Lateral Live-Load Distribution of Dual-Lane Vehicles with Nonstandard Axle Configurations

Abstract

The numbers and sizes of permitted vehicles that consist of at least four wheel lines have been increasingly crossing on highways and bridges. For safety purposes, it is necessary to understand how those truck loads are distributed to the primary bridge elements. It is widely accepted that the lateral live-load distribution factor (LDF) is affected by the spacing of adjacent wheel lines of truck loads. However, current specifications for the LDF are applicable only for vehicles that have standard axle configurations, whereas oversized vehicles usually have nonstandard wheel-line spacing. Furthermore, Iowa’s 5-ft (1.5-m) spacing requirement, which specifies that dual-lane trucks with interior-pair wheel-line spacing ≥5 ft (1.5 m) are allowed to have an axle weight up to 20 kips/lane (90 KN/lane), should also be evaluated. The objective of this paper was to investigate the impact of the wheel-line spacing of four-wheel dual-lane loads on lateral live-load distribution on slab-on-girder bridges. Twenty prestressed-concrete and 20 steel-girder bridges from the Iowa bridge inventory were sampled randomly. Two-dimensional linear elastic finite-element (FE) models of the selected bridges were established, and 22 types of single-axle four-wheel-line loads with variable adjacent wheel-line spacing were applied. The LDFs for the investigated bridges were determined on the basis of the extracted internal shears and moments of girders from the FE models and were compared with those estimated using current LRFD and load factor design (LFD) equations. For practical needs, axle weight limits for those dual-lane loads were determined on the basis of the calculated LDFs. It was found that both moment and shear LDFs decrease with increases in the outside and inner wheel-line spacing of the dual-lane loads. The current Iowa DOT practice with Iowa’s 5-ft (1.5-m) spacing requirement appears reasonable and adequate. The current LRFD equations significantly underestimated (or overestimated) moment LDFs for the interior (or exterior) girders of the four-girder bridges. Both the LRFD and LFD equations mostly underestimated (or overestimated) shear LDFs for interior (or exterior) girders. For safety concerns under dual-lane loading scenarios, moment LDFs for the interior girders of four-girder bridges and the exterior girders of more-than-four-girder bridges should be determined by averaging the estimations according to the lever rule and LRFD equations, and shear LDFs for the interior girders of bridges should be determined on the basis of the lever rule.