However, certain serviceability checks were left optional because they are not directly related to bridge safety, but are geared to protecting the long-term serviceability and durability of structures.
The load and resistance factor rating (LR FR) section of the manual is based on reliability theory to assure a certain level of safety for members. The American Association of State Highway and Transportation Officials (AASHTO) Manual for Bridge Evaluation was developed to assist bridge owners by establishing inspection, evaluation, load rating, and posting practices and procedures. Therefore, it is desirable to identify design practices, design truck assessments, and detailed code calibration procedures used in other countries to assure the safety and serviceability of newly designed bridges.
However, the calibration did not include serviceability calibration to assure bridge serviceability and performance, and it did not use comprehensive statistics available in the United States because the available weigh-in-motion (WIM) data was deemed unreliable. Present-day design specifications (load and resistance factor design (LR FD)) assure safety by analyzing the effect of heavy, legal trucks throughout the United States and comparing that effect to a protocol calibrated using limited but very reliable Canadian site statistics. These tools include an overall, integrated approach to bridge analysis, design, evaluation, and determination of load-carrying capacity (load rating). New, advanced tools and protocols are available to help bridge engineers better assess and assure safety and serviceability of highway bridges.