Research under NCHRP Project 12-26, "Distribution of Wheel Loads on Highway Bridges," considered numerous variables that affect the distribution of wheel loads and developed recommended design provisions. The recommended provisions apply to the Service Load and the Strength Design Methods, as well as to the structural evaluation of existing bridges. Load distribution criteria developed in this study include (1) simplified methods of analysis, including code formulas, and (2) more comprehensive and exact analytical models that are intended for computer-based application.

The effect of live load on the main longitudinal members of bridges is a function of the magnitude and location of truck wheel loads on the bridge deck surface and of the response of the bridge to these loads. Wheel load distribution factors allow engineers to analyze bridge response by treating the longitudinal and transverse effects independently, and empirical factors have been described in every edition of the AASHTO Standard Specifications for Highway Bridges since 1931. Prior research had produced a substantial amount of information on various bridge types and indicated a need for revision of the current AASHTO Bridge Specifications.

Research resulted in comprehensive specification recommendations for distribution of wheel loads in highway bridges. The findings of this study have been summarized in NCHRP Research Results Digest 187, "Distribution of Wheel Loads on Highway Bridges."

A PDF of the two-volume contractor's final report from the original project 12-26(1) is available here.