The seismic performance of underground linear structures is generally verified against member failure in the cross-sectional direction using two-dimensional seismic response analysis of soil-structure coupled systems. On the other hand, to verify the seismic performance of a single structure, a pushover analysis of the structure itself is sometimes performed. In a pushover analysis of a single structure, the force acting on the structure differs from that of a structure continuously surrounded by the surrounding ground, depending on how the boundary conditions, such as support points and loading points, are given. Therefore, in this study we proposed reasonable boundary conditions for pushover analysis of the cross-section of RC box culvert using a fiber model that considers material nonlinearity.