The buried box girders in the mountain city sewage pipeline system would be turned into simple-supported girders because of the removal of the foundation caused by landslide.This change of supporting would result in the failure risk of pipeline structure.The static properties of the buried box girders with small depth-to-span ratio in simple-supported condition are experimentally analyzed.The failure pattern

shear behavior and shear lag effect of the buried box girders in simple-supported condition are studied.The shortages in the present design codes for the calculation of shear capacities of the box girders are also discussed.It is shown that under the vertical uniformly distributed load

the crack distribution of the box girders is uniform

and no major critical diagonal crack is formed

and the box girders lose the bearing capacity in the form of concrete diagonal rod crushing.As the main shear-bearing components

the stirrups devote much to shear capacity of the box girder after concrete cracking.The strains of the longitudinal rebars are affected by those factors such as the redistribution of internal force and cracks

so only few areas of the box girders show the shear lag effect.The change of supporting form leads to the premature cracking of concrete and affects the serviceability of the sewage pipeline.