Live Load Distribution Factors in Two-Girder Bridge Systems Using Precast Trapezoidal U-Girders

Abstract

The lever rule method is often used to determine live load distribution factors (LDFs) in two-girder bridge systems because of the ranges of applicability implicit in the simplified equations of the AASHTO LRFD bridge design specifications. This method is typically more conservative than the simplified equations, which take into account key parameters such as beam spacing, span length, longitudinal beam stiffness, and slab thickness. The lever rule method for yielding LDFs does not take into account these key parameters and only considers simple span distribution; therefore, a certain degree of conservatism is implied. This study demonstrates that the lever rule method in determining LDFs in two-girder bridge systems using precast trapezoidal U-girders is not overly conservative, as comparisons between LDFs from finite-element analysis (FEA) and the lever rule method of the AASHTO LRFD show that the lever rule method produces values for LDFs that are closely reflective of the actual response for shear and to a lesser degree for flexure. The effects of live load on parameters that include flexural response, slenderness, and span length are also considered.