factors, implementation of load modifiers, load combinations, multiple presence factors, design vehicle live loads, distribution of live load to slabs and earth fill, live load impact, and live load surcharge. This study demonstrates that greater reliability and a more uniform factor of safety are obtained when the LRFD specifications are used.