Stress and Displacement Comparison and Stress Based Rock Sharing Assessment of an Exposed and Buried Crescent Rib Steel Bifurcation

Steel bifurcations used in water-conveyance systems respond differently to internal pressure when installed with and without external restraint. This study compared an exposed crescent-rib steel bifurcation (Case A) with an otherwise identical buried bifurcation surrounded by concrete and rock (Case B). Finite element models were developed using identical steel meshes and pipe-end restraints. Both configurations were subjected to an internal pressure of 11.57 MPa. The materials were treated as linear elastic, while frictional sliding and separation were permitted at the interfaces in the buried model. The rock elastic modulus was 4,000 MPa. Nodal-averaged von Mises stresses were examined at the inner surface, midsurface, and outer surface of the pipe shell and in the crescent rib. Maximum pipe-shell and rib displacements were also compared. The buried configuration reduced peak inner-surface, midsurface, and rib stresses from 753.09, 428.60, and 458.51 MPa to 229.35, 159.42, and 252.66 MPa, respectively. Maximum shell and rib displacements declined from 15.607 and 8.408 mm to 1.780 and 1.741 mm. The stress-reduction index calculated from mean midsurface shell stress was 58.28%, compared with 58.84% and 58.19% at the inner and outer surfaces. External restraint therefore altered both the magnitude and distribution of structural responses. This index describes the combined restraint of concrete and rock within the elastic model; it does not quantify the load carried independently by rock or establish a design coefficient.