NCHRP Report 575 contains the findings of research performed to develop recommended revisions to the legal loads for posting as depicted in the Manual for Condition Evaluation of Bridges and the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges. The report details the development of the new loads and includes recommended revisions to the manuals to incorporate these loads. The material in this report will be of immediate interest to bridge managers and load raters.
In the United States, trucks are typically allowed unrestricted operation and are generally considered “legal” provided they meet weight guidelines of Federal Formula B. Bridges are not usually posted if they have the capacity to carry the legal loads for posting described in the Manual for Condition Evaluation of Bridges or the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges.
During the past several years, the trucking industry has enhanced the load-carrying capacity of commercial vehicles by using a series of closely spaced axles. These axle configurations make it possible for shorter trucks to carry the maximum load of up to 80,000 pounds and still meet the requirements for Formula B. Nevertheless, these vehicles induce stresses in bridge structures that exceed the stresses induced by AASHTO Legal Loads for Posting. The result is that legally loaded trucks are overstressing some nonposted bridges.
The objective of this research was to reliably identify and quantify the types of short multi-axle legal vehicles operating on the public thoroughfares and the subset of these vehicles that cause overstressing. This information was used to develop new definitions for the AASHTO Legal Loads for Posting for use in the Manual for Condition Evaluation of Bridges and the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges such that bridges are more appropriately posted.
This research was performed by Lichtenstein Consulting Engineers, Inc. with contributions from Fred Moses, Gongkang Fu, and Michel Ghosn. The report fully documents the research leading to the recommended truck configurations and load factors.