Serviceability limits and economical bridge design

The objective of this research effort was to conduct state live-load deflection criteria and the AASHTO LRFD and AASHTO LFD specifications and the effect on economy. Of special concern is loss of economy when using High Performance Steel (HPS). The relationship between the LRFD and LFD specifications and the impact of LRFD was also examined.;Six states were selected for the serviceability comparisons. The states represent various levels of conservativeness. A set of ten in-service steel bridges (one with significant field test data) were used. The bridge parameters include: number of spans, length, width, girder spacing and geographical location.;The selected state deflection-limit practices were applied. The variations in state-specific loading, analysis and deflection limits for LFD and LRFD were studied. Finally, the design impact of state-specific practices was examined.;Results show that the AASHTO LFD and LRFD deflection criteria typically do not control design and, therefore, do not have a negative impact on economy of conventional or HPS bridges.;The relationship of AASHTO LRFD criteria to AASHTO LFD criteria was applied to determine the possible impact as states use LRFD. Variables studied were the differences in loading and analysis, where analysis differences are attributed to live load distribution, multi-presence of vehicles, and dynamic effects. The results computed show that LRFD deflections are slightly larger overall for single span bridges, but are reduced for multi-span bridges. However, the differences are relatively small in both cases. Thus, states using LRFD should not notice a significant change in the deflection criteria results from the LFD procedures. The strains and rating factors were also studied and slight variations in the specifications were observed.;Many states have adopted criteria more conservative than AASHTO, consequently, design may be controlled by deflections requiring more steel meet their standards. Some of the results are quite astonishing with significant cost. All ten bridges are in service and performing well. No deficiencies are apparent in either user comfort or deformation-induced damage. Conservative states maybe expending unnecessary materials and costs.;A study of the mechanistic deck strains expected in the deck over the piers at various load levels was conducted. The deflection limits are independent of the tensile strain produced at the piers. An alternate method of preventing structural degradation to bridge decks was also introduced to limit deck cracking.;This project is part of an overall research effort to produce rational deflection criteria (or other form of serviceability criteria) to limit user discomfort and deformation-induced structural damage. Current AASHTO criteria do not effectively meet that purpose. Additionally, many states have chosen to use more conservative deflection criteria than AASHTO. This results in more costly bridges and inconsistent design procedures. It also impedes the use of high performing materials such as HPS when deflections and not strength (safety) controls the design. Implementation of realistic and appropriate deflection limits over the nation’s bridge inventory will result in more efficient bridges. Conventional steel and HPS design will be more consistent and cost effective.