Live load distribution widths for reinforced concrete box sections

July 30, 2005
Publication: Sixth International Bridge Engineering Conference, Transportation Research Board, Boston, MA p 99-108
Author(s): Jesse Beaver McGrath, Timothy J. Atis Liepins

Abstract: AASHTO introduced the AASHTO "LRFD Bridge Design Specifications" in 1994. These load and resistance factor design (LRFD) specifications included new provisions for distributing live loads to reinforced concrete bridge decks that also apply to the design of reinforced concrete box sections with depths of fill less than 2 ft. Designs according to the LRFD specifications can be much more conservative than designs according to the AASHTO "Standard Specifications for Highway Bridges" and are inconsistent at a span of 15 ft. A finite element study sponsored by the Pennsylvania Department of Transportation investigated these issues and determined appropriate live load distributions for box sections subjected to live loads; the results are presented. The computer models were three dimensional for analysis of the distribution of live load effects along the length (flow direction). Box section models had spans of 8 ft, 16 ft, and 24 ft, and all had an 8-ft rise. The models had different slab and wall thicknesses, different haunch sizes, 0-ft and 2-ft depth of fill, different support reactions, live loading at the section midlength and free edge, and variation of longitudinal stiffness. The results indicate different distribution widths for positive bending moment, negative bending moment, and shear. Distribution widths for shear forces are the narrowest, followed by those for positive bending moment. The analysis results support the following proposed distribution of axle loads to the top slabs of box sections with less than 2 ft of fill: E(ft) = 8(ft) + 0.12 span (ft). Provisions are suggested to address multiple-lane loadings and distribution widths for the bottom slab.

Services: Structural Design