Abstract: This paper takes an actual  frame bridge as the engineering background, establishes a model of the bridge under design load, and analyzes the influence of the middle wall on the mechanical characteristics of the frame bridge. Comparative research shows that the extreme values of the top and bottom plates of the frame bridges decrease in vertical displacement, vertical axial force, shear stress; the extreme values of the curvature change and cross-sectional stress of the top plates decrease,  while those of the bottom places increase. The research also shows that the vertical displacement of the top plates is most obviously affected by the middle wall and the shear stress of the bottom plates is lest affected by the middle wall.

Key words: middle wall, mechanical characteristics, underpass bridge, finite element analysis