Framework for Simplified Live Load Distribution-Factor Computations

Abstract

The live load distribution-factor (LLDF) equations in the AASHTO-LRFD specifications were developed under National Cooperative Highway Research Program (NCHRP) Project 12-26. These equations include limited ranges of applicability, and when these ranges are exceeded, a refined analysis must be used. Additionally, the multiple-presence factors, bias, and variability with respect to the rigorous estimates are obscurely embedded. Herein, a simplified LLDF framework is provided. The effects of analysis uncertainty, variability, and multiple presence are separated and distinctly defined. This separation provides specification writers with the opportunity to use different multiple-presence and variability models. LLDFs were calculated using several simplified methods and grillage analyses for over 1,500 bridges. Based on the comparison, two simplified methods were further studied: adjusted uniform distribution and adjusted lever rule methods. Calibration factors were used to their improve accuracy. Seventy-four “test-the-limits” bridges (11 steel) were designed to test wide ranges of girder spacing, relative transverse to longitudinal stiffness, and bridge flexural-to-torsional stiffness. The focus of this paper is on the framework. The procedure and results are similar for other bridge types.