Abstract: In this paper, special behavior of single-cell trapezoidal box section is discussed particularly with longer cantilever slabs. Then, investigational reports on box section and other section in the United States and Canada are given with dimensions on cross-sections as well. An approximate methos is developed to compute the lateral forces due to longitudinal pretressing considering the losses due to frictionand wabbling effect. An illustrative example shows the procedures for estimating the lateral forces in top and bottom slabs.