Analysis of Live Loads on Culverts

January 30, 2008
Publication: Transportation Research Board 2008 Annual Meeting in Washington, DC
Author(s): Peterson D.L. Li G. Nelson C. McGrath, Timothy J.

Abstract: AASHTO Standard and LRFD Specifications differ in the manner that live loads are spread through fill onto culvert structures. Standard Specifications apply surface point loads, and spread loads at the rate of 1.75 times depths. The LRFD specifications apply live load through a tire footprint of 10 in. by 20 in. at the surface but attenuate with a lower coefficient (1.00 or 1.15 as a function of soil type) as the depth of fill increases. This paper reports a 3D, numerical model-based investigation of how live loads spread with depth, as a function of soil and culvert type. The investigation included the following tasks: selection of appropriate software and soil models, verification of model predictions, and 3D analysis of about 830 buried culverts to provide a basis for development of Simplified Design Equations for structural response. Modeling results show that live load spreading with depth depends upon depth, soil characteristics, and culvert characteristics. Pavements substantially reduce soil stress and structure forces, so the unpaved case controls. The distribution of vertical stresses on the plane at the crown of buried culverts varies substantially depending upon the soil properties, culvert characteristics and depth. A spreading constant of 1.75 does not adequately capture the results, a spreading constant of 1.15 is slightly unconservative at shallow depths, adequate for 24-in culverts at most depths, and conservative for larger culverts. Regarding bending moments in the structures, the crown bending moment has the greatest absolute magnitude. The peak negative bending moment is typically at the springline or above, as high as 60 degrees above the springline for large diameter culverts near the surface. Regarding thrusts in the structures, peak thrusts may occur anywhere from the culvert crown to the springline. Shallow burial tends to produce peak thrusts near the crown and deeper burial tends to shift the peak thrust closer to the springline. Invert thrusts are typically small,and may be either slightly negative or slightly positive.

Services: Structural Design
Markets: Highway & Tunnel