Comparison of AASHTO standard and LRFD code provisions for buried concrete box culverts

AASHTO developed the LRFD Bridge Design Specification, with the intent of replacing the Standard Specifications for Highway Bridges with a reliability based code that provides more uniform safety for all elements of bridges. Although many provisions in these two codes are the same, there are important differences that can have a significant effect on the amount of reinforcement required for buried precast reinforced concrete box culverts under some conditions. AASHTO LRFD Code provisions that differ from the Standard Specifications include load factors, load modifiers, load combinations that produce the maximum force effect, multiple presence factors, design vehicle loads, distribution of live load to slabs and through earth fill, dynamic load allowance, live load surcharge, and reinforced concrete design methodology for fatigue, shear strength, and control of cracking. These provisions are compared with the equivalent provisions from the AASHTO Standard code, and discussed in detail. Several combinations of culvert sizes and fill heights are evaluated using provisions from both the Standard and LRFD Specifications. A comparison of results show that in general, LRFD provisions result in increased design loads and reinforcement areas.