Investigation of AASHTO Live-Load Reduction in Reinforced Concrete Slab Bridges

Abstract

This paper presents the results of a three-dimensional (3D) finite-element study that investigated the effect of the multipresence factor of the load-reduction factors used in the AASHTO Bridge Design Specifications. Typical one-span, two-equal-span continuous, simply supported, three- and four-lane reinforced concrete slab highway bridges were selected for this study. AASHTO HS20 design truck loads are first placed transversally in all lanes, positioned side-by-side and close to one edge of the bridge slab; this fully loaded condition served as a reference case. Reduced loading patterns are then investigated using 3D finite-element analysis (FEA), with design loads in two out of three lanes (reduced 2/3), three out of four lanes (reduced 3/4), and two out of four lanes (reduced 2/4). The longitudinal bending moments and deflection results obtained for the FEA for reduced and fully loaded bridges are directly compared. Furthermore, a correlation between the reduced-load cases and AASHTO reduction factors or multiple-presence factors is made for concrete slab bridges. For the three- and four-lane bridge cases, AASHTO Standard Specifications generally correlate well with or overestimate the FEA-reduced maximum moments and edge-beam moments by up to 15% and 30%, respectively. This overestimation is more pronounced in short-span bridges. It is recommended that a reduction factor of 25% be applied only for reinforced concrete slabs with span lengths greater than 12 m (40 ft) and a reduction factor of 10% be applied for spans less than 12 m (40 ft). The AASHTO-LRFD overestimated the maximum longitudinal moments and edge-beam moments by up to 15% and 40%, respectively. This overestimation by AASHTO-LRFD, which is larger for longer spans, is increased to 40% and 55% when compared to the FEA results because of reduced moments. This research supports the current AASHTO-LRFD multiple-presence factors of 0.85 for three lanes and 0.65 for four lanes in estimating longitudinal bending moments in concrete slab bridges. The FEA results highlight the importance of considering span length in determining the multipresence factors when designing three-lane or more concrete slab bridges. This paper will assist bridge engineers in quantifying the adjustment factors used in analyzing and designing multilane reinforced concrete slab bridges.