Three-dimensional analysis of soil - Steel bridges

The present design of soil-steel bridges is based on plane-strain analysis by considering a slice of a unit width of the conduit wall and the surrounding soil. This two-dimensional analysis neglects the third-dimensional effect of the steel shell and the soil continuum which could be significant especially when the load varies in the longitudinal direction, as in the case of live load acting over a shallow cover. The structure is also subjected to a varying dead load due to the variation in the depth of cover from maximum at the middle part of the conduit to zero at the conduit edges. A three-dimensional finite element analysis is presented in this paper to examine the actual three-dimensional behaviour of soil-steel bridges. The thrust and bending moment around the conduit walls as well as the stability of a single conduit are presented and compared with the results obtained from plane-strain analysis. Also, the live load dispersion in the soil above the conduit is examined and compared with some present codes. The study leads to evaluation of the degree of approximation inherited with the practical approaches of the two-dimensional analysis.