Search

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...

The simplified equal distribution factor (EDF) method for live load shear presented in this study originates from Henry's method, a method that has been used in Tennessee for nearly forty years. Henry's method allows for equal distribution of live load effects in all beams. This study focused on a careful examination and modification of Henry's method by comparing shear distribution factors obtained from Henry's method with those from finite element analysis and other code-specified methods for actual bridges. Twenty-four Tennessee bridges with six different types of superstructures were used in the study. The effects of structural parameters on the shear distribution factors were also studied. Modification factors to Henry's method were proposed based on the comparison study. The proposed modification factors include structure type factors that are applied to different types of superstructures and a skew correction factor that is used to account for the effects of skew angle for skewed bridges. With proper modifications, the simplified EDF method can produce very reasonable and reliable distribution factors for live load shear.

A general refined analysis procedure for predicting the vehicular live load distribution on simply supported bridge girders is described. Load and Resistance Factor Design (LRFD) distribution factor formulas for flexure are carefully reviewed for modern prestressed concrete bridges made of I-girders or spread box girders with larger span-to-depth ratios. LRFD's results are compared with those obtained from the refined analysis method. Design recommendations are offered.