Continuous Load Scaling: New Method of Simulating Longitudinal Live Load Spreading for Two-Dimensional Analysis of Buried Culverts

Abstract

Because of their prismatic configuration, buried culverts are often designed and analyzed as two-dimensional (2-D) plane strain soil structure systems, as exemplified by the Culvert Analysis and Design (CANDE) finite element program. One major difficulty with 2-D analysis is that live loads are not infinitely long prismatic strips but are finite footprints that induce load spreading through soil in the in-plane and out-of-plane directions. Because traditional 2-D analysis permits only in-plane load spreading, the predicted soil stresses are increasingly overestimated as soil depth increases. The traditional approach to correcting the overestimate is to reduce judiciously the 2-D strip load; this process is called the reduced surface load (RSL) procedure. The reduction factor is dependent on an assumed load-spreading theory and a selected soil depth (H). RSL analysis results in soil stresses that are correct at the selected soil depth (H) but understressed for depths less than H and overstressed for depths greater than H. The new continuous load scaling (CLS) procedure introduced in this paper overcomes such shortcomings by continually diverting live load in the longitudinal direction as a continuous function of soil depth. This diversion is achieved by continuously increasing the plane strain unit thickness as a function of depth on the basis of an assumed load-spreading theory. CANDE solutions were obtained for both RSL and CLS procedures and compared with three-dimensional solutions for free-field conditions and for flexible and rigid culvert installations. The bottom-line finding is that the CLS procedure with the elasticity-based load-spreading method is far superior to all other 2-D methods.