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Appendix A – Amplifying Questions

Quantification of Safety

In the United States, structural safety is measured through a reliability-based uniform safety index (reliability index) for individual structural members and is based on live load data (frequencies and weights). In current design specifications, the index is selected on an average value of reliability indices of existing bridges, not on a desired level of safety. The reliability index is achieved by specifying calibrated load amplification factors and capacity reduction factors. Neither the live load data nor the reliability index has been revisited since the development of the present specifications.


In the United States, serviceability considers deformation, cracking, and stress limits of components. These criteria are based on past practices. Serviceability criteria are intended to give 75 years of service life, but the criteria used are not based on scientific evidence or research.

Safety and Serviceability—Operations

In the United States, structural safety of existing bridges is measured through two uniform safety indices for individual structural members that are based on live load data and the structural condition of members. Load capacity evaluations can be done at a higher national screening level (inventory level) or a local screening level (operating level). The index for the national inventory level is based on the design level of safety. The local operating level is based on a lower level of safety determined by the local jurisdiction’s experience with its existing bridges through smaller live load amplification factors.

Laws and Regulations Governing In–Service Bridges
Load-Carrying Assessment (Evaluation and Rating) of Bridges
Record Keeping

Refined Analysis—Design, Construction, and Operations

In the United States, bridge code longitudinal effects are uncoupled from transverse effects using empirical formulas for live load distribution. This uncoupling process allows simplified analysis of single members or sections. For complex bridges refined analysis (grillage and finite element analysis) may be used.

Research and Development
Page last modified on November 7, 2014
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