Improved Methods for Simulating Live Loads for Two-Dimensional Structural Analysis of Buried Culverts

Abstract

The current AASHTO LRFD Bridge Design Specifications stipulates a “special distribution width” for two-dimensional (2D) live-load analysis of reinforced concrete (r/c) boxes and arches with less than 2 ft of soil cover. This special distribution width allows a significantly greater reduction of the applied surface load than permitted for other culvert shapes and materials. Neither the AASHTO commentary nor the underlying developmental report provide the physical reasoning for the special distribution width, or why it only applies to r/c boxes and arches. This paper provides a clear, physical understanding of the three-dimensional (3D) phenomena associated with the special distribution width and the interaction with longitudinal load spreading through soil. This is achieved with the aid of a flat-plate model, representative of the top slab of a box culvert, with a variable line-load width. The closed-form solution reveals that the physical reason is “3D stiffness effects” (3DSE), which occur when the line-load width is relatively short compared with the culvert’s longitudinal lay length. Moreover, it is shown that 3DSE disappear when the line load reaches a special width called the “critical distribution width” or W_critical. W_critical is dependent on the culvert’s span and length, and is a key parameter along with parameter W_min needed to identify the limiting line-load width that evokes 3DSE. The key concepts of 3DSE, W_critical, and W_min are used to develop improved 2D analysis procedures using either the traditional reduced surface load approach or the more recent continuous load spreading approach.